pozzolanic cements

Cement & Concrete Composites 15 (1993) 185-214

Pozzolanic Cements* Franco Massazza

Italcementi, Via G. Camozzi 124, 24100 Bergamo, Italy

Abstract

Natural and artificial pozzolanas have been used to obtain hydraulic binders for over a thousand years. Hardening of pozzolanic cement pastes can result from the reaction between pozzolana and the lime that is added to the mix as hydrated lime or is produced following hydration of portland cement silicates. The pozzolanic reaction does not alter cement clinker hydration; it complements and inte- grates the hydration process because it results in a lower portlandite content and an increase in calcium silicate hydrates.

Besides reviewing the most recent investigations on pozzolana-containing cements, this paper shows that the behaviour of different types of pozzolana can be quite similar when they are blended and become hydrated along with portland cement clinker. Portland cement properties may undergo several qualitative modifications the extent of which substantially depends on the pozzolana/ clinker ratio. So, a maximum is reached in pozzolanic cements.

As in the case of pozzolanic cements, for which the current pozzolana content is about one third by weight of cement, the most outstanding variations induced in the behaviour of portland cement can be summarised as follows. Heat of hydration decreases whilst the rate of clinker hydration increases, paste porosity increases and permeability decreases, both portlandite content and Ca/Si ratio in C-S-H decrease and the C-S-H content increases.

Chemical and physical properties of pozzolanic cements eventually affect engineering ones. Early strength of both pastes and concretes decreases while ultimate strength is often found to exceed that of the reference portland cement.

If cements contain small amounts of very active pozzolana (silica fume, for example), both early

*Keynote paper presented at the International Conference on Blended Cements held in Sheffield in September 1993.

185

and ultimate strengths may be higher than those of the substituted cement.

Creep is found to increase definitely with increasing pozzolana content whereas shrinkage remains practically unaffected.

Chemical and microstructural variations in the paste also influence resistance of concretes to environmental attacks.

The low basicity and permeability resulting from the presence of pozzolana increase the concrete's resistance to lime leaching, sulphate and sea water attacks, and chloride penetration. Carbonation depth is practically unaffected. Pozzolana containing cements can help avoid expansion induced by alkali-silica reaction. Concrete resistance to freezing is not affected by the use of pozzolanic cement since it basically depends on the entrained air content.

The results of a variety of studies introducing a comparison between pozzolana-containing cements and corresponding portland cements can be summarised as follows: cements with appreci- able pozzolana contents perform better in the long term rather than at an early age.

In most cases, however, the differences between the two types of cements are not so marked and as a consequence both cements are interchangeable especially for the most common building types.

Keywords: Pozzolanic cements, chemical reactions, hydration products, microstructure, porosity and permeability, curing, shrinkage and creep, durability, carbonation, chloride penetration, chemical resistance, alkali-aggregate reactivity, frost resistance.

1 P O Z Z O L A N A S

The term 'pozzolana' has two distinct mean- ings. The first one indicates the pyroclastic rocks, essentially glassy and sometimes zeolitised, which

Cement & Concrete Composites 0958-9465/94/$7.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

186 Franco Massazza

occur either in the neighbourhood of Pozzuoli (the ancient Puteoli of the Roman times) or around Rome. ~ The second meaning includes all those inorganic materials, either natural or artificial, which harden in water when mixed with calcium hydroxide (lime) or with materials that can release calcium hydroxide (portland cement clinker). In this review the term 'pozzolana' will be referring to the latter meaning, definitely wider than the former, and will therefore embrace a large number of very different materials in terms of origin, composition and structure.

For a long time the use of pozzolanas has been mostly restricted to Italy, where considerable reserves of natural pozzolanas are found. In other countries the interest in these materials is of relatively recent date and has arisen due to the need for the disposal of fly ashes and silica fume.

This brief historical background can help explain why so many countries have long dis- trusted pozzolana-containing cements despite the millenary use of lime-pozzolana mortars and the almost century-old experience in pozzolanic cements. Anyway, the results of a variety of studies have substantially proved that pozzolanic cements, despite the lower initial rate of hardening as compared to portland cements, can yield a higher ultimate strength. Furthermore, they show a greater resistance to the attack of aggressive agents.

Literature concerning pozzolanas and pozzolana-containing cements is however too extensive to be summarised in this short review. For more detailed information the reader is referred to the proceedings of the major conferences on both

natural and artificial pozzolana-containing cements.2-t2

A technical classification of pozzolanas, pro- posed in 1976, is illustrated in Fig. 1.~3

Pozzolanas are formed by 'active' phases, capable of reacting with lime, as well as 'inert' phases, insensitive or little sensitive to its action.

As shown in Table 1, all active phases are thermodynamically unstable in as much as they are generally constituted by more or less altered glasses as well as amorphous constituents. The only exception is represented by zeolites which are crystallised minerals.

All active phases are rich in silica: this property represents the second most important feature of the materials having pozzolanic behaviour. Some of the typical composition data are summarised in Table 2. The crystalline phase content in pozzolanas varies very appreciably, e.g. between 15% and 35% in fly ashes. 22

The glassy portion of pozzolanas has a variable composition and, in addition to this, in natural pozzolanas it can be more or less altered, zeolitised or clayified. 3 As far as fly ashes are concerned, it is generally agreed that, among the different particles of the same fly ash, differences may also occur in the glass content as well as in the composition of the glass i tself . 23-25

2 L I M E - P O Z Z O L A N A M I X E S

Until the last century, lime-pozzolana mixes constituted the only response to the need for hydraulic mortars, i.e. mortars capable of harden-

I I

INCOHERENT J ROCKS

italian pozz. luffasche Santurin earth Vitreous rhyo- I i t e s

I FLY ASH

Fig. 1.

[ I,A,o,,L OZZOL AS J I I

I I I ROCKS ORIGIN (Altered) ROCKS I ! ! !

ICOUEREN| ROCKS I earthsltaIian vhite IHATERIAL$ Of I [ (Al t , , ,~ l I I °RGRNIC ORIGINI

I ARGILLIZEO HATERIAL$ Non active

I EOEITIZEO NATERIALS Trass Naples yellou tuff Canary tuff

k i I

I MATERIALS Of J $1RPLE OEPO$.

ARTIFICIAL POZZOLANAS

Oiatoua~eu~earths C|ays ~(nunactive)

INtX£O ORIGIN , Ruler Gaize I NAIR~L¥ [

BURNED CLAYS. Gliezh

1 I .J I Classification of pozzolanas.~ 3

Pozzolanic cements

Table 1. Mineralogical composition of some pozzolanas

187

Pozzolana Active phase Passive phase Ref.

Bacoli Glass Quartz, feldspars, augite 14, 15 Barile Partially decomposed Pyroxenes, olivine, mica, analcyme 14 Salone Glass, analcyme Leucite, pyroxenes, alkaline feldspars, mica 14 Gadolini Amorphous phase/glass Calcite, quartz, mica, kaolinite, feldspars 14 Vizzini Glass Feldspar, quartz, olivine, clay 16 Sacrofano Amorphous phase Quartz and feldspars 14 Maroc Opale Quartz, cristobalite, dolomite 15 Volvic Glass Andesine, quartz, diopside, magnetide 15 Neapolitan tuff Hershelite, analcyme (glass) Feldspar 15 Reintrass Chabasite, analcyme (glass) Feldspar 15 Fly ash Glass Mullite, Fe-spineil, hematite, quartz, carbon 15 Fly ash Glass Mullite, spinell, quartz, hematite, carbon 17

Table 2. Chemical analysis of pozzolanic materials

Pozzolana SiO, AI_,O~ Fe:O, CaO MgO sO, Na,O K,O TiO 2 CaCO~ Loss on Insoluble Ref. ignition residue

Bacoli(Naples) 53.08 17.89 4.29 9 .05 1.23 0 .65 3 . 0 8 7 .61 0.31 -- 3.05 25.82 14 Vulture(Potenza) 44.07 19.18 9.81 12.25 6.66 tr. 1.64 1 .12 0.97 -- 4.42 40-85 14 Salone(Rome) 46.84 18.44 10.25 8.52 4"75 tr. 1"02 6'35 0.06 -- 3'82 15.77 14 Casteggio(Pavia) 54.92 7.72 3-29 13-63 2.53 1.93 0.76 1.50 0 .18 22 .00 1 3 . 6 8 20.33 14 Vizzini(Catania) 50 .48 16.77 11 .59 4 .86 5.24 0 .15 0 . 8 7 0 .17 0.08 -- 9.68 3.75 14 Sacrofano(Viterbo) 85"50 3"02 0-44 0.58 tr. 0 . 7 7 0 . 1 6 0 .26 1.22 -- 7.94 8"40 14 Santorin earth 65"1 14.5 5.5 3"0 1.1 3"5 18 Rheinish trass 55'90 16.80 3.20 4.70 10.00 15 Opaline shale 65.4 10.1 4.2 4.6 2.7 6"3 19 Diatomite 86"0 2"3 1.8 -- 0'6 5.2 19 Rhyolite pumicite 65.7 15-9 2.5 3.4 1.3 3.4 19 Fly ash 41.49 22.14 9.74 9-48 4.98 1.24 0 . 9 4 2 .12 0.84 7'60 20 Fly ash 45.09 29"01 5.37 5'55 0.68 1.10 0 . 3 0 1 .99 1.44 7.34 20 Fly ash 48.59 28.21 5.94 1.15 2.15 0 .34 0 . 7 1 2 .33 1.44 7.44 20 Fly ash 41-94 18.44 9-47 14-20 2.27 2.63 1.53 21

ing in water and, at the same time, of resisting the attack of aggressive waters, sea water included. Due to their slow rate of hardening l ime-pozzolana mixes have gradually been replaced with pozzolanic cements. The l ime-pozzo lana-wa te r system is in any event simpler than the more complex c l inker -pozzo lana-wate r system and its thorough investigation can help with the interpre- tation of both behaviour and propert ies of pozzolana-containing cements.

2.1 Pozzolanic reaction The term 'pozzolanic activity' covers all reactions occurr ing among the active constituents of pozzolanas, lime and water. The definition, though approximate, is however acceptable f rom a technical and practical viewpoint. Notwithstand- ing the difficulty in following the evolution of pozzolana's active phases throughout the hydration process, the progress of the pozzolanic reac-

tion is commonly evaluated in terms of diminution of free lime in the system.

The term 'pozzolanic activity' includes two parameters, viz. the maximum amount of lime that a pozzolana can combine with and the rate at which such combinat ion occurs. Both factors depend on the nature of the pozzolanas and, more precisely, on the quality and quantity of the active phases. The heterogeneity of the pozzolana family as well as the complex phenomena occurring during hydrat ion can sufficiently explain the inability to model pozzolanic 'activity' and only enable general tendencies to be identified.

It can schematically be affirmed that the overall amount of combined lime essentially depends on the following:

(a) the nature of the active phases (b) their content in the pozzolana (c) their SiO2 content (d) the l ime-pozzolana ratio of the mix

188 Franco Massazza

whereas the combination rate depends on

(e) the specific surface (BET) of the pozzolana and

(f) temperature.

(a) In order to exemplify the role played by the various types of active phases, it should be remembered that within the zeolite family herschelite is more active than analcyme 26.27 and that zeolitic pozzolanas are considered to be more reactive than glassy ones.~ 5

(b) It is obvious that, other properties being equal, the larger the amount of combined lime, the higher the content in active phases and the lower the content in crystalline phases (quartz, sanidine, mullite, magnetite, etc.). A dependence but not a close relationship has been ascertained between these two properties 2~ inasmuch as the amount of combined lime also depends on:

(c) the SiO2 content of the active phases, which can range for instance between c. 45% and c. 85%. -~s Neither the role played by other elements, especially alumina, occurring in pozzolana, nor the long completion time of the pozzolanic reaction should however be neglected.

(d) Figure 2 shows that, for each pozzolana, the lime combined with 100 g of pozzolana increases as the lime-pozzolana ratio increases and that for each ratio a different equilibrium between combined and still free lime is established. 29,-~"

Figure 2 also highlights the fact that the amount of combined lime varies appreciably according to the different types of pozzolana used and also in relation to the lime-pozzolana ratio. Figure 314 illustrates how after 180 days of reaction pozzolanas have been able to combine with between 40% and 75% of lime with respect to their mass. In pastes, the lime combination is lower inasmuch as the hardened mass hinders the attainment of equilibrium conditions. 2s

(e) As shown in Fig. 3, the short-term activity essentially depends on the specific surface (BET) of the pozzolana, whereas in the long term the chemical and mineralogical composition of the pozzolana becomes of the utmost importance. 3~ It has been reported that the reaction rate of pozzolanic materials is proportional to the specific surface area squared/'

o,I A .r o (g o UJ U,I ri- M. U. O

7 0 ~ P = 3/1|

50

1

4

~) BACOLI 1) SALOtlE

30 3) SEGM 4) SACIROFANO

10

0 2 4 6 8

LIP = 1/1

2 4 6 8 TIME (DAYS)

L/P = 1/3

2 4 6

Fig. 2. Kinetics of reaction between Ca(OH)2 and some ltal- ian pozzolanas from: Bacoli (1), Salone (2), Segni (3) and Sacrofano (4), with different initial lime/pozzolana (L/P) weight ratio. 29

7O

z< so

o so

40

~ 30 '

~" ao O1

10

o m2"/g

Pozz.n.1 23 4 5 6 • HYDiRATION UNOER SHAKING O STATIC HYDRATION

Fig. 3. Calcium hydroxide combined versus specific surface. L/P ratio 80:100, water/solid ratio 2:174

(f) The temperature increase emphasises the rate of lime-natural pozzolana reaction though it does not modify the differences in behaviour due to the different nature of the pozzolanas. Between 50°C and 90°C, most lime is already fixed after ! day of reaction 29 whereas, above 70°C, combined lime tends to decrease.

As is shown in Fig. 4, evidence of this inversion is also found in fly ashes at approximately 60°C 3° and is likely to be due to the variation in the composition of hydrated phases.

Conversely, the amount of combined pozzolana, expressed in terms of silica which becomes soluble in acid, always

Pozzolanic cements 18 9

°I Z T

4O

TIME (DAYS)

Fig. 4. Lime reacted for various times and at various temperatures. CaO :fly ash = 20:80 weight ratio. 3°

increases with temperature. Whenever a given temperature is exceeded, the C/S or C/A ratio of hydrated phases tends to decrease.

2.2 The reaction products The reaction between pozzolana and lime basically produces the same compounds which are formed on hydration of the clinker constituents as well as of portland cement. The differences are minor and, in general, affect the amount rather than the nature of the phases.

2. 2. i Natural pozzolanas Natural pozzolanas react with lime forming calcium silicate hydrate (C-S-H) and hexagonal aluminates C4AHI3, 32-34 probably carbonated. 35 When some natural pozzolanas, more or less clayified, react with a saturated Ca(OH)2 solution having a w/s ratio = 100, then tetracalcium aluminate hydrate C4AHt3 , its derived carbonated compound as well as C-S-H and hydrogarnets are formed. Tetracalcium aluminate hydrate is sometimes present with gehlenite hydrate 36 although the presence of both compounds is regarded in contrast with the relations of equilibrium existing in the CaO-AI203-SiO2-H20 system. 37 This coexistence might however result from a meta- stable equilibrium in as much as the presence of C2ASH ~ and C4AHI3 or (C4AHI3-C3A.C~.HI2) has also been reported by other authors. 26

As a function of time and of the increase in the amount of fixed calcium, hydrogarnet tends to prevail on both aluminate and carboalumi- nate. 36.37 The variability of the C/S ratio of C-S-H seems to depend on the type of pozzolana, the temperature of curing and the analytical method used. As an example, with an opal-based pozzo-

lana, ratios ranging between 0.87 and 0.75 have been obtained also as a function of the curing temperature, whereas, with a glassy pozzolana, ratios turned out to be substantially higher, namely between 1.35 and 1.75. 6 An electron microprobe has given values ranging between 0.75 and 0.8515 whereas chemical analysis has shown values ranging between 1.2 and 1.7. 28 The variability of the C/S gel ratio can be ascribed to the non-stoichi- ometry of the C-S-H whilst the existence of different calcium silicate hydrates is evidenced by microscopic examinations. 34

At temperatures between 50°C and 90°C, the pozzolanic reaction leads to the formation of amorphous C-S-H which is similar to that obtained upon hydration of portland cement. 29

When gypsum occurs in pozzolana-lime pastes, ettringite is formed. 38 Whenever specific percentage contents of calcium sulphate are exceeded, the formation of ettringite causes paste disintegration. 39

2.2.2 Fly ashes mixed with lime When fly ashes are mixed with lime, C-S-H, C4AHI3 and CzSAH 8 are formed. 6 If silicoalu- minate fly ashes contain sulphates, besides C-S-H and C4AH13 , ettringite is also formed. 15 CaAH|3 decreases with time whereas the CaAcH12 content increases. 4° High-lime fly ashes can harden without a further addition of lime 41 since they contain variable amounts of free lime which, upon mixing, are transformed into Ca(OH)2. From a practical viewpoint, these fly ashes correspond to the artificial hydraulic limes obtained by mixing hydrated lime with pozzolana. 42

Hardening is therefore the outcome of the pozzolanic reaction and of the occurrence of specific hydraulic compounds which, as in the case of C:S, are formed during coal combustion. When high lime contents occur, also C2ASH8 is also eventually formed. 43 If, however, lime is entirely or mainly combined with A120 3 and SiO2 in the glass, it becomes unreactive. As a consequence, no prominent pozzolanic reaction is evidenced and, at least for three months, no C-S-H is formed. 4° After three days the surface of the fly ashes appears to be slightly covered with hydrates, some of which protrude from the surface of the fly ash particles.

2.2. 3 Silica fume and Ca(OH)2 Data concerning the reaction occurring between silica fume and Ca(OH)2 are rather limited. Owing to the high reactivity of silica fume, free lime dis-

190 Franco Massazza

appears between the 7th and the 28th day 44 therefore earlier than what generally occurs in natural pozzolanas and fly ashes. The reaction favours the formation of rather well-crystallised C-S-H I? 4 The reaction between silica fume and Ca(OH)2 solutions is very rapid and causes a phase to precipitate and turn into C-S-H very quickly. 45

2.2. 4 Other types o f pozzolana The same compounds are also formed with other types of pozzolana. The reaction products of burnt kaolin with lime are mainly gehlenite hydrate (C2ASHs), calcium silicate hydrate (C-S- H) and small quantities of calcium aluminate hydrate (C4AHI3). 32,4(''47 At higher temperatures and suitable lime concentrations, the tetracalcium aluminate hydrate turns into cubic C3AH ~.

ing between 15 and 20 A, whereas fly ash-lime mixes show only a slight discontinuity. 48

2.4 Strength of lime-pozzolana pastes The most remarkable practical consequence produced by pozzolanic reaction consists in the gradual hardening of the pozzolana-l ime pastes (Table 3). 38 Strength increases as the amount of combined lime increases; 28,42 however, as is shown in Fig. 6, 28 there is no general relationship between the two parameters, just a simple correlation within each pozzolana. An extreme case is represented by clayey materials which, despite their ability to fix lime, do not harden appreciably. An interesting example referred to in Ref. 6 shows that the strength of a material rich in clay minerals

2.3 Porosity and microstructure The specific surface (BET) of the pastes increases gradually with time up to 35-10"0 m2/g after 90 days of curing. 2s

The hydration products obtained by dispersing l ime-pozzolana mixes in water (water-to-binder ra t io= c. 12) can be as much as 150 m2/g after 1 year of curing, according to the type of pozzolana and the l ime-pozzolana ratiosJ ~ At least up to a ratio of 0.6, combined lime always increases as the ratio increases, whereas the specific surface decreases, sometimes dramatically, once it has attained a peak value. Whatever the type of pozzolana, there always exists a direct relation between porosity and specific surface of hydrated products (Fig. 5).4s A difference has however been evidenced between a typical natural pozzolana and a fly ash: natural pozzolana-l ime mixes show a clearly defined discontinuity in pore radii rang-

• VIZZINI TUFF 0.75 • BACOLI POZZ.

• FLY ASH A • D FLY ASH B • 4B •

i B 0 • Ae • ~ '

C] I I I I

).00 30 60 90 1 2 0 x 1 0 4

SPECIFIC SURFACE ( ¢ ~ ' / g )

Fig. 5. Relationship between porosity and specific surface of the hydration products: water/solid = 12. 4~

E 0.50

>- I.-

0 ~) 0 .25 o.

Table 3. Compressive strength of mortars containing Segni pozzolana 3s -- values expressed as kg/cm:

7)'rne UaSo 4 (%)

0 2.5 5 7.5 10

Ca(OH) 2 = 15% 7 days 16 31 28 56 65 28 days 51 85 108 120 140 6 months 122 138 175 170 180 1 year 135 156 180 181 163 2 years 150 165 187 186 148

Ca(OH)2 = 25% 7 days 8 22 30 28 51 28 days 49 55 125 144 150 6 months 168 175 233 239 215 1 year 190 2(18 266 268 2(12 2 years 212 239 28(I 275 187

200 8 5

-r" 150 4 I.-

ul

10C 6 i,i >

II: 50 o.

I I I I 0 10 20 30 40

COMBINED Ca(OH)2(g/100 g of ~ z z . )

Fig. 6. Mechanical strengths of 17 mm x 17 mm samples against combined calcium hydroxide. Lime-pozzolana pastes (40: 100). 2~

Pozzolanic cements 191

can be as much as approximately one third of the strength found in materials having real pozzolanic behaviour. For this reason technical assessment of pozzolana must be carried out by measuring the strength of the lime mix (or portland cement mix) and not by determining the amount of fixed lime present. Fixed lime is only a useful means to help explain the hardening phenomenon. 5,39

The strength of lime-natural pozzolana pastes is remarkably increased by the addition of gypsum (Table 3). This should however not exceed certain levels (e.g. 5%) 49 if disintegration of specimens due to the formation of large quantities of ettringite is to be avoided.

High-lime fly ashes behave like hydraulic binders. If they have a suitable composition, they need neither lime nor cement to harden. Such mixes always run the risk of rapid setting and thus require the addition of a suitable setting retarder.

If gypsum is added, the strength will vary between 12.4 and 4.34 MPa after 28 days of curing. After 120 days, strengths will still be increasing and practically doubling, s°

3 POZZOLANIC CEMENTS AND POZZOLANA CEMENTS

When added to a portland cement, pozzolana reacts with the calcium hydroxide formed during hydration of clinker calcium silicates. As a result of this reaction, the final portlandite content of pastes made up of pozzolana-containing cements is always lower than that found in control portland cement pastes. This applies to natural pozzolanas,~ 5.5 J fly ashes j 7.52.53 and microsi l ica . 54-56

Pozzolanic cements are by definition mixes of portland cement and pozzolana which, if dis- persed in water and kept under certain conditions, eventually produce solutions unsaturated with calcium hydroxide. 57,58 Conversely, pozzolana cements do not comply with this requirement inasmuch as their pozzolana content is insufficient to combine -- in terms of both quality and quantity -- most of the free lime released by calcium silicates thus giving unsaturated solutions.

3.1 The hydration of pozzolanic cements Clinker (plus gypsum) and pozzolana show different reaction processes and react at different ages. As is known, the pozzolanic reaction becomes apparent after 7-15 days since mixing

occurred, 59,6° that is to say, as soon as 80% of alite 60'61 and 60-80% of C3A+C4AF have reacted. 6° This fact influences both the microstructure and properties of pozzolanic cements. This is the reason why the hydration processes of clinker and pozzolana can be investigated separately but always taking into account that the two materials exert a reciprocal influence.

As a rule, the addition of pozzolanas accelerates the initial hydration of the clinker contained in pozzolanic cements. This behaviour is neatly apparent whenever the measured parameters, namely rate of heat evolution and total heat of hydration, are referred not to the clinker + gypsum + pozzolana system but to the former two only, i.e. whenever the measured values are multiplied by the dilution factor. In such cases, the second peak of heat evolution is higher than that of the non-diluted cement. This is always true with natural pozzolanas 6 and silica f u m e 56,62 whereas it is not always so with fly ashes, whose peak of heat evolution can be slightly higher, 52 unaltered 63 or lower 62 than that of the control portland cement. In this latter case, the retarding effect can be ascribed to the presence of unburnt coal in the ashes.

The progress of hydration in pozzolanic cements cannot be expressed in terms of Ca(OH)2 content, as can be done -- still with some reserva- tions -- with portland cements since the portlandite content in the pastes is the result of at least three concurring phenomena:

• acceleration of the hydrolisis of the calcium silicates of clinker,

• pozzolanic reaction, • modification in the composition of the

hydrated phases.

This concurrence can help explain the behaviour of the portlandite content in cement pastes which increases at first and then decreases (see Fig. 7). 2o Consequently, one would be driven to assume that natural pozzolana is more active than the fly ash and calcareous filler inasmuch as they leave smaller amounts of free CaO in the pastes.

However, from Figs 8 and 9, 20 where the activity of pozzolana is expressed in terms of both combined water and strength, it is particularly apparent that this conclusion is wrong. For the purpose of obtaining more consistent results, the only conclusion one can draw is that any pozzolanic addition is likely to modify the reaction course, the composition of the hydrated phases as well as the microstructure.

192 Franco Massazza

14 uJ

12

10

~ 8

c~ 6 o o v -

~ 4 -r o t~ o 2

Limestone 30%

P o z z ~

I I 1 / I 13 7 28 60 90

TIME (DAYS)

Fig. 7. Free portlandite in blended cement pastes.:"

14

o~

8

¢3 4 uJ _z ¢Q =E 2 o 0

13 7 28 60 9 o TIME (DAYS)

Fig. 8. Combined water in blended cement pastes. -~(~

7 o

E so

40

3o

~ e

o. 1Q

0 t,..} i I

3 ~ 2'8 9o TIME (DAYS)

Fig. 9. Mechanical strengths of blended cements. 2~)

3.2 Hydration products in pozzolanic cements The reaction products formed during the hydration of pozzolanic cements are the same as those occurring in portland cement pastes, although differing in content. They are:

• ettringite,

• tetracalcium aluminate hydrate (often carbonated),

• monosulphoaluminate, • C-S-H, • Ca(OH)2.

Ettringite is rapidly formed in cements containing natural pozzolana a~ or fly ashes, i7,64 The persistence of ettringite which has not turned into monosulphate was observed at 28 days in fly-ash containing cements ~7 and at 1 year in cements containing natural pozzolanasJ 9 The picture is actually much more complex than it might seem because ettringite can also disappear after 3 days2 Moreover, its conversion into monosulphate was also observed in SO3-poor fly ashesJ 2 As a consequence, ettringite's conversion into monosulphate depends on the amount of SO 3 available.

Tetracalcium aluminate hydrate is often carbonated either because it has been contaminated by the CO2 contained in the atmosphere a9 or because of the occurrence of calcite in cement. ~ 7

The hydration of alite is accelerated by the presence of pozzolanic materials. The formation of C-S-H is so rapid that is already visible after 24 h s and it is even more rapid if pozzolana is made up of microsilica. ~5 The hydration of both C3S and C2S is associated with the formation of portlandite.

In pozzolanic cements, besides the C-S-H which is formed due to the hydration of clinker silicates, the C-S-H originating from the reaction between pozzolana and hydrolysis lime also occurs. The composition of this latter type of C-S-H is different from that originating from C3S and C2S, the reason being due to different conditions of formation. In pozzolanic cements the C-S-H has a lower C/S ratio and an alumina content greater than that of portland cement pastes (Table 4). ~

The variability in the composition of C-S-H has also been stressed by methods other than EMPA. As an example, optical microscopy showed that in hydrated pozzolanic cement pastes the calcium silicate hydrate has different refraction indexes

Table 4. Composition of C - S - H in cement pastes with and without 40% blending component (w/c = 0.40, 293 K, age 4 years) by EMPA s

C/S A/C C](S + A) Na20 K,O (%)

OPC 2-03 0-06 1.81 0"03 0'1 [ Fly ash cement 1.01 0.21 0-84 0.24 0.33 Slagcement 1.62 0.44 (I.96 0.23 0.30


which clearly indicates a difference in the chemical composition. 7] This conclusion is supported by the different degree of polimerization of the silicate anion, greater in pozzolanic cement than in portland cement pastes. 65

In cements containing high-lime fly ashes, C2ASH8 (gehlenite hydrate) besides C-S-H is also f o r m e d . 64 Anyway, if gypsum is added to cements, the gehlenite hydrate gradually decreases and eventually disappears after 60 days.

In the short and medium term, the presence of portlandite in hardened pozzolanic cement pastes is justified by a low initial rate of pozzolanic reaction. Only in the long term should the pozzolanic reaction be overwhelming thus making free lime disappear. Table 563 shows however that portlandite does not disappear even after 4 years of curing. It is therefore evident that there must be other reasons beside those mentioned earlier in Section 3.1.

3.3 Portlandite content The overall Ca(OH)2 content of pozzolana containing pastes is always lower than that of control portland pastes when referred to the binder (portland cement+pozzolana). It may however be higher, 17 l o w e r 2°,66 o r even the s a m e 52,53"67 when the content is referred to portland cement alone. In any case, in the first month, the portlandite content is more or less equivalent to that formed in portland cement multiplied by the dilution factor.

The long-term persistence of portlandite may appear incompatible with the presence of pozzolana since the common Italian pozzolanas, as well as fly ashes, can combine an amount of Ca(OH)z being as much as about 50% of their weightJ 4 Henceforth, the portlandite content of hardened cement pastes should be very low. The reality being different, the persistence of portlandite

must be ascribed not to a lack of pozzolanic reaction but to the difficulties encountered by both portlandite crystals and pozzolana particles in reacting in very stiff pastes where they are found to be covered and shielded by hydrated compounds.

This assumption is sustained by the following:

• Free lime is progressively less in pastes, mortars and concretes respectively, i.e. in materials having increasing a/c ratios and thus greater porosity and permeability (Fig. 10). 51

• As portland cement fineness increases, combined lime first increases and then decreases (Fig. 11).67

• Combined water is always larger in blended cement pastes than in control cement pastes (Table 6). 17

• In spite of the significant Ca(OH)2 content of pastes, pozzolana has substantially and remarkably reacted after 28 d a y s . 6°.68

To explain the contradiction between the high free lime content and the considerable progress in

1) Paste 2) M o r t a r 3) Conc re te u)

" 8 °

o

U

1 . I - ~

- - 3 r

WN u l

" 0 3 7 28 60 90 ~

TIME (DAYS)

Fig. 10. Free portlandite content of paste, mortar (1:3) and concrete (1:6) samples cured at 40°C (water/cement ratio=0.5). 35% of portland cement replaced by two types of pozzolanasP t

Table 5. Free lime content in portland cement with different amounts of pozzolana. Pastes cured for 4 years 63

Free lime

A TD Franke method method

~%) (%)

Portland cement 10.2 Portland cement + 10% Segni Pozzol. 7.6 Portland cement + 20% Segni Pozzol. 5"3 Portland cement + 30% Segni Pozzol. 2.9 Portland cement + 40% Segni Pozzol. 1-1"5 Portland cement + 50% Segni Pozzol. 1

9"65 7"20 5"4 3"1 1"3 0"9

14

o e= o 10 a u.i z ,,n :z 6 o (.)

2

FLY ASH M

i I I I I I 3000 4o00 sooo sooo rooo 8ooo

PORTLAND CEkF.NT FINENESS (cm2/g)

Fig. 11. Ca(OH)2 combined by fly ashes versus the specific surface (Blaine) of control portland cements. 67

194

Table 6. TGA/DTG data for PC-fly ash pastes 7

Franco Massazza

Paste Age Ca (OH): (per l OO g)

total binder

Content Bound water Content (per lOO g) (per lOO g) (per lOO g)

PC total binder 1'('

Control PC 2 h 0-94 0.94 2.82 2-82 5 h 6.(t5 6.(t5 9.(10 9.00 1 day 15.05 15.05 14.75 14.75 3 days 18.12 18.12 19.53 19.53 7 days 19.16 19.16 21.39 21.39

28 days 23.94 23.94 26.85 26.85

4 515 2 h 0.40 0.46 2.34 2.69 5 h 1.53 1.76 4.13 4.75 1 day 11.16 12.84 13.56 15.61 3 days 17.76 20.44 19.55 22.49 7 days 18.57 21.37 20'95 24.11

28 days 18-94 21.79 24.34 28.01

4 5311 2 h (I 0 2.44 3"33 5 h 2'45 3"35 4.54 6"20 1 day 8"70 11.89 12" 17 16"62 3 days 12'84 17'54 17-21 23.51 7 days 12"94 17'68 19.25 26'29

28 days 15-(16 20"57 23' 14 31'60

1(/15 2 h 1.(t8 1-24 2'8(t 3"23 5 h 1"34 1"55 4.72 5.43 1 day 11.(12 12.70 13"82 15"93

3 days 16'47 18'98 19-93 22-97 7 days 16.35 18.84 20"60 23.74

28 days 20-73 23.89 27-9 l 32.16

1 030 2 h () 0 2.23 3.05 5 h (t.96 1-32 3.54 4.85 1 days 8.91 12.21 14.13 19.36 3 days 12.77 17.49 18.94 25.95 7 days 12.53 17.16 19.5(/ 26-72

28 days 19-42 26.60 22.38 30.66

pozzolanic reaction it is necessary to assume that pozzolanas will have to subtract lime from the adjacent C - S - H . 69 As in the case of the C3S-pozzolana mixes, this model requires that:

(i) a C/S gradient exists which, starting from portlandite crystals, would cross C-S-H to reach the fly ash particle; and that

(ii) the C-S-H in pozzolanic cements has a C/S ratio being lower than that of ordinary cements.

Figure 12 70 and Table 4 ~ show how both requirements are complied with. These two conclusions are far from being insignificant, inasmuch as they can contribute to explaining the higher flexural strength of pozzolanic cements as well as their improved resistance to aggressive ions.

3.4 Paste microstructure The paste microstructure reveals the different rates of clinker hydration and pozzolana reaction.

After 1 day, the reaction has only involved clinker and plenty of ettringite crystals as well as C-S-H rims, apparently amorphous, are evident. ~7'7~ It would seem that fly ash particles provide a suitable nucleation site for the growth of the hydrated compounds. ~7 After 28 days, the fibres of C-S-H have filled all pore spacings.~ 7

After 1 day the particles of the pozzolanic materials seem to have been attacked. It is sup- posed that some chemical interaction between the surface of the pozzolana and the ions dissolved in the mixing water must have occurred. A reaction between a large well-formed crystal of CH in con- tact with a fly ash particle can also be observed. 7-" Only at later stages does the outer part of the pozzolana particles appear altered and is replaced by a thin layer of newly-formed products. After a 28-day hydration, the outer part of the natural pozzolana's grains is transformed into a rim of new hydrated products. Reflected fight microscopy has shown that in some cases this rim is made by two different layers. 7~


3 days

4.0 . . . . ~ '

3 .d 2 ~ 6 13

2.0 14

1.0

_o ,~ 0 1 2 3 4 5 6 re , 91, days

i i i

O H Y D R A T E S

~zl 1 ~ 2 J3 pozz. 3.11" 1 2.C ~ 1

2 3 4 5 6 7 8

1.0

o ; ~ a ~ ; ~ DISTANCE (p.m)

Fig. 12. C/S ratio in blends of C3S and Beppu white clay pozzolana. TM

SEM examination of fly-ash in cement has suggested that the rims are initially made up of a continuous and uniform layer of calcium hydroxide which is in turn covered with calcium silicate hydrate. 73 As far as hydration of high-lime fly ashes is concerned, a model has recently been produced which can be applied, with only minor changes, to all pozzolanic materials. 64

The fly ash spheres are consumed from the outer surface inwards thus forming a rim of new products. The rim is gradually replaced with radi- ating bundles of fibrous C-S-H perpendicular to the surface of the particles. This mechanism implies a topochemical action as well as through a solution growth with a thin, amorphous, migrating interfacial region situated between the honey- comb C-S-H and the residual fly ash spheres.

Beneath the rims, the surface of the particles is smooth and signs of chemical attack are barely present. This could suggest that the so-called 'inert' phases of pozzolana are also dissolved by a strong alkaline environment.

Thus, the progress of the pozzolanic reaction, which in turn determines the progress of the mechanical properties, must take place through this double layer, probably by diffusion of the various ionic species present.

3.5 Porosity and permeability of pozzolanic pastes The total porosity, assessed by mercury intrusion, is generally higher in pozzolanic cement pastes than in portland cement pastes manufactured with the same w/c ratio. 17'66'74-77 Though

decreasing with time, the porosity is always higher than in portland cements. After a 7-month curing, differences can range between 15% and 60% according to the type of pozzolana. TM

The pore size distribution of the two types of cement are different and mesoporosity of pozzolanic cement pastes is displaced towards the smaller pores. In fact, the Maximum Continuous Diameter (MCD) and the Threshold Values (TV) are lower in pozzolanic cement pastes than in portland cement ones.

When considering porosity values, it must be taken into account that experimental results could be altered by various factors.

O The cumulative mercury intruded pore volume measured on 105°C-oven-dried specimens is higher than that found in the same specimens from which water was extracted by solvent replacement. TM

• Vacuum drying at 70°C and drying at 20°C with decreasing relative humidities highlight different porosity values . 79

• From mercury intrusion measurements, the pore distribution in hydrated blended cement mixes is somewhat distorted and the porosity is often overestimated due to the structural damages occurring at high pressure.80,81

Some experimental evidence indicates that part of the pores of cement pastes containing fly ash or silica fume are not intruded by mercury. 66 This would mean that pozzolana-containing cements give a definitely higher porosity. As suggested by test results, pozzolanic cement pastes are relatively richer in larger pores whose entrances are however smaller than in portland cements pastes. Conversely, permeability of pozzolanic pastes is tendentially lower 17,66,74 (Table 774).

A general correlation might be expected between permeability and porosity since both of them vary with time according to similar equa- tions. In reality, such a correlation is only valid for each single cement. 82 Lack of a general correlation is to be ascribed to differences in the micro- structures between pozzolanic and portland cement pastes.

The apparent contradiction in terms of (higher) porosity and (lower) permeability of pozzolanic cements can be explained through the following model. 83

• In most pozzolanic cements, hydration occurring during the first 7-15 days almost exclusively involves the clinker+gypsum

196 Franco Massazza

Table 7. Porosity (%) and permeability coefficients (m 2) of cement pastes 74"

Cement paste Curing

7days

Porosity Permeability

7 months

l'orosity l'ermeability

Pti. cement 19.70 2.36 10-17 Ptl. cement + filler 26.10 1.29 10-17 Ptl. cement + fly ash 31.90 1.70 10-17 Ptl. cement + Bacoli pozz. 26"60 1'92 10-17

15"10 15"30 17"40 13"30

3'00 10-17 1"26 10-17 0'51 10-17 0"77 10-17

"Dried at 70°C for 16 h under vacuum at the residual pressure of 5 mbar.

fraction. Within this time c. 80% of alite has r e a c t e d . 7°

• Part of the hydration products of clinker is formed and develops on site, whereas the remaining part deposits on the pozzolana particles.

• When the pozzolana starts reacting, its particles are surrounded by a porous but already stiff structure. Only a part of the hydration products can therefore form on site.

• Since there is no evidence of a growing pressure, part of their hydrated compounds must form elsewhere and precipitate in the water-filled capillary pores. This process can take place due to the fibrous radial structure of C-S-H which covers the pozzolana particles and allows dissolution and transporta- tion phenomena to occur.

• The volume of the precipitating mass is small and is therefore unable to fill the larger pores. It is however sufficient in amount to obstruct the thin connections existing between the larger pores or at least to reduce span thereof.

• As a consequence, the porosity of pozzolanic cements is still higher even after completion of the pozzolanic reaction whereas permeability is reduced.

The microstructure of pozzolanic cement pastes is slightly but neatly different from that of portland cement pastes. Nevertheless, the microstructural differences are such that they determine the well-known differences in the engineering properties of both types of cement and mainly regarding:

• mechanical properties • durability

3.6 Mortar and paste strength Pozzolana starts reacting somewhat belatedly with the calcium hydroxide produced by clinker

hydration and therefore, at least initially, it behaves like an inert diluting agent towards the portland cement with which it has been mixed. Consequently the partial replacement of portland cement for pozzolana generally reduces the initial rate of cement hardening, though at longer curing ages the situation can be reversed. This conclusion applies to natural pozzolanas, ~4 high -~5 and low-lime fly ashes ]7,86'87 and silica fume. 88,89 A typical example of this behaviour is given in Fig. 13. 51

The effect of pozzolana on strength depends on a number of factors the most significant of which are:

• pozzolana content, • type of pozzolana, • pozzolana grading and specific surface, • type of cement, • strength class of cement, • curing, • temperature.

1401

X 121] I'-

Z

10(] tLJ > u) u) lu 8¢ E ¢1.

0 6 0 ul >

.,I m 4 0 I l I

1o 20 3o ,~ 's'o

POZZOLANA CONTENT iN CEMENT, %

Fig. 13. Effect of substituting portland cement for pozzolana on the compressive strength of ISO mortar. Values expressed as percentage of the 28-day strength of reference cement. -~[


3.6.1 Pozzolana content Too high pozzolana contents reduce strengths at all ages and must therefore be avoided unless other reasons, such as the need for a low heat of hydration or a greater durability, suggest the opposite. Of course, the real behaviour of a pozzolana-containing cement depends also on the type of portland cement and pozzolana used. In other words, there is an optimum pozzolana content that must be determined on a case-by-case basis and that depends on a number of factors.

The 28-day-strength reduction caused by the replacement of pozzolana for portland cement is in any case lower than that obtained by diluting the control portland cement with an inert filler. 9°'91 Generally, the optimum pozzolana content refers to the 28-day strength. If the main target to be met is different, i.e. a slower/higher heat of hydration, higher ultimate strength or improved durability, the 'optimum contents' should be varied accordingly.

3.6.2 Type of pozzolana Figure 14 9~ shows that the strength development of pozzolanic cements depends on the type of pozzolana used 86"91,92 and that the strength of mixes containing 20% of pozzolana is always higher than that of the control sample which contains 20% of ground quartz. 9° The case of Sacrofano pozzolana illustrated in Fig. 14 must be considered as particular and not as common since this type of pozzolana is very active.

The strength loss caused by the partial replacement of cement for pozzolana is sooner or later recovered, depending both on the type and content of pozzolana. For example, a cement containing 15% of rhyolitic glass slightly exceeds the strength of the control portland cement already after 7 days. 88 Owing to its fineness and composition, microsilica is even more effective since, when added in amounts ranging between 5 and 10% by weight of cement, strengths are increased by 15-25% after 7 days of curing and by 4-12% after one year. Conversely, with certain poor- quality materials the strength-pozzolana content ratio curve lacks the maximum peak and the strength of the control portland cement is always higher than that of blended cements. 93

3.6.3 Pozzolana particle size distribution In the short and long term a remarkable reduction in strength is associated with a decrease in pozzolana f ineness . 85'86'92 The role played by the particle size distribution is illustrated in Fig. 15. 92 The effect is even more pronounced at earlier ages.

Figure 16 s5 shows that the 0-20 fraction reduces 7-day strengths by 50% and 90-day strengths by 15%. The fractions with higher particle diameters reduce the initial strength exten- sively and neither after 90 days of curing is this loss of strength recovered. ~5 In the latter case, high-lime fly ashes behave slighly better probably because they contain hydraulic constituents.

Conversely, the strength is substantially improved if ground fly ash instead of graded frac-

~ tions of fly ashes is used. In this case, if the grind- Silica fume

so / f ~ O t , t o m . e a r t h ing fineness of the fly ash has proven suitable (Blaine Specific Surface = 453 m2/kg), the 90-day

/ / / ~ Seg.I

° o ° , r o , A

" r l ~ Sacrofano 75 ¢c 30 ~ 7 ~tm

20 50

2O

1 0

25 46

0 10 20 30 40

TIME (DAYS) 0 30 60 90 120

PERIOD OF CURE (DAYS) Fig. 14. Strength development of different pozzolana- cement pastes. 20% replacement; 4 x 4 x 16 specimens. Fig. 15. Mortar strength development rates for fly ashes of Curing: 77 h in water and then in air at 65% RH at 20°C. 9° various median diameters. (~m)? 2

198 Franco Massazza

[%] too ] r].,, ce° Fractions

80 [~ 0-20pm

l~ 20-40pm

60 D 40-60pm

40 ~ ~ ~] > 60pm

7 days 28 days 90 days

Fig. 16. Relative compressive strength of binders containing 30% fractionized low-calcium fly ash. ~

o.

"1" I - 0 Z uJ m I - u) uJ > U) (n m O. : i 0 0

80

6O

40

20

- - - f t c = 0 m f / c = 0.25

f / c = 0.50

w / ( ¢ + t ) = 0.50

8 d i l f e ren t fly ashes

0 ~ 0 7 28 90

AGE (DAYS)

Fig. 17. Compressive strength of mortars without (w) and with fly ash (f) added to portland cement (c)? 6

strength can be higher than that of the control cementY 5 When, however, the fineness of fly ashes exceeds certain limits, the compressive strength will decrease instead of increasing. 85

The substitution of 5% of a active materials (fly ashes, slags) -- accounting for 30% of the cements under examination -- for silica fume does not improve the 7-90 day strengtas whereas the substitution of practically inert materials (crystalline slags, ground quartz) causes a neat improvement already at 28 days. 9~

3.6.4 Type of cement The strength of mortars depends not only on fly ash properties but also on the portland cement used. Partial replacement of portland cements of the same strength class, with a 28 day compressive strength ranging between 45,5 and 49,7 MPa for the same fly ashes leads to compressive strengths that do not match those of the control cements. ~6 This means that the behaviour of a pozzolana cannot be assessed by using a portland cement alone and confirms the difficulty of using a given cement class to set up a reference mortar. The partial substitution of blasffurnace slag cements for fly ashes reduces the strength to a greater extent than what usually happens in portland cements. ~('

3.6.5 Cement strength class Fly ashes in cements have different effects according to the strength class of portland cement being used. An example is given by 86 who blended a 35 and 45 class portland cement made with the same clinker with 25% of 8 different types of fly ash. All mortars containing the former cement reached the same strength as the control fly ash- free mortars after approximately 60 days and exceeded it after 90 days (see Fig. 17). 86

100

....--1 +2 =~ 90 ~ 3

80 I- ~ 70 ul --> endslcm21g)

- , Series 1 3450 3920 4640 Series 2 4080 4550 5420

5O Ser i es3 6810 7110 7590

3 TIME (DAYS)

Fig. 18. Mean values of relative compressive strengths, with respect to control portland cements. Series 1, 2 and 3 made with blends of one portland cement and one fly ash, ground separately at three different finenesses. 95

With the class 45 portland cement, mortars containing fly ashes were not capable of attaining the same strengths as the control cement even after 90 days. 86 In other words, it would seem that the lower the fineness of the control cement, the sooner pozzolanic cements reach or approximate to the strength of the control cements. 95

3.6.6 Length of curing The strength drops when pozzolana partially replaces portland cement. Figure 1895 shows that the substitution of 30% of cement for fly ashes can reduce the early strength of the control cement by 50%, therefore exerting a greater effect than that calculated by dilution rule. 95 For this reason the curing of pozzolanic concretes and mortars needs more care than the curing of portland cements. Provided that curing is correctly carried out, the difference in strength between pozzolanic and portland cements decreases with age and eventually disappears or changes in sign. The moment of


recovery depends on the fineness of both portland cement 95 and pozzolana. 85

The lower hardening rate caused by the replacement of portland cement for pozzolana does not create major problems when pozzolanic cements are prepared directly at the cement works, because here the cement manufacturer takes all necessary steps to ensure that blended cements have strength values complying with standard specifications. When pozzolana is added to cement at the building site where the properties of pozzolana and cement cannot be further modi- fied, the only possible corrective measure for opti- mizing the properties of the mix is to increase the cement content.

3.6. 7 Plasticising and superplasticising admixtures The workability is only slightly affected when pozzolanic cements contain natural pozzolanas or fly ashes. When, however, cements contain very fine materials such as microsilica and diatoma- ceous earths, batches require the addition of large amounts of superplasticisers.

Although the dosage of admixtures increases proportionally to the quantity of microsilica present, it is still possible to replace portland cement for silica fume (s.s.-20 m2/g) and very finely ground silica (s.s. = 12 and 20 m2/g SiO2 = 87%) up to 25%. In spite of the presence of the admixture, microsilica reduces the initial compressive strength in virtually all mixes. This loss is gradually attenuated and after 28 days the strength of all mixes is higher than that of the control cement. 86 It is reported that the highest strengths have been obtained not with silica fume but with finely ground silica of equal fineness. 89

Even when associated with superfluidising admixtures, silica fume requires an optimum dosage. Generally, an amount of about 15% is sufficient, this value being far lower than those required by natural pozzolanas and fly ashes. 89

The addition of a superplasticiser (condensed sulphonate melamine) to pastes containing between 20 and 35% of fly ashes does not change the strength but allows a lower w/c ratio to be used and thus higher strengths to be attained. 96

4 POZZOLANA CONTAINING CONCRETE

4.1 Concrete strength The strength of pozzolana-concretes is affected by the same factors influencing the strength of

both pastes and mortars. In this case, obviously, the influence exerted by aggregates -- which con- stitute most part of the concrete -- as well as by the cement content must also be considered.

4.1.1 Optimum content As far as pastes and mortars are concerned, concretes also need an optimum content of pozzolana to attain the best performances. After 3 days of curing, a 15% replacement of a portland cement for 15% fly ash gives in the majority of cases a higher compressive strength than that of the control cements However better development of early strengths is to be attributed to a physical rather than chemical cause. As is known, in fact, the pozzolanic reaction has already started at 3 days but it has certainly not progressed very far. A twofold substitution, amounting to as much as 30% of fly ash, reduces early strengths and the strength loss is recovered only after 40-90 days . 97

Owing to its high reactivity, the pozzolanic effect of silica fume starts earlier, i.e. after 7 days at 2 0 ° C 98'99 and after 2 days at 35°C . 98

4.1.2 Chemical and mineralogical composition The chemical and mineralogical composition of both natural and artificial pozzolanas varies noticeably from type to type and this can help explain why, even by using the same portland cement, concrete strength depends on the type of pozzolana used.~ 00

The decrease in early strengths -- a typical feature of concretes containing siliceous fly ashes -- may not occur despite a considerable replacement of cement with high-lime fly ash (CaO=30.3%), 1°~ (CaO=22%)J °2 If 30% of portland cement is replaced, increases in strength amounting to as much as 85% and 35% can be obtained by a 28-day and 8-year curing respect- ivelyJ °3 This behaviour of high-lime fly ashes depends on their content in hydraulic compounds.

The reactivity of microsilica is basically pozzolanic in character and therefore more similar to that of the most common vitreous pozzolanas and fly ashes.

Up to 7 days, the 5, 10 and 20% substitution of a portland cement for silica fume does not substantially modify the strength of concretes containing 300 kg/m 3 of cement and having a w/c ratio of 0.7 although, with respect to the control cement, workability is noticeably reduced. Sub- sequently, the difference between the strengths of the four cements is the more substantial, the larger the amount of silica fume (Fig. 19). 99

200 Franco Massazza

60

A

~ so

4 0

zo

20 UJ h- a

o

- 4 50

3 ~ 40

2 ~ 30

20

Slgca fume Slump 10

(1) 300/0 15

(2) 285115 14 0 (3) 270/30 10 (4) 240160 6

0 I I I 7 2 8 9 O

TIME (DAYS)

Fig. 19. Concrete strength versus time. Cement composition and slump indicated in the figure; w/c = 0.7; no plasticiser. 99

Silica fume gives a major contribution to strength development between the 3rd and the 28th day at 20°C. 1°4

4.1.3 Curing Every concrete -- and the same applies therefore to pozzolana-containing concretes -- requires proper curing which is essential to exploit all potential capabilities of the cement used. The strength development of concretes containing pozzolanas is more adversely affected by very short curing periods under water than plain portland cement-containing concretes. H~°,l°4 Such behaviour depends on the fact that, at least during the first 7 days of curing, the most common pozzolanas do not take part in the hydration process and they solely act as a diluting agent and increase de facto the w/c ratio. It is not by chance that the weight loss of air stored concrete diminishes as the fly ash content is decreased 1°5 and the curing period is prolonged.

Fly-ash-containing concretes, initially moist cured for 7 days and subsequently stored at 65% R.H. for 90 days, were able to attain and exceed the strength of the control portland cement concrete.~°5

As far as curing of pozzolanic cements is concerned, emphasis is generally laid on the relatively lower early strengths while other aspects are often forgotten. Concretes containing pozzolanas in general and fly ashes in particular ~°6 continue increasing their strength even after 28 days, an age which is commonly taken as a reference for concrete quality. After 7 years of water curing, a concrete containing 46.6% of fly ashes was found to have developed a more than twofold strength as compared to that found at 28 days.

F l y ash ~ ~ ~ 2 0 %

.._..--- ~ ~ ~ - - 4 0 %

/ 7 " - - ..... ---" u % . f / _ ~ - 2 0 % ~ 40%

- - Grade 25

I I I I I

20 40 60 80 100

AGE (DAYS)

Fig. 20. Effect of concrete grade and fly ash content on compressive strength. Portland cement replaced by 20 and 40% fly ash.~°~

4.1.4 Quality The lower quality of fly ashes seen in recent years has suggested that fines in fly ashes should be separated from coarser particles. Strength is thus improved as a consequence of the decrease in the maximum diameter of the particles (20, 10 and 5/am) and a 15% replacement of cement can give concretes having 7-day strengths roughly equivalent to that of the addition-free control concrete. After 91 days of curing, all fly-ash-containing concretes yielded strengths equal to or, if finer, greater than those of the control specimens.l°7

4.1.5 Temperature Blended cements are more sensitive to temperature than portland cements. Anyway, if the mixes have been proportioned so to give concretes having the same 28-day strength, the 1-3 day strength of pozzolana-containing concrete will not substantially differ from that of concrete containing portland cement only. 9~

4.1.6 Partial substitution The partial substitution (20-40%) of a cement for fly ashes exerts a different effect according to the cement content. For example, with a cement content of 285 kg/m 3, the 91-day strength is lower than in the control concrete, whereas with a cement content of 345 kg/m 3 it is increased between 28 and 91 days. In any case, the initial strength was shown to be lower than that of the control specimen (Fig. 20). L°8

4.1.7 Plasticizers and superplasticisers Plasticizers and superplasticisers are often used in concrete technology on account of the many advantages they offer in terms of strength, workability and durability. When concrete contains microsilica, their utilisation becomes essential as


the water demand increases almost linearly with increasing microsilica contents in cement (see Fig. 21). 1°4 Furthermore, microsilica renders concrete more cohesive and viscous. 1°4

Since the use of admixtures is often associated with variations in the mix composition it is rather difficult to relate their effects with other parameters of the concrete itself. Loss of strength in concretes, for example, caused by replacing portland cement with 30% fly ashes, can be recovered by adding silica fume, and a water reducing agent (Fig. 22109). Conversely, a ligninsulphonate-based plasticising admixture was not able to regain the strength loss caused by a 10% addition of silica fume to cements containing either 10% or 25% of fly ashes. 98 This clearly indicates that a successful outcome is related to the type of admixture selected.

A parameter that should not be neglected is the aggregate inasmuch as a cement containing 25%

~ 5o

z ° so

i .° m 2o

~ lO

No admixtures j R ; 3 8

~ SP 30

r I I I I I 10 20 30 40 50

MICROSILICA CONTENT (kg/m3)

Fig. 21. The influence of microsilica addition on the water demand of mixes with no water reducing admixtures. Cements: OPC (SP 30), RHC (RP 38), pozzolana cement (RP 38 containing 20% fly ash). 1°4

A

i '° 3©

Control+20% silica fume - -20%

c e m e n t ~ " ~ ' " ' ~ " - "

[ / ¢ 70% PII cem.+30% fly ash (com'eol,)

9'1

t J

,3 ~ 2'. 5~ AGE (DAYS)

Fig. 22. Compressive strength of concrete w/(c + f) = 0.60, superplasticiser and AEA) °9

with

of fly ashes and tested with various aggregates has given different strengths. With a 10% addition of silica fume, strengths are found to decrease with some aggregates whereas they are increased with certain others. In the former case, the diminution is not recovered even after 2 years of water storage whereas in the latter two cases the positive effect is maintained, l°°

In the presence of microsilica equal slump does not mean equal workability) °4 In order to ensure the same workability it is always recommended that microsilica-containing concrete should have a slump higher than 3-5 cm.

4.1.8 Early strengths Fly ashes and microsilica are usually added to concrete to replace part of the cement content. Often cost-effective, this practice raises some perplexities from a technical viewpoint since the early strengths are remarkably reduced. Con- versely, the use of pozzolanas to compensate for any deficiency in fines in the aggregates is always advantageous and in some cases necessary. At least between 2 and 91 days, concrete strengths are increased if fine aggregates instead of portland cement are replaced. The amount of aggregate to be replaced with fly ashes in order to obtain better results depends on curing as well as on the w/c ratio.110

4.1.9 High strength concretes In pozzolana-containing concretes the relatively low rate of initial hardening does not pre- vent concretes with 28-day strengths higher than 60 MPa, i.e. high strength concretes, from being manufactured. As shown in Fig. 23,111 this can also be obtained by replacing 50% of portland cement with fly ashes. In order to achieve the best results the binder content must be high, the w/c+f.c, ratio low, the workability improved through a superplasticiser, the fines content adequate. When evaluating the advantages of a large substitution of the portland cement, one should consider that the heat of hydration, shrinkage and creep will also be reduced. 111

4.1.10 Efficiency factor Sometimes the replacement of portland cement for pozzolana is not effected as a simply ponderal equivalence but it is best accomplished by using the so-called efficiency factor K, generally assumed to be 0.3. The efficiency factor K is defined as the mass of portland cement that makes the same contribution to the strength of

202 Franco Massazza

~" 10(] a, I

so

40

Cement 233

PFA 233 Fines 659 C o m s e 1035 SuperpL

WATER/(CEMENT+PFA)RATIO

0.32 0.421 0.614 .--. 4 0 0 i ! I ¢

I Fog curing i t

, ~300

I I ~ 200 365 d

o 174 133 kg

174 133 " 720 761 "

1093 1121 "

(%C+PFA) I .8 1.8 1.7 % Water 145 148 163 kg

Fig. 23. Mix proportioning chart for fly ash concrete containing a superplasticiser and 50% cement replacements, t~

concrete as a unit mass of fly ash. According to this procedure, portland cement is not replaced with the same amount of fly ashes on a weight basis but with a given amount of ash multiplied by K.IO5

28-day water cured !

Fly ash ontscd (%)

I I I I I I I /~01 I

20 4O 6O 80

DRYING TIME (DAYS)

Fig. 24. Drying shrinkage versus age of concretes having different fly ash contents. Grade 35 mixes."~s

Table 8. Shrinkage and expansion after 2 years of concretes having different composition and similar workability ~ ~ 2

Target W/C Binder Shrinkage h Expansion strength content" Environment (10 -~) (MPa) (kg/m 3)

Laboratory Outside sheltered

20 0'615 266 485 422 260 40 0-423 348 460 385 233 60 0'320 466 475 393 201

4.2 Drying shrinkage Shrinkage, expansion and creep are widely recognised as fundamental properties of concrete. It is therefore necessary to know exactly how pozzolanas can influence these parameters. In this respect, it should be remembered that a large percentage of cracks forming in the concrete structures is due to the restraints opposed to the shrinkage.t ~ 2

It is generally assumed that drying shrinkage is not substantially influenced by pozzolanas in themselves. Variations, if any, should be ascribed to changes occurring in the water demand and in the microstructure of cement pastes (Fig. 24). ,~8

Shrinkage does not depend on the strength of the concrete: this is true either if the binder (cement +fly ash) content and the w/c ratio are constant "~7,~°s or if the workability is kept constant by modifying the composition and thus the strengths.J J 2

As a rule, shrinkage depends on the cement content and the w/c ratio. 114 Variations with the same sign in both parameters make shrinkage increase or decrease, whereas variations having opposite signs contrast reciprocally. Table 8 ~2 shows that the shrinkage of three concretes having different compositions and strengths is basically the same due to the contrasting action exerted by

"50% Portland cement -- 50% fly ash. ~'Superplasticiser 1.8% of binder content.

the cement content and the diminution of the w/c ratio. This diminution has been brought about by the use of a superfluidising agent.

The independence of shrinkage from concrete strength is remarkable inasmuch as it is obtained by substituting 50% portland cement for fly ashes. ~ i 2

Table 8 shows that expansion of concretes stored in a fog chamber (RH= 95%) is neatly lower in concretes with higher strengths. In this case the difference could be ascribed to the lower w/c ratio of the high strength concrete and therefore to the lower capillary porosity which hinders and reduces water penetration from the outside.

Curing conditions exert a noticeable effect on shrinkage since, as illustrated in Table 8, air storage of specimens being only protected against rainfalls or sunlight has caused a lower shrinkage.

In conclusion, the final shrinkage of fly-ash- containing concretes having different compositions lies in the 100 x 10 -6 interval./12

Air-classified fly ashes (<20, < 10 and <5 ktm) have given similar results since shrinkage differences never exceeded 100 × 10- 6107


T=ble 9. Typical values of concrete shrinkage (#m/m) 84

Cement Storage (days)

1 3 7 28 90

325 Portland 33 86 170 360 476 425 Portland 37 86 158 354 501 525 Portland 60 114 180 358 497 325 Pozzolanic 44 97 206 480 609 425 Pozzolanic 44 100 185 420 540

Replacement of various amounts of fly ash (20%, 30% and 50%) in cement does not substantially modify the drying shrinkage since the differences fall within the 50 x 10 -6 interval. ~°~ Drying shrinkage is larger for cements replaced by 20 and 40% of fly ashes as compared to the control cement but only when the curing period is short (7 days for example).~°8

Shrinkage is larger in concretes made up with natural-pozzolana- containing cements (see Table 9). 84 This feature, which distinguishes natural pozzolanas from fly ashes, can be ascribed to the much more porous microstructure of natural pozzolanas. Water reducing agents have dimin- ished shrinkage in both concretes, either with or without fly ashes.~ ~ 5

4.3 Creep The deformation under load (creep) of concrete is an important time-dependent parameter involving structural mechanics inasmuch as creep influences the loss of pretension, the long-term deformation of structures and the load displace- ment from the concrete to the reinforcement. Factors affecting creep are various and can be dis- tinguished as internal or external. Internal factors depend on the properties of concrete, while external ones are related to environmental conditions.

In the first group the composition of concrete and therefore of the cement itself is found. Creep generally increases as the replacement for fly ash increasesJ °z Sometimes, however, the effect is more marked for small than for large replace- mentsJ °8 Also in fly ash-containing concretes, creep increases as the strength decreases as well as load and time increase.

Figure 25 ~0~ illustrates the influence exerted by high-lime fly ashes on creep. With regard to creep, the behaviour of blended cements is substantially different from that observed for the strength and shrinkage of concretes. Substitutions up to 50% for fly ashes cause minor variations in strength and shrinkage but modify creep heavily. 1°1 Con-

l g00

4

700 Z ,<

u~ 500 0. w (control) tu n- f.a. replac. 0 300

( 3 ) 3 0 % - ( 4 ) so % "

100

I I I I I I

0 60 160 260 360 TIME (DAYS)

Fig. 25. Influence of creep of a high-lime fly ash as a replacement; w/(c + f) = 0.38J °l

cretes with slightly different compositions designed to attain the same workability have shown that samples containing fly ashes had a specific creep approximately 50% higher than that found in fly-ash-free samples. ]]5

5 DURABILITY

Though it might seem oversimplified, we could affirm that while the salient property of portland cements is their higher rate of hardening, the main property of pozzolanic cements is their greater resistance to chemical attacks. As is known, the durability of concrete depends on a number of chemical and physical causes, the most outstanding of which are:

(a) carbonation, (b) leaching, (c) chlorides, (d) sulphates, (e) reactive aggregates, (f) freeze/thaw, (g) thermal variations

As far as items (b)-(e) are concerned, pozzolanic cements behave better than portland cements, whereas with respect to items (a), (f) and (g) neither cement shows substantial differences.

When concrete is attacked by external chemical agents, the first and most useful defence is impermeability. In this respect, as mentioned earlier in Section 3.4, pozzolanic cements behave far better than portland ones.

5.1 Carbonation The carbon dioxide contained in the air is potentially dangerous to concrete inasmuch as

204 Franco Massazza

CO 2 is capable of attacking all hydrated compounds of cement. In the cement-water-carbon dioxide system the stable phases are calcium carbonate, silica, alumina and iron oxide hydrates. 116 In dense and strong concretes, however, carbonation reduces porosity and permeability while increasing strength and resistance to sulphates. Carbonation is more detrimental to the existing reinforcement than to the concrete in itself inasmuch as it reduces the pH of the pore solution. As a consequence, should carbonation reach the reinforcement, the iron is no longer protected by the alkalinity of the pore solution and, in the presence of oxygen and humidity, it oxidises and becomes corroded.

The reaction of cement paste with carbon dioxide can nonetheless improve several properties of concrete. In dense and compact concrete, actually, carbonation reduces total porosity and specific surface of cement pastes as well as permeability. ~3 Carbonation increases the resistance to sulphates z ~ 7 and to the attack of aggressive ions in general. ]~s Carbonation is a time-dependent phenomenon but once this parameter has been fixed it will also depend on the following:

• cement content, • w/c ratio, • curing period, • ambient humidity, • partial pressure of CO2, • temperature, • microcracks,

that is to say, on all the factors affecting the density, compactness and permeability of concrete.

According to some researchers,J ~')-~2 J the depth and rate of carbonation also depend on the type of cement although more recent experiments have demonstrated that concretes made up of pozzolana-containing cements behave in the same way as portland cements provided that:

(a) the curing period is long enough, (b) the strengths of concretes containing differ-

ent types of cements are the same.

Whenever the depth of carbonation is to be reduced, a correct curing is a primary requirement for all concretes but it becomes particularly important for all blended cements which have a relatively slow rate of initial hardening. As a matter of fact, if wet curing is limited to 1, 3 and 7 days, fly-ash containing cements carbonate to a greater extent than portland cements ~22 and espe-

cially if carbonation is forced (4% of CO~, HR=50%) . 121 If the substitution of portland cement for fly ash is modest (16%), differences in the carbonation depth are also small. J2~

After 14.5 months the carbonation depth of concretes is sligthly lowered by the addition of 10% of silica fume. If the mixing water is reduced through a suitably devised plasticiser, so as to keep workability constant, a 20% replacement also makes carbonation depth diminish.12~ Carbo- nation depth definitely increases with increasing replacement of portland cement for silica fume. This is valid for both 1 day and 28 day curing but it should be remembered that the water content increases as replacement increases. ~ 25 These conclusions could be criticised inasmuch as they were obtained with concretes having different composition and strength or by accelerated carbonation procedures.

It seems unreasonable to measure strengths on wet-cured specimens after 28 days and carbonation depth on wet-cured specimens after 1-7 days only and then stored in a dry ambient for a long time. If pozzolana -~2~' or fly-ash-containing ~27 cements are compared on the basis of the concrete strength, carbonation depth is shown to diminish linearly as the 28-day strength of the concrete decreases (Figs 2612j and 2712s).

The converse relation between compressive strength and carbonation depth is not valid as in the case of silica fume because substitution up to 30% of the control portland cement makes the strength of both mortars and concretes, as well as carbonation depth, increase proportionally. ~ 29

5.2 Leaching Water can decompose any of the hydrated compounds present in cement. ~3° For this reason,

Series A Series B

• CONTROL

3C • WATER-REOUCEO L1

• FLY ASH

Z o_

20

7-DAY MOIST CURED

0 10 oo-~v MOmT Z

90-DAY MOIST CUREO (PLAIN, W-R)

2;0 2;0 3;0 3;0 CEMENT CONTENT (kg/m3)

Fig. 26. Depth of carbonation versus cement content of fly ash concretes (accelerated testing). ~ 2~

Pozzolanic cements 2 0 5

M A T T H E W S

50

OPC concrete I - - - - Pfa concrete

, ~ CC = correLcoeff.

n.

= \ cc-o.87 " ~ o. 20 \

cc.o.' c-o-- CC-0.79 IO-DA¥ W A Y E R - C ~ E D /

I~ AtR-STORED CUBES

I i I I

0 10 20 30 40 50 DEPTH OF CARBONATION (ram)

Fig. 27. Depth of carbonation versus 28-day compressive strength.12~

el 4

0~ u) 0 9 3 4

;3 o 2

1

0

A

S

I I I I

20 40 60 80 10J0

~ A

B

U N B R U S H E D S A M P L E S

I

0 20 40 60 80 100

DAYS IN ACID WATER

Fig. 28. Mass loss of concretes exposed to carbonic acid water. Mix A, OPC 312 kg/m3; mix B, OPC 170 kg/m3 +fly ash 170kg/m3.135

if the concrete is porous and water is abundant or continually renewed, all the lime -- including the one present in silicates and aluminates -- will be leached and the residue will be made up of SiO2'xH20 and AI203"YH20. The leaching of lime increases both the porosity and the permeability of the cement paste. This increase, in turn, accelerates the leaching of the lime.

The amount of lime extracted from pozzolanic cement pastes is far less than the amount of lime released by portland cement. TM Leached lime decreases as the pozzolana/clinker ratio increases. ~ 32

As compared to portland cements, hardened pastes made up of pozzolanic cements are more leaching-resistant due to three closely related reasons:

(a) they contain only 3-6% of Ca(OH) 2 (Ref. 51) as against the 20-22% of portland cement pastes;

(b) they contain more calcium silicates and aluminates in gel-like form:

(c) they have reduced permeability.

The above reasons reduce the progress of hydrolysis. 51

Although acid waters containing aggressive CO2 increase the rate of lime leaching, pozzolanic cements still behave better than portland o n e s . j33:34 Figure 28135 shows that as in the case of unbrushed samples the mass loss is c. 40% higher in portland concretes than in pozzolanic ones. Obviously, even with pozzolanic cements, the more delayed cement hydrolysis and lime leaching are, the less permeable is the concrete.

5.3 Chloride penetration Both concrete and reinforcement can be harm- fully affected by chloride action. In the first case the most important consequence is represented by the leaching of the free calcium hydroxide 136-~39 which makes concrete more porous and less strong. In this respect, pozzolanic concretes are more resistant to leaching because they are less permeable and the only leachable lime is the fraction not yet combined with pozzolana 139 and still accessible to water.14°

More dangerous and harmful is the chloride which, when penetrating into concrete, reaches as far as the reinforcement. If the C1-/OH- ratio near the reinforcement steel dips below a treshold limit of 0.3,~41 passivation is destroyed and corrosion becomes inevitable, if water and oxygen are present.

To protect reinforcement from chlorides, therefore, less permeable concretes and adequate cover thicknesses are required. In addition to these general precautions, efficacious protection is provided by the type of cement used, particularly by cements containing natural pozzolanas, ~42:43 fly a s h e s 137"144-147 o r silica fume.89,129,147-151

Figure 29129 shows how sensitively lower chloride penetration is in cement pastes containing silica fume. 15°

There are several reasons underlying such good behaviour of pozzolanic cements. In general, these are to be ascribed to the slower rate at which chloride ions diffuse throughout pozzolanic cement pastes. 145,]48A49,151-~54 The lower penetra-

206 Franco Massazza

A E g

UJ 0 n-

o . J " r o U. o 2; I - O. . 1 0

Z o

n- I.-

W O.

20

15

10

4 weeks

Silica fume

0 0 10 20 30

8 weeks

i i i /

o 0 10 20 3 I I

o lo 3'o SILICA FUME CONTENT (%)

Fig. 29. Effect of silica fume content on penetration depth of chloride ion? 29

Table 10. Coefficients for diffusion of chloride ion into cement pastes ~ -~'~

Sample D(cm's / x lO "~) W/C Temp. Ref

Portland cement 1-23 10 155 2-51 25 4"85 0"4 40 0"83 10 0'90 25 0'97 40

Pozzol. cement

Portland cement 19.2 20 154 (RHPC) 13.2 30

28.3 50 Fly ash cement 9'2 0-5 20

(22% PFA) 11"6 30 13'7 50

Portland cement 15.1 20 (SRPC) 19-3 30

37-0 50

Portland cement 4.47 (OPC)

Pozzol. cement 1-47 (070 OPC/30 PFA)

Slag cement 0.41 (35 OPC/65 BFS)

Portland cement 10'00 (SRPC)

25 156

0-5 25

25

25

bility of the C1- ion into pozzolana containing cement pastes indiscriminately involves natural pozzolana] 52,~55 fly ashes 151A 53,154,156 and silica fume.~49.153 The effective diffusion coefficient Deee of chloride is reduced as the pozzolanic material content increases. For example, 30% substitution of a portland cement for fly ash decreases the Dec f by one order of magnitude. ~53 Effective diffusion coefficients measured by several authors on cement pastes are given in Table 10.126 Though data are not directly comparable as samples were prepared in different ways and tested under dif-

Table 11. Coefficients for diffusion of chloride ion into concretes ~ 5~

Sample D(cm-'s ' x I0 '~) Temp. 111//( (°c)

Ptl cem. concr. 1'65 25 0'5 (vibrated) Ptl cem. concr. 3.24 25 0'6 (non-vibrated) Pozz. cem. concr. 1.05 25 0-5 (vibrated) Pozz. cem. coner. 2-26 25 0'6 (non-vibrated)

ferent conditions, it is quite clear how efficacious pozzolana-containing cements can be in hindering chloride ion penetration.

The fact that the Dec f coefficient decreases with increasing pozzolana content would recommend the use of pozzolanic cements, in which pozzolana amounts to approximately 33%, instead of using pozzolana-containing cements, in which the pozzolana content is lower and not well defined. Dec f coefficients are still lower in concretes made with pozzolanic cements than in those manufactured with portland cements (see Table 11 ).~55

5.4 Sulphate Calcium, sodium, magnesium and ammonium sulphates are, in increasing order of hazard, harmful to concrete as they can lead to concrete swell- ing and, consequently, cracking. The degree of deterioration depends on the intensity of the attack as well as on the characteristics of the concrete.

CaS04 reacts with calcium aluminate hydrates thus forming expansive ettringite (3CaO'A120 3" 3CaSO 4" 32H20). Na2SO4 reacts with calcium hydroxide and forms expansive gypsum (CaSO 4- 2H20) which, in the presence of aluminates, may in turn lead to the formation of ettringite. MgSO 4 reacts with all cement compounds, thus decomposing cement and subsequently forming gypsum and, at a later stage, ettringite.

Since the fundamental condition enhancing the formation of expansive compounds is the occurrence both of aluminate hydrates and calcium hydroxide in the paste, low C3A contents are needed for minimising the risk of sulphatic expansion. In Italy, pozzolanic cements are preferred 49 because the C 3 A content of the clinker is diluted by pozzolana and because the pastes contain lower amounts of Ca(OH)2. Ferric-pozzolanic


cements, made with c 3 m - f r e e clinker are recommended in case of very strong attacks.

Figures 3051 and 31 ]57 illustrate the positive action exerted by natural pozzolanas and fly ashes. Also a 20-25% substitution of an ASTM type F cement for different high-lime fly ashes decreases sulphate expansion. Concretes immersed in a 10% Na2SO4 solution for 27 months gave expansion values ranging between 0.008 and 0.116%, therefore substantially lower than those of the control c o n c r e t e ( 0 . 7 2 8 % ) . 15s,159

The effect of pozzolana is more or less marked depending on the type and quantity of pozzolana being used. The greater sulphate resistance of pozzolanic cements is probably related to the composition and structure of the pores in hydrated pastes. Also an interaction between sulphates and capillary pores is very likely to occur.

5.5 Sea water Sea water has a high salt content which is dangerous to the durability of both ordinary and

/~m/m

+10000

Ptl cem. - - - - - - - ~ / z O Ptl cem.+10% pozz.

z ~ + 5000 " +3o~ " - ~ / \ . " + 4 ~

X w

0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

200o ~ ; , ; ~o i o l o o 2 ; o ' ' ' ' 5000 2000 500 1000

STORAGE TIME (DAYS)

Fig. 30. Effect of substituting portland cement for pozzolana on the expansion of 1:3 mortar. Samples 2 cm × 4 cm × 25 cm stored in 1% MgS04 solution? ~

reinforced concretes. Although sea-water-induced deterioration is less intense than one would assume from its composition, CI-, SO42- and Mg 2+ still represent a danger and require a properly designed and manufactured concrete.

An important contribution to concrete protection is afforded by pozzolanic cements. The major role played by these cements was stressed in 1951 during a survey of maritime structures built at several Italian harbours with pozzolanic m o r t a r s 16° and has since been repeatedly confirmed. 161 It has actually been shown that portland cements allow a chloride penetration from two to fivefold higher than that of blended cements. 162 F i g u r e 32,163 in which the variation in chloride concentrations with depth in sea walls is given, shows that the use of fly ashes in concrete can reduce C1- concentrations.

It can thus be said that, history, experiments and research all recommend pozzolanic instead of portland cements for maritime works.

5.6 Alkali-aggregate reaction Over the past 50 years, several harmful chemical reactions between aggregates and portland cements have been noted. The most common reaction is the one occurring between some types of silica (opal, chalcedony and tridymite) and cement alkalis. From this reaction a gel made of alkaline and alkaline-earth silicates is formed which tends to absorb water and therefore to swell.

There is no complete agreement about this conclusion; it is however assumed that the expansive reaction does not occur with portland cements containing less than 0.6% of Na:O alkali

0.20 10%soak test

(1) 25% M-6498 0.1( (2) 25% M-6569

(3) 25% M-6730 (4) 25% M-6754 . ~ 5

0.1; (5) No fly ash o ¢) Z

0.08 4

0.04

- 1

0 .00 0 400 800 1200 1600 2000

TIME (DAYS)

Fig. 31. Sulphate expansion of concretes containing low- calcium fly ash (soak test)? 57

0.16 Q .o o c g

~ 0.10 v

z w F- Z 0 U ~ 0.05 n- O ,.J

0

OPC concrete

m Pfa concrete \ \

\ \

\

I I I I i I 5 10 15 20 25 30

AVERAGE OEPTH OF SAMPLE (mm)

Fig. 32. Chloride concentration profiles for concretes from Aberthaw sea wail. ~63

208 Franco Massazza

equivalent. Unfortunately, technology and anti- pollution regulations in force in many countries make this target too expensive. ~59 On the other hand, the problem cannot be always solved by changing aggregates. A solution is however offered by cements containing fly ashes or natural pozzolanas.

This might seem a contradiction because natural pozzolanas can contain up to 11% of alkalis and fly ash up to 5%. In practice, however, expansion is actually reduced and brought down within acceptable levels. 164-166 The final result will depend on the quality and quantity of the addition. Pozzolana and fly ashes shall therefore be chosen and their contents proportioned with the utmost care . 5t,]64,165 Figure 33167 highlights on the one hand the efficacy of pozzolanas in reducing expansion and on the other the different effects according to the type selected. Similar curves are given in Refs 168 and 169 for natural pozzolanas. In Italy there are only few examples of alkali/ aggregate reactions, despite the existence of reactive aggregates. This condition could partly be due to the wide use of pozzolanic cements.

So far there is no established explanation for the better behaviour of pozzolanic cements with respect to the alkali-silica reaction. It has however been suggested that the presence of free ~ 7o crystalline iT( Ca(OH)2 can be a prerequisite for the development of a destructive alkali-silica reaction. This condition can help explain why the inhibition of expansion requires a large replacement of portland cement for pozzolana. It also implies that a pozzolanic cement with a substantially higher pozzolana content must be used instead of pozzolana cement.

1 . 5

~e 1.0 / a/c = 3.5

/ u) Z

0., II ""3

0 100 200 3()0 TIME (DAYS)

Fig. 33. Expansion with age for specimens in which part of the aggregate had been replaced by an equal volume of pfa such that pfa/(c + pfa) = 0.3 (by weight)J 6y

5.7 Frost action As a rule, concrete does not have a good frost resistance due to the presence of water-filled capillary pores. If water freezes, concrete increases in volume and tends to expand. The opposite happens when the temperature rises above 0°C. Repeated freeze/thaw cycles acceler- ate the material's decay.

A suitable protection against frost can easily be obtained by introducing microscopic air bubbles into concrete which are able to accommodate water being pushed forward by the ice in the process of being formed. For this reason, natural pozzo lanas , 172 fly ashes j45 and silica fume ~49,t53 present in cement do not reduce frost resistance, provided that the concretes have a suitable entrained-air content and similar strengths. This behaviour is exemplified in Fig. 34 H)~ which shows that only a 50% substitution of portland cement for fly ashes ultimately decreases frost resistance.

A large replacement of portland cement for silica fume (20-30%) makes the freeze/thaw resistance decrease also in the presence of en t r a ined air. 173'174 The reason for this lies per- haps in the differences involving the specific surface as well as the factor of air void spacings in concretes.~7~

Laboratory test results have been confirmed in the field. In the Alpine regions of Italy, many dams built with pozzolanic cements are in an excellent state of preservation after decades of service despite the large number of freeze/thaw cycles they are subjected to yearly. ~75 As a matter of course,

I

100 y

98 D- 3: (~ Fly ash Lu

06 . J

E ao~ Z 94

OU'o ~ 92 ~ ' 3 0 %

~ 5 0 % i i i l i /

0 200 400 600 800 1000 1200

NUMBER OF CYCLES

Fig. 34. Residual weight of air entrained concrete versus number of freeze-thaw cycles. Percentages of cement replacement for fly ash are indicated. Samples cured for 14 days before exposure. 1"~


when employing pozzolanic cements, one should remember that due to the slow initial hardening, frost exposure of pozzolanic concretes should be somewhat delayed as compared to portland cements.

6 CONCLUSIONS

The properties of pastes, mortars and concretes made up with pozzolanic cements are not qualita- tively different from those obtained from portland cements. This is the reason why pozzolanic cements can be used in most applications as an alternative to portland cements. The differences between the properties of the two types are mainly quatitative and must be thoroughly known to attain the best performances.

Compared to portland cements, pozzolanic cements show:

• lower early strengths, • higher ultimate strengths, • similar shrinkage, • higher creep, • lower heat of hydration, • greater porosity, • lower permeability, • greater resistance to lime leaching, • greater resistance to sulphate and sea water

expansion, • inhibition of the alkali-silica expansion, • similar freeze/thaw resistance.

All these properties are linked to differences involving the chemical and mineralogical composition of cement, the mechanism of hydration, the composition and microstructure of the paste.

In conclusion, portland cements are preferred whenever the rapid attainment of high strengths is deemed useful or necessary, while pozzolanic cements find their best applications when requirements of durability are regarded as a priority. In most cases, however, both cements are interchangeable.

REFERENCES

1. Sersale, R., Structure des pouzzolanes et des cendres volantes et hydratation des ciments aux pouzzolanes et aux cendres volantes, in 7th Int. Congr. on the Chem- istry of Cement(Paris, 1980), vol. 1, IV-1/1-21

2. Lea, EM., The chemistry of pozzolanas, in Proc. of the Syrup. on the Chemistry of Cements (Stockholm, 1938), 460-90

3. Malquori, G., Portland-pozzolan cement, in Proc. of the 4th Int. Symp. on Chemistry of Cement (Washington, 1960), vol. II, 983-1006

4. Kokubu, M., Fly ash and fly ash cement, in Proc. of the 5th Int. Syrup. on the Chemistry of Cement (Tokyo, October 7-11, 1968), vol. IV, 75-105

5. Massazza, E, Chemistry of pozzolanic additions and mixed cements, in 6th International Congress on the Chemistry of Cement, (Moscow, September 1974)

6. Takemoto, K., Uchikawa, H., Hydratation des ciments pouzzolaniques, in 7th Int. Congr. on the Chemistry of Cement, (Paris, 1980), vol. I, IV-2/1-2/21

7. Sersale, R., Structure et caractrrisation des pouzzolanes et des cendres volantes, in 7th Int. Congr. on the Chemistry of Cement, (Paris, 1980), vol. I, IV- 1/3-1/29

8. Uchikawa, H., Effect of blending components on hydration and structure formation, in 8th International Congress on the Chemistry of Cement, (Rio de Janeiro, September 22-27, 1986), vol. I, 249-80

9. Regourd, M., Caracteristiques et activation des pro- duits d'addition, in 8th International Congress on the Chemistry of Cement, (Rio de Janeiro, September 22-27, 1986), vol. I, 199-229

10. 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete (Monte- beUo, July 31-August 5, 1983) vol. I-II, ACI SP-79, ed. V.M. Malhotra.

11. Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete (Madrid, April 21-25, 1986), vol. I-II, ACI SP-91, ed. V.M. Malhotra.

12. Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete (Trondheim, June 18-23, 1989) vol. I-II, ACI SP-114, ed. V.M. Mal- hotra.

13. Massazza, F., Chemistry of pozzolanic additions and mixed cements, Il Cemento, 1 (1976) 3-38

14. Costa, U., Massazza, F., Factors affecting the reaction with lime of Italian pozzolanas, in 6th International Congress on the Chemistry of Cement, (Moscow, September 1974)

15. Mortureux, B., Hornain, H., Gautier, E., Regourd, M., Comparaison de la rractivit6 de diffrrentes pouzzolanes, in 7th Int. Congr. on the Chemistry of Cement, (Paris, 1980), vol. III, IV-110/115

16. Costa, U., Massazza, E, Influence of the thermal treat- ment on the reactivity of some natural pozzolanas with lime, ll Cemento, 3 (1977) 105-21

17. Berry, E.E., Hemmings, R.T., Langley, W.S., Carette, G.G., Beneficiated fly ash: Hydration, microstructure, and strength development in portland cement systems in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 241-73

18. Mehta, P.K., Studies on blended portland cements containing Santorin earth, Cement and Concrete Research, 11(1981)507-18

19. Mielenz, R.C., Witte, L.E, Glantz, Od., Effect of cal- cination on natural pozzolans, ASTM Special Technical Publication, 99 (1950) 43-91

20. Costa, U., Massazza, F., Natural pozzolanas and fly ashes: Analogies and differences, in Proc. Syrup. on Effects of Fly Ash Incorporation in Cement and Con- crete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 134-44

21. Massazza, E, Cannas, M., Ricerche sul Comporta- mento idraulico di ceneri volanti del carbone sulcis, ll Calore, 1 (1961) 2-8

22. Uchikawa, H., Uchida, S., Hanehara, S., Effect of character of glass phase in blending components on

210 Franco Massazza

their reactivity in calcium hydroxide mixture, in 8th International Congress on the Chemist~. of Cement, (Rio de Janeiro, September 22-27, 1986), vol. IV. 245-51

23. Watt, J.D., Thorne, D.I., Composition and pozzolanic properties of pulverised fuel ashes J. Appl. Chem., 15 (1965) 585-604; 16 (1965) 33-9

24. Hemmings, R.T., Berry, E.E., Speciation in size and density fractionated fly ash, in MRS Symp. on Fly Ash and Coal Conversion By-products: Characterization, Utilization and Disposal H, vol. 65 (Boston, December 2-4, 1985), Material Research Society, Pittsburgh, PA, 91-104

25. Terrier, P., Moreau, M., Recherche sur le m6canisme de l'action pouzzolanique des cendres volantes dans le ciment, Revue des Matdriaux de Construction, 613 (1966) 379-96

26. Sersale, R., Sabatelli, V., Sull'attivith 'pozzolanica' delle zeoliti: 1. Reattivitfi dell'herschelite con soluzione d'idrossido di calcio, in Rend. dell'Accademia di Scienze Fisiche e Matematiche della Societti Nazionale di Scienze, Lettere ed Arti in Napoli Serie 4 XXVII (1960) 263-86

27. Sersale, R., Sull'attivit/~ 'pozzolanica' delle zeoliti: II. Reattivit/~ dell'analcime con soluzione d'idrossido di calcio, in Rend. dell'Accademia di Scienze Fisiche e Matematiche della Societ6 Nazionale di Scienze, Lettere ed Arti in Napoli Serie 4 XXVIII ( 1961 ) 45-67

28. Massazza, F., Costa, U., Factors determining the development of mechanical strength in lime-pozzolana pastes, in XII Conference on Silicate bldustry and Sili- cate Science, (Budapest, June 6-11, 1977), vol. 1, 537-52

29. Collepardi, M., Marcialis, A., Massidda, L., Sanna, U., Low pressure steam curing of compacted lime-pozzolana mixtures, Cem. Concr. Res., 6 (1976) 497-506

30. Buttler, F.G., Walker, E.J., The Rate and extent of reaction between calcium hydroxide and puiverised fuel ash, in Int. Symp. on The Use of PFA in Concrete. (Leeds, April 14-16, 1982) w)l. 1, ed. J,G. Cabrera and A.R. Cusens, 71-81

31. Massazza, E, Pozzolanas and pozzolanic cements, in Cement-Br. 1/1980-81 (Zagabria, March 13. 1980), 3-17

32. Rodger, S.A., Groves, G.W., The microstructure of tricalcium silicate/pulverized fuel ash blended cement pastes, Advances in Cement Research, 1 (1988) 84-91

33. Turriziani, R., Prodotti di reazione dell'idrato di calcio con la pozzolana: I, La Ricerca Scientifica, 8 (1954) 17(19-17

34. Tavasci, B., Struttura della malta di calce e pozzolana di Bacoli, 1l Cemento -- Rivista Tecnica della Costruzi- one, 8-9 (1948) 114-20

35. Millet, J., Hommey, R., Etude mindralogique des pfites pouzzolanes-chaux, Bull. Liaison Labo. P. et Ch., 74 (1974) 59-63

36. Amicarelli, V., Sersale, R,, Sabatelli, V., Attivith 'pozzolanica' dei prodotti piroclastici 'argillificati', Rend. dell'Accademia di Scienze Fisiche e Matematiche della Societd Nazionale di Scienze, Lettere ed Arti in Napoli -- Serie 4 vol. XXXIII 1966, 257-82

37. Dron, R., Experimental and theoretical study of the CaO-AI203-SiOz-HzO system, in 6th Int. Congr. on the Chemistry of Cement, (Moscow, September 1974) Supplementary Paper Section II, 16

38. Turriziani, R., Schippa, G., Malte di pozzolana, calce e solfato di calcio: I, La Ricerca Scientifica, 9 (1954) 1895-903

39. Parissi, E, II controllo delle pozzolane, ll Cemento Armato -- Le lndustrie del Cemento, 1944, 45

40. Aitcin, EC., Autefage, E, Carles-Gibergues, A., Vaquier, A., Comparative study of the cementitious properties of different fly ashes, in Proc. 2nd bu. Conf on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986), vol. 1, ACI SP- 91, ed. V.M. Malhotra, 91 - 114

41. Massazza, E, Cannas, M., Ricerche sulla preparazionc di calci idrauliche artificiali utilizzando le ceneri volanti del carbone Sulcis -- Nota 1, ll Calore, I/2 (1962) 9-15; 58-64

42. Government Decree of 31 August 1972, Norme sui requisiti di accettazione e modalit6 di prova degli agglomerati cementizi e delle calci idrauliche

43. Carles-Gibergues, A., Thenoz, B., Vaquier, A., M6ca- nisme d'hydratation d'une cendre de lignite caicaire, in 7th Int. Congr. on the Chemist O, Of (~ment, (Paris, 1980), vol. III, IV-53/59

44. Buck, A.D., Burkes, J.E, Characterization and reactivity of silica fume, in l'roc, of the 3rd Int. Conf on Cement Microscopy, (Houston, March 16-19, 1981), 279-85

45. Grutzeck, M.W., Atkinson, S., Roy, D.M., Mechanism of hydration of condensed silica fume in calcium hydroxide solutions, in 1st Int. Conj. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, July 31-August 5. 1983) vol. 2, ACI SP-79, ed. V.M. Malhotra, 643-64

46. Malquori, G., Cirilli, V., Azione della calce sul caolino disidratato e sullc pozzolane naturali, La Ricerca Scientifica ed il Progresso Tecnico, 2--3 (1943) 85-93

47. Murat, M., Hydration reaction and hardening of cal- cined clays and related minerals: 1 -- Preliminary investigation on metakaolinite, Cem. Concr. Res., 13 (1983) 259-66

48. Tognon G., Ursella, P., Combined lime and specific surface area of the hydration products of iime-pozzolana and lime-fly ash mixes, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Concrete, (Boston, November 16-18, 1981 ), Material Research Society, Pittsburgh, PA, 145-54

49. Turriziani, R., Les pouzzolanes ltaliennes et les ciments pouzzolaniques: II, Silicates lndustriels, 23 (1958) 265-70

50. Sullentrup, M.G., Baldwin, J.W. Jr., High lime fly ash as a cementing agent, in 1st bu. Cbnf on the Use of Fly Ash, Silica Fume, Slag & Other Mineral By-Products in ('oncrete, (Montebello, Juli 31-August 5, 1983), ACI SP-79, ed. V.M. Malhotra, 321-31

51. Massazza, E, Costa, U., Aspetti dell'attivita pozzolanica e propriet/~ dei cementi pozzolanici, II Cemento, 1 (1979) 3-18

52. Abdul-Maula, S., Odler, l., Hydration reactions in fly- ash-portland cements, in Proc. Symp. on Effects of Fly Ash bworporation in Cement and Concrete, (Boston, November 16-18, 1981 ), Material Research Society, Pittsburgh, PA, 102-11

53. Diamond, S., Sheng, Q., Olek, J., Evidence for minimal pozzolanic reaction in a fly ash cement during the period of major strength development, in Pore Struc- ture and Permeabili O' of Cementitious Materials, (Boston, November 28-30, 1988) 1989, 437-46

54. Chatterji, S., Thaulow, N., Christensen, P., Puzzolanic activity of byproduct silica-fume from ferro-silicon production, Cem. Concr. Res., 12 (1982) 781-4

55. Nelson, J.A., Young, J.E, Additions of colloidal silicas and silicates to portland cement pastes, Cem. Contr. Res., 7 (1977) 277-82

56. Cheng-Yi, H., Feldman, R.E, Hydration reactions in portland cement-silica fume blends, Cem. Concr. Res., 15(1985)585-92


57. Government Decree of 3 June 1968, Nuove norme sui requisiti di accettazione e modalit~ di prova dei cementi

58. CEN EN 196/Part V ( 1987) Methods of testing cement - - Pozzolanicity test for pozzolanic cements

59. Mohan, K., Taylor, H.F.W., Pastes of tricalcium silicate with fly ash -- Analytical electron microscopy, trimethylsilylation and other studies, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Con- crete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 54-9

60. Dalziel, J.A., Gutteridge, W,~., The influence of pulverized-fuel ash upon the hydration characteristics and certain physical properties of a portland cement paste. Cement and Concrete Association -- Technical Report 560, 1986, pp. 28

61. Copeland, L.E., Kantro, D.L., Hydration of portland cement, in 5th Int. Symp. on the Chemistry of Cement, (Tokyo, October 7-11, 1968) 1969 vol. II, 387-421

62. Meland, I., Influence of condensed silica fume and fly ash on the heat evolution in cement pastes, in 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag & Other Mineral By-Products in Concrete, (Montebello, July 31-August 5, 1983) ACI SP-79, voi. 1, 665-73, ed. V.M. Malhotra.

63. Turriziani, R., Rio, A., Osservazioni su alcuni criteri di valutazione dei cementi pozzolanici, Annali di Chimica Applicata, XLIV (1954) 787-96

64. Grutzeck, M.W., Roy, D.M., Scheetz, B.E., Hydration mechanisms of high-lime fly ash in portland-cement composites, in Proc. Symp. on Effects of Fly Ash Incor- poration in Cement and Concrete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 92-101

65. Massazza, F., Testolin, M., Trimethylsilylation in the study of pozzolana-containing pastes, I1 Cemento, 1 (1983) 49-62

66. Hooton, R.D., Permeability and pore structure of cement pastes containing fly ash, slag, and silica fume, in Blended Cements, (Denver, June 27, 1984), ASTM STP 897, ASTM Philadelphia 1986, 128-43

67. Costa, U., Massazza, F., Some properties of pozzolanic cements containing fly ashes in 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag & Other Mineral By- Products in Concrete, (Montebello, July 31-August 5, 1983) vol. 1, ACI SP-79, ed. V.M. Malhotra, 235-54

68. Roy, D.M., fly ash and silica fume chemistry and hydration in Proc. 3rd Int. Conf. on Fly ash, Silica fume, slag, and natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1 ACI SP-114, ed. V.M. Malhotra, 117-38

69. Massazza, F., Microstructure of hydrated pozzolanic cements, in First International Workshop on Hydration and Setting, (Dijon, 3-5 July 1991)

70. Ogawa, K., Uchikawa, H., Takemoto, K., Yasui, I., The mechanism of the hydration in the system C3S pozzolana, Cem. Concr. Res., 10(1980)683-96

71. Tavasci, B., Cereseto, A., Struttura del cemento pozzolanico idrato La Chimica e l'Industria, 31 (1949) 392-8

72. Ghose, A., Pratt, P.L., Studies of the hydration reactions and microstructure of cement-flyash pastes, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Concrete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 82-91

73. Diamond, S., Ravina, D., Lovell, J., The occurence of duplex film on fly ash surface, Cem. Concr. Res., 10 (1980) 297-300

74. Costa, U., Massazza, F. Permeabilith di paste di cemento portland e pozzolanico: relazione con la tessi-

tura porosa, in Convegno AITEC 'La durabilitd delle opere in calcestruzzo', (Padova, 8-9 ottobre 1987), vol. 1, 67-73

75. Manmohan, D., Metha, P.K., Influence of pozzolanic, slag and chemical admixtures on pore size distribution and permeability of hardened cement paste, Cement, Concrete and Aggregates, 3 ( 1981 ) 63-7

76. Feldman, R.F. Pore structure formation during hydration of fly-ash and slag cement blends, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Con- crete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 124-33

77. Feldman, R.F., Pore structure formation during hydration of fly-ash and slag cement blend in Proc. Syrup. on Effects of Fly Ash Incorporation in Cement and Con- crete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 124-33

78. Italcementi S.p.A. Laboratorio Chimico Centrale -- Internal Report, 1990

79. Costa, U., Massazza, F., Remarks on the determination of the pore distribution of portland and pozzolanic cement pastes, in Proc. on the first Int. Congr. Pore Structure and Construction Materials Properties, (Versailles, September 7-11, 1987), ed. J.C. Maso, 159-66

80. Feldman, R.E, Significance of porosity measurements on blended cement performance, in 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag & Other Mineral By-Products in Concrete, (Montebello, July 31-August 5, 1983) vol. 1, ACI SP-79, ed. V.M. Malhotra, 415-33

81. Feldman, R.E, Pore structure damage in blended cements caused by mercury intrusion, Journal of the Amer. Ceram. Soc., 67 (1984) 30-3

82. Costa, U., Massazza, F., Permeability and pore structure of cement paste, Materials Engineering, 1 (1990) 459-668

83. Massazza, F., Microscopic structure and concrete durability, in La Fissuration des B~tons et la Durabilit~ des Constructions, (Saint-R6my-l~s-Chevreuse, 31-8/ 2-9-1988), pp. 12

84. Massazza, F., Costa, U., Aspetti dell'attivith pozzolanica e propriet/~ dei cementi pozzolanici, Il Cemento, l (1979) 18

85. Giergiczny, Z., Werynska, A., Influence of fineness of fly ashes on their hydraulic activity, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzo- lans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 97-115

86. Sybertz, F., Comparison of differem methods for testing the pozzolanic activity of fly ashes, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzo- lans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 477-97

87. Abdul-Maula, S., Odler, I., Hydration reactions in fly- ash-portland cements, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Concrete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 102-11

88. Asgeirsson, H., Gudmundsson, G., Pozzolanic activity of silica dust, Cement and Concrete Research, 9 (1979) 249-52

89. Kohno, K., Aihara, F., Ohno, K., Relative durability properties and strengths of mortars containing finely ground silica and silica fume, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 815-26

90. Chatterji, S., Collepardi, M., Moriconi, G., Pozzolanic property of natural and synthetic pozzolans: A comparative study, in Proc. of the CANMET/ACI: 1st Int.

212 Franco Massazza

Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. l, ACI SP-79, ed. V.M. Malhotra, 221-33

91. Costa, U., Massazza, E, Natural pozzolanas and fly ashes: Analogies and differences, in Proc. Symp. on Effects of Fly Ash Incorporation in Cement and Con- crete, (Boston, November 16-18, 1981), Material Research Society, Pittsburgh, PA, 134-44

92, Watt, J.D., Thorne, DJ., The composition and pozzolanic properties of pulverised fuel ashes: lI1 -- Pozzo- lanic properties of fly ashes as determined by chemical methods, J. Appl. Chem., 16 (1966) 33-9

93. Delvasto, S., Pozzolanic activity and characteristics of Colombian materials, in Proc. 2nd hit. (?)nf on I~Tv Ash, Silica fitme, Slag, and Natural Pozzolans in Con- crete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Malhotra, 77-89

94. Regourd, M., Mortureux, B., Hornain, H., Use of condensed silica fume as filler in blended cements, in Proc. of.the CANMET/A(Z" 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete. (MontebeUo, Quebec, 31 July-5 August 1983) vol. I, ACI SP-79, ed. V.M. Malhotra, 847-65

95. Costa, U., Massazza, E, Some properties of pozzolanic cements containing fly ashes, in Proc. of the ( 'ANMET/ A(7." 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag attd Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983)vol. I, ACI SP-79, ed. V.M. Malhotra, 235-54

96. Hooton, R.D., Permeability and pore structure of cement pastes containing fly ash, slag, and silica fume, in Blended Cements, (Denver, June 27, 1984), ASTM STP 897. ASTM STP 897, ASTM Philadelphia 1986, 128-43

97. Cabrera, J.G., Plowman, C., The influence of pulverized fuel ash on the early and long term strength of concrete, in 7th Int. Congr. on the Chemistly ()[ Cement, (Paris, 1980) vol. 3, IV-84-92

98. Maage, M., Strength and heat development in concrete: Influence of fly ash and condensed silica fume, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natu- ral Pozzohms in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 923-40

99. Sandvik, M., Gjorv, O.E., Effect of condensed silica fume on the strength development of concrete, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, aim Natu- ral Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 893-901

100. Ronne, M., Effect of condensed silica fume and fly ash on compressive strength development of concrete, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 175-89

101. Yuan, R.L., Cook, J.E., Study of a class C fly ash concrete, in Proc. of the CANMET/ACT: 1st Int. Conf. on the Use of Hy Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. I, ACI SP-79, ed. V.M. Malhotra, 307-19

102. Day, R.L., Joshi, R.C., Langan, B.W., Ward, M.A., Examination of the use of high proportions of high- lime content fly ash in construction, in 2nd Int. Conf. on Ash Technology and Marketing, (London, Septem- ber 16-21, 1984), 377-85

103. Papayianni, J., Strength and bond data for Greek high- lime fly ash concrete, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Con- crete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Malhotra, 367-86

104. SeUevold, E.J., Radjy, F.E, Condensed silica fume (microsilica) in concrete: Water demand and strength development, in Proc. of the CANMET/ACI: 1st Int. Conf on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. ll, ACI SP-79, ed. V.M, Malhotra, 677-94

105. Thomas, M.D.A., Matthews, J.D., Haynes, C.A., The effect of curing on the strength and permeability of PFA concrete, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, AC1 SP-114. ed. V.M. Malhotra, 191-217

106. Hansen, T.C., Long-term strength of high fly ash concretes, Cement and Concrete Research, 20 (1990) 193-6

107. Ukita, K., Shigematsu, S., Ishii, M., Improvements in the properties of concrete utilizing classified Fly Ash, in l'roc. 3rd Int. Conf on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 219-40

108. Sri Ravindrarajah, R., Tam, C.T., Properties of concrete containing low-calcium fly ash under hot and humid climate, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 139-55

109. Carette, G., Malhotra, V.M., Early-age strength development of concrete incorporating fly ash and condensed silica fume, in Proc. of the CANMET/ACI: 1st hit. Conf. oll the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. II, ACI SP-79, ed. V.M. Malhotra, 765-84

110. Hobbs, D.W., Influence of fly ash upon the workability and early strength of concrete, in Proc. of the CAN- MET/ACI: 1st hzt. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebelio, Quebec, 31 July-5 August 1983) vol. 1, ACI SP-79, ed. V.M. Malhotra, 289-306

111. Swamy, R.N., Mahmud, H.B., Mix proportions and strength characteristics of concrete containing 50 per- cent low-calcium fly ash, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Con- crete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Malhotra, 413-32

112. Swamy, R.N., Mahmud, H.B., Shrinkage and creep behaviour of high fly ash content concrete, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume. Slag, and Natural Pozzolans in Concrete. (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 453-75

113. Kropp, J., Hilsdoff, H.K., Influence of carbonation on the structure of hardened cement paste and water transport, in Werkstoffwissenschaften und Bausanier- ung, (Esslingen, September 6-8, 1983) 1983, 153-7

114. Odman, S.T.A., Effects of variations in volume, surface area exposed to drying, and composition of concrete on shrinkage, in Le Retrait des Betons ttydrauliques, (Madrid, 20-22 mars 1968), pp. 20

115. Cripwell, J.B., Brooks, J.J., Wainwright, P.J., Time- dependent properties of concrete containing pulverized fuel ash and a superplasticizer, in 2nd Int. Conf. on Ash Technology and Marketing, (London, September 16-21, 1984), 313-20

116. Verbeck, G., Carbonation of hydrated portland cement, in Cement and Concrete, (Los Angeles, Sep- tember 19-20, 1956) 1958, ASTM STP 205, 17-36

117. Arber, M.G., Vivian, H.E., Inhibition of the corrosion of steel embedded in mortar, Australian Journal of Applied Science, 12 ( 1961) 330-8


118. Regourd, M., Carbonation acceleree et resistance des ciments aux eaux agressives, in RILEM Int. Symp. Carbonation of Concrete, (Fulmer, April 5-6, 1976), 5.3-3

119. Meyer, A., Investigations on the carbonation of concrete, in 5th Int. Syrup. on the Chemistry of Cement, (Tokyo, October 7-11, 1968) 1969 vol. HI, 394-401

120. SeUevold, EJ., Nilsen, T., Condensed silica fume in concrete: A world review, in Supplementary Cementing Materials for Concrete 1987, ed. V.M. Malhotra, Minis- ter of Supply and Services Canada 165-243

121. Ho, D.W.S., Lewis, R.K., Carbonation of concrete incorporating fly ash or a chemical admixture, in Proc. of the CANMET/ACI: 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. I, ACI SP-79, ed. V.M. Malhotra, 333-46

122. Kasai, Y., Matsui, I., Fukushima, Y., Kamohara, H., Air permeability and carbonation of blended cement mortars, in Proc. of the CANMET/ACI: 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. I, ACI SP-79, ed. V.M. Malhotra, 435-51

123. Paillere, A.M., Raverdy, M., Grimaldi, G., Carbonation of concrete with low-calcium fly ash and granulated blast furnace slag: Influence of air-entraining agents and freezing-and-thawing cycles, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 1, ACI SP- 91, ed. V.M. Malhotra, 541-62

124. Vennesland, O, Giorv, O.E., Silica concrete -- protection against corrosion of embedded steel, in Proc. of the CANMET/ACI: 1st Int. Conf. on the Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. II, ACI SP-79, ed. V.M. Malhotra, 719-29

125. Skjolsvold, O., Carbonation depths of concrete with and without condensed silica fume, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzo- lans in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 1031-48

126. Massazza, F., Pozzolanic cements: an answer to durability problems, in 3rd NCB Int. Seminar on Cement and Building Materials, (New Delhi, January 21-25, 1991) Vol. 4, VIII- 13/VI-27

127. Nagataki, S., Ohga, H., Eun Kyum Kim, Effect of curing conditions on the carbonation of concrete with fly ash and the corrosion of reinforcement in long-term tests, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Mal- hotra, 521-40

128. Matthews, J.D., Carbonation of ten-year old concretes with and without pulverised fuel ash, in 2nd Int. Conf. on Ash Technology and Marketing, (London, Septem- ber 16-21, 1984), 398, pp. 12

129. Yamato, T., Soeda, M., Emoto, Y., Chemical resistance of concrete containing condensed silica fume, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natu- ral Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 2, ACI SP- 114, ed. V.M. Malhotra, 897-913

130. Lea, EM., The Chemistry of Cement and Concrete, Edward Arnold, London, 1970, 727

131. Santarelli, L., Cesareni, C., Contributo allo studio della resistenza chimica dei cementi al dilavamento da parte di acque pure, L'Industria ltaliana del Cemento, 4 (1949) 100-4; 5 (1949) 126-9

132. Goggi, G., Ulteriori progressi nel controilo del dilavamento dei cementi da parte di acque pure, L'Industria Italiana del Cemento, 12 (1960) 394-400

133. Bertacchi, P., Deterioration of concrete caused by carbonic acid, in RILEM Symposium -- Durability of Concrete, (Praga, 1969)vol. 4, C-159-68

134. Backes, H.-P., Carbonic acid corrosion of mortars containing fly ash, in Proc. 2rid Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Malhotra, 621-36

135. Ballim, Y., Alexander, M.G., Carbonic acid water attack of portland cement-based concretes, in Proc. of the 5th Int. Conf. on Durability of Building Materials and Components, (Brighton, 1990), E. & EN. Spon, London, 179-84

136. Maultzsch, M., Vorg~nge beim Angriff von ChloridlS- sungen auf Zementstein und Beton, Material und Tech- nik, 3 (1984) 83-90

137. Rechberger, P., Electrochemical determination of chloride diffusion coefficients, Zement Kalk Gips, 11 (1985) 679-84

138. Chatterji, S., Mechanism of the CaCI2 attack onport- land cement concrete, Cement and Concrete Research, 8 (1978) 461-8

139. Feldman, R., Chen-Yi, H., Resistance of mortars containing silica fume to attack by a solution containing chlorides, Cement and Concrete Research, 15 (1985) 411-20

140. Turriziani, R., Rio, A., Osservazioni su alcuni criteri di valutazione dei cementi pozzolanici, Annali di Chimica Applicata, 44 (1954) 787-96

141. Diamond, S., Chloride concentrations in concrete pore solutions resulting from calcium and sodium chloride admixtures, Cement and Concrete Aggregates, 8 (1986) 97-102

142. Collepardi, M., Marcialis, A., Turriziani, R., La Pene- trazione degli agenti anti-gelo nelle paste di cemento, I1 Cemento, 3 (1972) 143-50

143. Cussino, L., Curcio, F., Saccone, R., Alunno Rossetti, V., Cemento pozzolanico per calcestruzzi durabili ad elevata resistenza, in Convegno AITEC 'La Durabilitft delle Opere in Calcestruzzo' (Padova, October 8-9, 1987)

144. Lukas, W., Chloride penetration in standard concrete, water-reduced concrete, and superplasticized concrete, in Developments in the Use of Superplasticizers, ACI SP-68, ed. by V.M. Malhotra, 1981, pp. 253-67

145. Gebler, S.H., Klieger, P., Effect of fly ash on the durability of air-entrained concrete, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 1, ACI SP- 91, ed. V.M. Malhotra, 483-519

146. Roy, D.M., Kumar, A., Rhodes, J.P., Diffusion of chloride and cesium ions in portland cement pastes and mortars containing blast furnace slag and fly ash, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 1423-44

147. Plante, E, Bilodeau, A., Rapid chloride ion permeability test: Data on concretes incorporating supplementary cementing materials, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 1, ACI SP-114, ed. V.M. Malhotra, 625-44

148. Marusin, S.L., Chloride ion penetration in conven- tional concrete and concrete containing condensed silica fume, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 1119-33

149. Kumar, A., Roy, D.M., Retardation of Cs ÷ and CI- diffusion using blended cement admixtures, Journal of the American Ceramic Society, 69 (1986) 356-60

214 Franco Massazza

150. Gautefall, O., Havdahl, J., Effect of condensed silica fume on the mechanism of chloride diffusion into hardened cement paste, in Proc. 3rd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Trondheim, June 18-23, 1989) vol. 2, ACI SP-114, ed. V.M. Malhotra, 849-60

151. Gautefall, O., Effect of condensed silica fume on the diffusion of chlorides through hardened cement paste, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 2, ACI SP-91, ed. V.M. Malhotra, 991-7

152. Collepardi, M., Marcialis, A., Turriziani, R., La cinetica di penetrazione degli ioni cloruro nel calcestruzzo, // Cemento, 4 (1970) 157-64

153. Byfors, K., Influence of silica fume and flyash on chloride diffusion and pH values in cement paste, Cement and Concrete Research, 17 (1987) 115-30

154. Hansson, C.M., Strunge, H., Markussen, J.B., Frolund, T., The Effect of Cement Type on the Diffusion of Chlo- ride, Nordic Concrete Reserch Publication No. 4, 1985, 70

155. Collepardi, M., Marcialis, A., Turriziani, R., Penetra- tion of chloride ions into cement pastes and concretes, Journal of the American Ceramic Society, 55 (1972) 534-5

156. Page, C.L., Short, N.R., E! Tarras, A., Diffusion of chloride ions in hardened cement pastes, Cement and Concrete Research, 11 ( 1981 ) 395-406

157. Dunstan, E.R., Performance of lignite and sub-bitumi- nous fly ash in concrete -- A progress report, Report REC-ERC-76-1 -- U.S. Bureau of Reclamation 1976. Quoted in: Berry, E.E., Malhotra, V.M., Supplementary Cementing Materials for Concrete 1987, ed. V.M. Malhotra, 117 and 159

158. Manz, O.E., McCarthy, G.J., Effectiveness of western U.S. high-lime fly ash for use in concrete, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986) vol. 1, ACI SP-91, ed. V.M. Malhotra, 347-65

159. Smith, R.L., Raba, C.E Jr., Recent developments in the use of fly ash to reduce alkali-aggregate reaction, Alka- lis in Cbncrete ed. V.H. Dodson, ASTM STP-930, American Society for Testing and Materials, Philadel- phia 1986, 58-68

160. Greco, L., Mazzetti, C., Periani, P., Constatazioni recenti e precauzioni nuove nei riguardi della decom- posizione delle malte e dei conglomerati cementizi nell'acqua di mare, Giornale del Genio ()'vile, Jan (1951)3-14

161. Guyot, R., Ranc, R., Varizat., A., Comparison of the resistance to sulfate solutions and to sea water of different portland cements with or without secondary constituents, in Proc. of the CANMET/ACI: 1st Int. Conf on the Use of Fly Ash, SUica Fume, Slag attd Other Mineral By-Products in Concrete, (Montebello, Quebec, 31 July-5 August 1983) vol. l, ACI SP-79, ed. V.M. Malhotra, 453-69

162. Gjorv, O.E.. Vennesland, O., Diffusion of chloride ions

167.

from seawater into concrete, Cement and Concrete Research, 9 (1979) 229-38

163. Thomas, M.D.A., A comparison of the properties of OPC and PFA concretes in 30-year-old mass concrete structures, in Proc. of the 5th Int. Conf. on Durability of Building Materials and Components, (Brighton, 1990), E. & EN. Spon, London, 179-84

164. Nixon, EJ., Gaze, M.E., The use of flyash and granulated blastfurnace slag to reduce expansion due to alkali-aggregate reaction, in Proc. 5th Int. Conf. on Alkali-Aggregate Reaction in Concrete, (Cape Town, 1981), $252-32

165. Hobbs, D.W., Expansion due to alkali-silica reaction and the influence of pulverised fuel ash, in Proc. 5th hu. Conf. on Alkali-Aggregate Reaction in Concrete, (Cape Town, 1981), $252-30

166. Oberholster, R.E., Westra, B.W., The effectiveness of mineral admixtures in reducing expansion due to alkali-aggregate reaction with Malmesbury Group aggregates, in Proc. 5th Int. Conf. on Alkali-Aggregate Reaction in Concrete, (Cape Town, 1981 ), $252-31 Hobbs, D.W., Influence of pulverized-fuel ash and granulated blastfurnace slag upon expansion caused by the alkali-silica reaction, Magazine of Concrete Research, 34 (1982) 83-93

168. Mehta, P.K., Studies on blended portland cements containing Santorin earth. Cement and Concrete Research, 11 (1981)507-18

169. Massazza, F., I1 ruolo delle aggiunte al cemento nella durabilit~ del calcestruzzo, ll Cemento, 4 (1987) 359-82

170. Chatterji, S., The role of Ca(OH)2 in the breakdown of portland cement concrete due to alkali-silica reaction, Cement and Concrete Research, 9 (1979) 185-8

171. Chatterji, S., Thaulow, N., Christensen, P., Jensen, A.D., Studies of alkali-silica reaction with special reference to prevention of damage to concrete. A preliminary study, in Oth hu. Conf. Alkalis in Concrete, (Copenhagen, June 22-25, 1983), 253-7

172. Mehta, EK., Natural pozzolans, in Supplementary Cementing Materials for Concrete 1987, ed. V.M. Malhotra. Minister of Supply and Services Canada 1-33

173. Yamato, T., Emoto, Y., Soeda, M., Strength and freezing-and-thawing resistance of concrete incorporating condensed silica fume, in Proc. 2nd Int. Conf. on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, (Madrid, April 21-25, 1986), vol. 1. ACI SP- 91, ed. V.M. Malhotra, 1095-117

174. Galeota. D.. Giammatteo, M.M., Marino, R., Volta, V., Freezing and thawing resistance of non air-entrained and air-entrained concretes containing a high percentage of condensed silica fume. in Second CANMET/ A (7 hit. Conj. on Durability of Concrete, (Montreal, August 4-9, 1991) 1991, Vol. 1, ACI SP-126. ed. V.M. Malhotra, 249-61

175. Massazza, F., Oberti. G., Durability of pozzolanic cements and Italian experience in mass concrete, in 2nd C¼NMET/AC1 bit. Conf. on Durability Of ~m- crete, (Montreal, August 4-9. 1991)