118

AIR ENTRAI NfoIENT OF MORTAR

NEIL BENINGFIELD RMC Mortars Limited ,

Holly House , 74 Upper Holly Walk, Royal Leamington Spa , Warwickshire , UK

ABSTRACT

The mechanism of air entrainment is considered.

Research work on many of the factors affecting the air content of mortars is discussed . The effect of mix consistence , mixing time , sand composition and grading , clay content , temperature , cement particle size and cement chemistry a r e quantified. It is seen that the relationships found were not always in accord with existing literature although th is was often predominately arientated to concrete rather than mortar .

The effects of entrained air on the plastic and hardened mortar properties are discussed .

INTRODUCTION

An earlier literature survey by the Author(l) showed that the factors affecting the amount and type of air entrained and the effect of this air on the properties of mortar are rarely quantified in the literature.

Much air entrained mortar is produced without due regard to changes in the factors that can alter the total amount of air or the stability af that air.

The majority o f these influences are addressed and relationships shown linking the effects of mix design and other factors on the amount af air entrained .

THE FACTORS AFFECTING AIR ENTRAINMENT

Mixing Time For the most widely used generic type of air entrainment , that based on vinsol resin , air content increases to a peak quite rap idly and then

..

....

119

declines as shown in Graph 1.

Mix Consistency and Mixing Time Graph 1 also shows that a mix of much gre ater consistence, i.e . much wetter , loses air at about the same rate as a drier mix o

Sand Grading: Graph 2 shows that fine sand loses air much more rapidly than coarse when subjected to prolonged mixing.

In order to obtain the same air content however , for both mixes , much more agent was required for the fine than for the coarse and so t he comparison could not be made on the basis of identica l original mixes.

It may be therefore that air loss relates directly to the amount of agent used and this relationship rather than a grading related one is the

primary one .

Graph 2. The effect of change in Graph l . The effect o f prolonged mlxlng a1: t'NO àifferent consistences. sand graàing on t he 'Tlixing cime

effect: .

13

14 ?ine Sanà

12 A Coarse Sanà

~l

:::: ~ O

<::

*' 9

a

7 a

d.b . 9.4

d. b. 12.75

i 6 24 32 2.0 TIME =N ,HNUTES

12

8

ô

i2 TIME

FACTORS CAUSING AIR LOSS WITH TIME

24 36 IN ,üNUTES

It had been suggested that the loss of air was caused by the mi x drying but careful weighing of a mix, complete with mi xer bowl and blade for accuracy , showed that only small amounts of water were lost. Restoring accurately that loss did not restore the air contento

Various authors have reported particle attrition, leading to increased fines, as a reason for air loss with time. (2)

ela

120

Mixes were therefore made using a soft oolitic material and a hard siliceous sand, mixed for 30 minutes and the air content loss and grading change measured . This showed a slightly greater increase in fines for the oolite , as would be expected, but the air loss was in the same order for each , as shown in Graph 3.

Graph 3. The effect o f aggregace ~ardness o n the mixing ~ime effec~ .

16

14

12

~

lO

!O:: y 3 <:

~

6

4

o 4

o \ \0

X

"-"­

"-"-" o

" , , ,

x

"­ , , "

8 12 16 20 24 ,':ME :N :.1INUTES

Silceous sand

Ool ite

'" "

28

It is believed that this demonstrates without doubt that sand attrition is not a factor in causing air loss . Water loss also having earlier been ruled out this leaves two further poss ible influences , chemical reactions associated with the commencement of hydration and the breaking apart of agglomerates of cement and/or clay , material too fine to be meaningfully identified by the ordinary sieve analysis used .

In order to investigate whether breakdown/attrition of clay was a factor a sample of sand was washed clean of alI less than 75~m particles, such that alI clay had been removed.

This showed exactly the same characteristics of loss with time as the untreated product containing clay and showed the clay not to be a causative factor in the air loss. Graph 4 shows this with the clean sand plotted together with an untreated contraI test for comparison.

It was concluded therefore that , as neither aggregate attrition, water loss nor the reduction of clay agglomeration caused the air loss

~

-

121

it was associated with cement hydration or particle size/agglomeration effects .

Graph 4. The mixing time effect fo r sand 'IJi th the minus 75\JlTI fraction

o::: --;

< ~

16 removed.

10 ~. 8

Ô

4

x control sand untreated sand all:> 75~m

OL-____ ~ __________________ _

a 12 ::'6 20 24 28 TIME IN MINUTES

In order to investigate the inf1uence of cement on air loss with time a series of mixes were made with varying cement contents and subjected to pro1onged mixing using a fixed amount of air entraining agent . These clearly showed the influence of cement content in depressing the air. In addition , analysis of the form of the graphica1 re1ationship, rep1icated many times , a1ways revea1ed the same characteristic forms, especial1y at higher air contents .

The form of the relationship, shown in Graph 5 , may be seen to comprise three interconnecting phases.

These can be considered as propagation , initial 10ss which is rapid at first and then lessens and final 10ss, the period at which the rate of 10ss increases, marked1y so if considered as a function of initia1 air contento This 1ast phase is particu1ar1y noticeab1e at high air contents, 1ess so or perhaps even absent at low air contents.

It was seen that there was a re1ationship between cement content and mixing time in two specific areas, the propagation phase and the loss phase.

-----------------------------

122

Both the time to reach a maximum , i.e . the length of the propagation phas e and the total air gained increased with an increas e in cemen t content o Indeed , at low cement contents there was of cours e a i r loss and not ai r ga in.

It is bel ieved that these phenomen a are caused by the adsorption of AEA on to the cement . This adsorption appears preferential , in that the AEA reacts first with the c eme nt such that it doe s not e ntrain air. It then appears to progressively desorb and begin to entra i n air . The reason behind this phenomenon and its reversal is unclear .

Graph S . The effec~ of c emen~ content on the mixing time effect us ing o.oge .-'lEA on to caI dry mix .

36

32

O O. P.C

c::: ~ : 6

"" ':t:== -----......... 33% O. P.C === 50% O.P. C

ôô% O.P. C

d

O 3 16 2d 32 dO TIME IN MINUTES

d8

Repeat ing this work at dosages of AEA that were a fixed percentage of cement weight gave the form of relat i onship shown i n Graph 6 . It is in tere sting to no te that the final a ir con tent after what is hypothesised as desorption has taken place was virtually the same at between 9 . 2% and 11 . 2% f or a l I four mixes .

The final f actor affecting air loss with time that was invest i gated was the initial air content o It was f oun d that the amount lost was in the same order at between 3 & 4 . 7% , for mixes of i nitial air contents that varied widely from between 27% down to only 6% .

Mix Consistence It is generally accepted in the fieId of Concrete Technology that air content inc r eases with slump (consistence) (3 , 4,5,6) but some sources qualify this statement by restricting it to a maximum slump of between 150

...

...

123

and 200mm (7,8,9 , 10)

high slumps. One source , the PCA (11) reports a decrease at very

z

6 u

33% 50% õõ% 14%

8 16 24 32 40 48 TI~~E [N 'lINUTES

op c opc :>pc opc

Graph 3. ~he e!'~ec~ o f cemen~ concent fo r 0 . 1% AEA on cemenc ~eight .

The Author ' s work showed a gradual and slight reduction in air content as consistence increased and this showed a linear relationship with air loss directly proportional to increase in water contento

It is suggested that this is due merely to an expansion of the total volume of aqueous phase, the actual amount of air bubbles r emaining the same, t hus reducing in percentage terms .

Temperature An empirical relationship developed by RMC Mortars Limited is in agreement with data reported by Hercules Inc (12) and is shown in Graph 7 .

Graph 7 ~he ~ela~~onship a f tempera~ure ~o sne amount of air e nt~ained .

.-----2

2.0

~ mp iri~al ~~la~lonsnip

developed jy ~MC ~or~ars Lt d

- - - - - Lla 1:a due ':0 t-iercules =nc.

TEMPERATURE °C OL-____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ~8 20 2 2 24 26 28 30 32

124

This has proved satisfactory in practice for admixtures based both on vinso l resin and on synthetic surfactant formulations.

Air Entraining Agent Concentration : carried out in this area , and accordingly presented here , although the general form considered .

A great deal o f work has been a comprehensive treatment is not of the relationship is

There are grounds for treating the relationship between AEA addition and air content as linear for constant cement content o At modest air contents it is commonplace so to do as a convenience when marketing or using the admi x ture ; thus the common re l iance on admixtur e dosage expressed as a percentage of the cement content o

Although widely used t his approach is incorrect , the correct relationship having been àiscussed by, amongst others , Backstrom et al (13) anà Lauer (14)

The present research showed that the assumption o f linearitv is barely justified at lower addition levels . This region is the straight or nearly straigh t part o f the hyperbolic or parabolic relationship shown in Graph 8 .

Graph 8 . The effec~ of AEA co ncentration on air conten~ .

l 40 ~4% OPC

30 ~ ?/33% OPC x < 20

~

10

O 2 4 6 8 10 12 l4

AMOUNT OF . .l,EA AS % OF CEMENT .I/EIGHT

The non l inear form of the relationship , with an increasing falling off of the effectiveness of the AEA until a plateau is reached , is probably due to adsorption and to the formation of AEA rich micelles in the aqueous phase .

CEMENT PAfi~ICLE SIZE

Previous workers have ground a single specimen of cement progressively more finely (15,16,17) or useà fine and coarse cemen t from different works (18 ) .

125

It was thought worthwhile to carry out a brief investigation to confirm the effect using a range of specific surfaces that could in practice be used on site .

The cements used were all from the same works, they were not subjected to laboratory grinding and were therefore in every respect ordinary production cements . Graph 9 shows that increase in cement fineness depressed the amount of air entrained and that this effect held for widely varying air contents .

This data shows that increasing the cement specific surface from 280 to 440m2 /kg reduced the air content by a virtually constant amount, 2.4% on average , regardless of the amount actually present.

Graph 9. ~he effect of cement fineness 8n air con~ent for 5 dif[erent AEA le11el s .

20 ____ 0 .2% AEA

"------------- O. 016%"EA lã

12 ~ 0 . J1% ."EA • .:r:;

< 3 ~ õi'. :].004% AEA

4 ----- 0 . 002% AEA

o 280 320 360 400 440

CEMENT SIZE AS S?ECIFIC SURFACE m' Ik g

CEMENT CHEMISTRY

It is alfftcult to obtain a cement of different chemistry without other differences in e.g. particle size , but it proved possible , by using a foreign cement, to obtain a material with exactly the same specific surface as one of those already tested but with very different composition . This material was a Danish white cement with a specific surface of 430m2 /kg , virtually identical to the 436 of the rapid hardening Portland cement previously used . The difference in composition is shown in Table 2 .

Although Chatterji (19) reported that white cement produced the same air content as Portland cement , his materiais varied in specific surface thus making a comparison of less value , in the writer's view.

In the present investigation there was a defini te decrease in the amount of air entrained with the grey cement , such that the white cement mi x entrained 22% more, as shown in Graph 10.

L

126

TABL::: 2 Comoarison of Lhe composi tion of the ,,,h i te c ement and th e RHPC.

?ROPSRTY

5PSC:FIC SüRFACE (mlkg ) 5iO, CaO 50, C, S C, S C1 .-\

C.1 AF

Na, O

'I/HITE CEMENT

430 24 69

1 . 9 80

9 4 1

0 .2

CEMENT TYPE

RHPC

.136 20 63

3 . 4 :59 13 10

6 . 6

O. S

TYE'ICAL OPC (FOR COMPARISON ONLY )

365 20 64

2.3 59 ~ 4

10 S . 3

0.7

Grap h 10. The effecc o f c hange in cemenc composi tion on a ir contento

20

16

12

,..., 8 <

*-4

O ,J04 , 008 , 012 ,016 .020

AEA% DF TOTAL ORY ~T .

SAND PARTICLE SIZE

x '"hi te c emenc

::lHPC

In order to i nvestigate the effect of individual sand particle sizes, without interactive effects of a complete range of particles , a ' standard' sand was s ieved in to s eparate sizes .

Each size was tested on its own to investigate the a ir entrained for a cement:sand mix of totally single sized aggregate and the results are shown in Graph 11 .

-

127

~raph ~l. The effec~ OI sana ~ar~~cle size 8n air concent :or single sized sand.

30

20

<

~ o

150 300 600 1. 18 2 . 36 8 . S. SEIVE SIZE ( ~m /mm )

The results confirmed that sand particles in the so called inte rmed iate range promote air entrainme~t bu t the present work gave the maximum air content with 1 . 18mm particles , compared with the 300~m found most effective in other work ( 20) .

Graph 12 shows fur ther work on th is topic and it is seen that the coarser sand can attain much higher air contents than the fin e compar i son .

Graph 12. The effec1: o f sand ;Jar'::icl e si ze on air com:en t . 24

:::

<

20

15

/, 36 I I

iTlffi sanà

150 11m sand

o L-__________ ~ __________ __

. 02 . 0 4 . 06 . 08 0 . 1 . 12

% AEA ON !OTAL JRY ~T .

128

Sand Composition and Particle Shape Although previous work has shown tha( ~and composition, e.g . whether polar or non- polar can affect air content 14 the writer failed to validate this thesis and obtained the same air contents with both limestone and silica sand.

With respect to particle shape, sands artificially graded to the same grading but with rounded particles and angular particles were shown to entrain similar amounts of air.

THE AFFECT OF ENTRAINED AIR ON MORTAR

There are two classes of effect of the air . These being the effect on the plastic or wet properties of the mortar and the hardened or dry properties. Each is considered hereafter.

The Plastic Properties The ease af use perceived by the brick or blocklayer is enhanced as is the homogeneity/resistance to segregation and bleeding and to some extent the cohesion and adhesion.

An obvious effect is reduction in unit mass again leading to easier usage.

These effects increase with increasing air contents up to leveis that are grossly excessive for proper usage, this by virtue of the concomitant reduction in desired hardened properties as the excessively high leveis are reached.

Thus the maximum is imposed, not by the plastic properties but by the hardened ones.

The Hardened Properties As air content increases the resistance to freeze thaw cycling generally increases. Although sometimes thought to relate directly, the air content is itself only a secondary function and it is the bubble spacing factor that is directly related to enhancement of freeze thaw resistance. The air content is a function of this and of the size of the bubbles.

Thus small air contents can provide excellent freeze thaw resistance provided that their bubble spacing factor is sufficiently low. This will be so for low air contents, if the bubbles are of very small size and this approach is not practicable with the current air entraining agent technology; the bubbles are much larger than desired for this purpose to be fulfilled .

It is of interest that although it was long suggested that suffici ent air was required to accommodate the we l l known volume changes of water at 01' about freezing point Li tvan (21) amongst others has shown that this is not really the case , a much more sophisticated m~chanism of diffusion obtains .

With the bubble sizes generally available currently there is little reduction of the desired hardened properties at up to perhaps 15 to 20% of

0lIlIIII

-

129

air . Indeed, sometimes a modest increase in strength is recorded at low and medium leveIs due to water cement ratio reductions.

However, at leveIs of 20 to 25% air reduction in strength becomes noticeable and it is suggested that the prudent maximum is at about the 20% leveI .

1.

2 .

3 .

4.

5 .

6.

7.

8 .

9.

10 .

11.

12.

REFERENCES

Beningfie l d, N.E. Air Entrainment of Mortar - a bibliography. Paper submitte d to the 8th International Brick/Block Masonry Conference Ireland ' 88 .

Burg, G.R .U . Slump loss, air loss and field performance of concrete. ACI Journal , Title No. 80 - 34, July- August 1983 , pp . 332- 339.

British Ready- Mixed Concrete Association. Air entrainment and ready- mi xed concrete . BRMCA, Middlesex, March 1972 . Technical Report No. 2.

Wr ight , P.J . F . Entrained air in concrete . Proc . Inst i tution of Civil Engineers , Vol. 2 , Par t 1 , January - November 1953 . Paper No. 5915 , pp . 337- 358 .

Fulton , F . S. Concrete technology - A South African handbook. Portland Cement Institute , Richmond , Johannesburg , South Africa, 1969 .

Bartel , F.F. Air content and unit weight . ASTM STP 169B ' Significance of Tests & Properties of Concrete & Concrete Making MateriaIs ', Chapter 10, pp . 122- 131.

Brown , B. V. Use of air entraining agents in concrete production. RMC Technical Services Ltd., Mi ddlesex, Jan 1981 . Tech Note 116 .

Brown, B.V. Air entrainment - Part 2 . Concrete, January 1983, IM/07/3 - No . 81 , pp . 45- 46 .

Suthe r land , A. Air entrained concrete . C&CA 1974 .

Russell , P. Concrete admixtures. Eyre & Spottiswoode Ltd. , Margate , 1983.

Portland Cement Association . Concrete Information Ser i es - Ai r entrained concrete. Concrete, July 1968 .

Hercules Powder Company . Vinsol air entraining agents for cement products. Hercules Powder Co ., Delaware , USA . Bulletin PC- 178.

13.

14.

15 .

16 .

17 .

18 .

19 .

20 .

21.

130

Backstrom, J . E . et aI. Origin, evolut ion and effects of the air void system in concrete. Part 2 - Influence of type and amount of air entraining agent. Proc . ACI V. 55 , ACI Journal Vol . 30 , No. 2 , Title No . 55- 16 , August 1958. pp. 261- 272.

Lauer , K.R. The mechanisms of air entrainment in mortars . Thesis for Doctor of Philosophy, Purdue Universi ty , August 1960 University of Mi crofilms Inc . Ann Arbor , Michigan, USA . Mie. 60- 6119 .

Mayfield, B. & Moreton , A. J . Effect of fineness of cement on the air entraini ng proper t i es of concrete . Civil Engineering & Public Works Review , January 1969 , pp. 37- 41.

Scripture , E . W. et aI . Effect of temperature and surface area of the cement on air entrainment. Proc . ACI V.48, ACI Journal , Vol . 23 , No . 3 , Ti tle No . 48- 15 , November 1951, pp. 205- 210.

Greening , N. R . Some causes for variation in required amount of air entraining agent in Portland cement mortars . Journal of PCA Res. & Dev . Labs., Vol 9 , No . 2 , PCA , USA , May 1967 .

Hall, W.K. An investigation into the influence of the fineness of current production cements and cement contents on air entrained concrete . 4 th Convention, Association of Concrete Technologists , 1 - 3 June 1976 .

Chatterji , S . et aI. The characteristics of air- bubble systems in hardened ......... air entraining agents - plasticiser combination . Silicates Industriels, July - August 1978 , pp . 153- 156 .

Mielenze, R . C. et aI . Origin , evolution and effects of the air void system in concrete . Part I - Entrained air in unhardened concrete . Proc. ACI V.55, ACI Journal Vol . 30 , No . 1 , Title No. 55 - 5 , July 1958 . pp . 95- 121 .

Li tvan , G. G. Frost activiti es in cement paste. July - August 1973 .