20320140503037

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN

0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 317-326 © IAEME

317

RESIDUAL COMPRESSIVE STRENGTHS OF FLY ASH CONCRETES

EXPOSED TO ONE YEAR OF ACCELERATED SULPHATE ATTACK

Rajamane N. P.1, Dattatreya J. K.

2, D. Sabitha

3

1Head, CACR, SRM University, Kattankulathur (Ex-SERC), 2

Professor, Dept. of Civil Engg, SIT, Tumkur (Ex-SERC), 3

Scientist, CSIR-NGRI, Hyderabad

ABSTRACT

The presence of sulphate ion causes deterioration of concrete structural components exposed

to marine environments or placed in soils and groundwater contaminated with sulphate salts. This

aspect of concrete deterioration is being studied for decades. Sulphate attack on concrete is a

complex process involving many factors such as cement type, sulphate cation type, sulphate

concentration and exposure period. The sulphate ions react with C3A and Ca (OH)2 , to produce

expansive and/or softening types of deterioration. The sulphate attack in marine environment gives

rise to expansive ettringite, gypsum, and brucite and sometimes is associated with calcite formation.

The use of blended cements incorporating supplementary cementing materials and cements with low

C3A content is becoming common to prevent the deterioration of concrete structures subjected to

aggressive sulphate environments, even though it has been reported that the limitation on C3A

content is not the general answer to the problem of sulphate attack. This paper presents the results of

an investigation on the performance of fly ash blended cement (made in cement plants or on site)

concrete mixtures with immersion period of up to 12 months in environments characterized by the

presence of mixed magnesium-sodium sulphates. The concrete mixtures of basically, M25 grade,

comprise one Portland cement (OPC) (with w/c=0.55), one Portland Pozzolana Cement (PPC) i.e.

factory blended cement contacting fly ash (with w/c=0.55), and one on site fly ash based blended

concretes (water-to-binder ratio of 0. 50). Deterioration of concrete due to mixed sulphate attack

(sulphates of sodium and magnesium) was evaluated by assessing concrete visual inspection, weight

loss and strength loss at periodic intervals throughout the immersion period of 12 months. The test

data shows that sulphate attack cannot be prevented by only any one of three common factors: low

w/c, low calcium hydroxide content, or low C3A content. Lowering w/c has a deleterious effect on

the resistance of concrete exposed to magnesium sulphate, as at low values of w/c, there is limited

pore space to accommodate the products of reactions with sulphate, namely, magnesium silicate

INTERNATIONAL JOURNAL OF CIVIL ENGINEERING

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ISSN 0976 – 6308 (Print)

ISSN 0976 – 6316(Online)

Volume 5, Issue 3, March (2014), pp. 317-326

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hydrate (which has no adhesive properties) and gypsum. The test data in the present paper shows that

a reduction in the w/b from 0.55 of OPCC to 0.50 in FAC has not helped FAC which had 10%

strength loss between 180 and 360 days as compared to 5% strength loss in OPCC. However, an

increase of w/b ratio from 0.50 in FAC to 0.55 in PPCC, has indeed caused higher strength of loss of

30% between 180 and 360 day, but only 10% loss in FAC. Interestingly, during the period of 180 to

360 days, at w/b ratio of 0.55, the OPCC had performed favourably as against fly ash containing

PPCC. Thus, adoption of one parameter alone among many such as w/c ratio, cement type, fly ash

content etc may not be sufficient to ensure sulphate resistance. Any sulphate resistance study, longer

period, say of one year is essential, to arrive at rational decisions on sulphate attack

1.0 INTRODUCTION

Sulphates of sodium, potassium, magnesium and calcium which occur in soil or ground water

may be carried into inner sections of concrete by either ionic diffusion or capillary absorption of their

salt solutions and cause disruptive forces leading to cracking or spalling of concrete [Lawrence,

1990; Newmann, 1990; Mark Richardson, 2002; Neville, 1995]. They react with the hydration

products of cement paste, viz. calcium hydroxide (CH), Calcium Alumino hydrate (CAH), and even

calcium silicate hydrate (CSH). Sodium sulphate attacks CH in an acidic type reaction forming

gypsum, NaOH and water. In flowing water, CH can be completely leached out. Sulphates also

react with the C3A component of cement forming an expansive product called ettringite

(3CaO.Al2O3.3CaSO4.32H2O), which results in cracking of concrete. MgSO4 attacks CSH as well as

CH & CAH forming gypsum, Mg (OH)2 (brucite) and aqueous silica. Because of the very low

solubility of Mg (OH)2, the reaction proceeds to completion, so that, under certain conditions, the

attack by MgSO4 is more severe compared to other sulphates. The critical consequence of MgSO4

attack is the destruction of CSH. In addition to disruption and cracking, sulphate attack also leads to

loss of strength due to reduced cohesion in Hydrated Cement Paste (HCP) and in the bond between

HCP and aggregate. Concrete attacked by sulphate has typical whitish appearance. The attack

usually starts at the edges and corners and is followed by progressive cracking and spalling, reducing

the concrete to a friable mass. Sulphate attack occurs only when the sulphate ion concentration

exceeds a certain threshold. It is about 0.5% for magnesium sulphate and 1% for sodium sulphate.

A saturated solution of magnesium sulphate leads to severe deterioration, but for low water-cement

ratio concretes, it takes 2-3 years for the usual appearance of distress.

The three principal means for reducing sulphate attack are : (1) limiting C3A and C4AF

content of cement (2) use of pozzolanic additions (3) prevention of ingress of sulphates by making

dense, low-permeability concrete. It is now well accepted that limiting C3A and C4AF is not the

ultimate answer to the problem of SO4 attack and blended cements may be preferable from this point

of view [Mangat, 1992 and 1995; Neville, 1995; Lawrence, 1990; Osbome 1994; Torii, 1994].

The resistance of concrete to sulphate can be tested in the laboratory by storing the specimens

in a solution of magnesium sulphate or sodium sulphate or in a mixture of the two. Alternate wetting

and drying accelerate the damage due to crystallisation of salt in the pores of concrete. However

there is no agreement on the choice of wetting and drying durations. The effect of expansion can be

estimated in terms of loss in strength of specimen, change in dynamic modulus Edyn, magnitude of

expansion, loss of mass and visually observed deterioration. ASTM C 1012 tests for 1:3 cement

mortar immersed in 2.5% SO4 solution can be used to assess the effect of using various cementitious

materials in the mix. However, the test is slow and since mortar and not concrete is tested, some

typical effects of mineral fillers in the aggregates are not reflected. Other ASTM tests based on C

452-894 and C 1038-89 are also related to mortar and intend to identify the excessive sulphate

content of an OPC rather than attack by expansive sulphate.



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Some of the recent studies [Newmann, 1990; Al Amoudi 1995] on long term performance in

extreme climates (combined attack of sodium and magnesium sulphate in extreme temperatures)

indicate that the aluminate contents of cement and CRMs are both critical factors that control the

performance and even dense concretes with low water-cement ratio could be affected in very adverse

environments.

This paper describes the test results of three types of mixes of M25 grade concretes with

binders being OPC, OPC+25% FA, and factory produced fly ash containing Portland Pozzolana

cement (PPC). The data is expected to show the influence of presence of fly ash on the sulphate

resistance of concretes and residual compressive strength measured after the predetermined

accelerated sulphate exposure time was taken as parameter to quantify the degree of the

deterioration.

2.0 CONCRETE MIXTURES INVESTIGATED

• Grade: M25

• Types of concretes mixtures:

I. Ordinary Portland Cement Concrete (OPCC) Mixture,

II. FA based Portland Cement Concrete (FACC) Mixture with FA (% by mass) contents in

binder portions of the concretes being 25%. Class F fly ash from a coal based thermal plant

near Chennai, admixed with OPC during preparation of concrete mixes in the laboratory.

III. Portland Pozzolana Cement Concrete (PPCC) Mixture.

Minimum slump of about 100 mm was maintained in the concrete mixes to facilitate

easier compaction by a table vibrator. The curing period of the specimens before subjecting them to

accelerated exposure test was 56 days (28 days water curing followed by 28 days of storage in

ambient conditions in the laboratory). This period was expected to provide sufficient time to enable

the pozzolanic reactions of FA within the concrete matrix so that there is a significant contribution

from FA to the microstructure refinement of the concrete and realistic durability characteristics of fly

ash based concretes could be assessed. The properties of cements and fly ash are described in Tables

1 (a) and 1 (b) respectively.

Table 1 (a): Physical and Chemical Properties of OPC and PPC

Nature of Tests Cement type

43 grade OPC PPC1

1. Chemical composition in percent by mass (IS:4032)

(a) Silicon dioxide, SiO2 21.65 29.07

(b) Insoluble residue 1.08 28.87

(c) Iron Oxide, Fe2O3 4.77 3.10

(d) Total Calcium Oxide, CaO 62.48 47.84

(e) Aluminium Oxide, Al2O3 5.65 13.89

(f) Potassium Oxide, K2O 0.43 0.46

(g) Sodium Oxide, Na2O 0.20 0.30

(h) Magnesia, MgO 0.8 1.11

(i) Sulphur calculated as sulphuric anhydride,

SO3 2.48 2.54

(j) Loss of Ignition 1.71 1.64

Specific surface,m2/kg 333 362

3. Compressive Strength (IS:4031)

(a) 3 days, MPa 32.7 29.0



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Table 1 (b): Physical and Chemical Properties of Fly Ash

The mix proportions including water-cement ratio (w/c) were selected based on exposure

conditions stipulated in IS: 456 and guidelines for mix design of ACI Committee report 211 [ACI

211, 2005]. Based on the several trial mixes produced in the laboratory with the available materials

strength and workability performance of these mixes, the proportions for OPC control mixture

(OPCC) of M25 grade was finalised with water-cement ratios (w/c) of 0.55. The same water-cement

ratios and mixture proportions were adopted for PPCC mixtures also. As most of the durability

characteristics of concretes in general are more governed by water-cement ratio (or water-binder

ratio) rather than by the strength, the selection of same water-binder ratios for OPCC and PPCC can

be considered as rational. However, suitable modifications were made in the water-binder ratios

(b) 7 days, MPa 36.3 35.5

(c) 28 days, MPa 52.3 57.3

4. Setting Time (IS:4031)

(a) Initial 2hrs. 20min. 1hr. 20min.

(b) Final 3hrs. 45min. 3hrs. 45min.

5. Soundness

(a) Expansion after boiling 3 hr in Lechatlier’s

Mould 0.5 0.5

(b) Expansion after Autoclave Test 0.02 Nil

Drying Shrinkage, percent - 0.07

7. Water required for standard consistency,

percent 26% 30.5%

8. Temperature during the period of testing 28°C 28°C

9. Relative humidity during the period of test 64% 64%

S.No.

NATURE OF TESTS Results Obtained

1. Chemical analysis Percent by mass

1.1 Silicon dioxide, (SiO2) + Aluminium Oxide,(Al2O3) +Iron Oxide,

(Fe2O3) 94.84

1.2 Silicon dioxide, (SiO2) 60.39

1.3 Magnesium Oxide, MgO 1.85

1.4 Sulphur calculated as sulphuric anhydride, (SO3) 0.27

1.5 Sodium Oxide, (Na2O) 0.18

1.6 Loss of Ignition 1.12

1.7 Aluminium Oxide,(Al2O3) 33.71

1.8 Iron Oxide, (Fe2O3) 0.74

1.9 Calcium Oxide, (CaO) 1.06

1.10 Potassium Oxide, (K2O) 0.12

2. Fineness

Specific surface,m2/kg 300

3. Test for Lime Reactivity

Average compressive strength, N/mm2 4.72

4. Test for Drying Shrinkage

Average Drying Shrinkage, percent

0.02

5. Test for Soundness

Soundness by Autoclave expansion, percent

0.01



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(w/b) and mix proportions in case of FACC mixture taking into account their cementing efficiency at

28 days and their lower specific gravity in order to achieve required target strengths and workability.

Details of the mixture proportions are presented in Tables 2 (a) and 2 (b).

Table 2: Mixture Proportions for M25 Grade Concrete

B = Binder = Cement + Fly Ash = C + FA

Table 3: Strength Test Results

3.0 DETAILS OF EXPERIMENTAL INVESTIGATIONS

3.1 Strengths

The test results given in Table 4 indicate that there is a negligible increase in compressive

strength between 90 and 180 days and also between 180 and 360 days, as compared to that between

Materials (kg/m3)

OPCC, PPCC FACC

Mix Id

OA, P1A OAF25

Binder (B) 360 396

Cement (C) 360 297

Fly Ash (FA) - 99

Sand (S) 648 581

Coarse aggregate (CA) 1152 1152

Water (W) 198 198

w/b =W/B 0.55 0.50

Proportions (B:S:CA:W) 1:1.8:3.2:0.55 1:1.47:2.91:0.50

Mix ID OA OAF25 P1A

w/b 0.55 0.5 0.55

Grade M25 M25 M25

Age (days) Compressive strength, MPa

3 18 18 26

7 23 25 31

28 32 33 42

56 41 41 49

90 42 46 51

180 42 48 53

360 43 52 54

Flexural strength, MPa

28 5.1 5.1 5.8

90 5.9 6.5 6.4

180 5.9 6.8 6.6

360 5.9 7.9 7.3

Split tensile strength, MPa

28 2.9 2.9 2.6

90 3 3.3 3.3

180 2.9 3.5 3.5

360 3 3.6 3.5



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28 and 90 days and as expected the rate of increase in strength at longer age is very less, especially

for OPCC specimens. FACC specimens registered slightly higher increase (Figures 1).

Figure 1: Compressive Strength Development of Concrete Specimens with Age

OPCC specimens had almost reached their maximum strength by the end of 90 days where as

FACC specimens required a longer period of about 180 days to reach their maximum strengths. It

may be noted here that after 28 days of water curing, no external curing by application of moisture or

water was undertaken. The specimens gained their strength due to the water available within the

matrix after the specimens were taken out of water at the end of 28 days of curing. After the

specimens are exposed to ambient conditions, there is a gradual loss of water from the specimens and

there could be a limit on the age up to which there is a continuous development of strength. In the

present study, it was observed that the refinement of the concrete matrix due to the presence of fly

ash slows down the drying process due to the tortuosity of the pores and thereby allows the strength

development to proceed for longer duration than in the concretes without fly ash.

The test results indicate that the required 28-d compressive strengths were achieved for both

OPCC and FACC mixes. This is evident from the strengths of 32 to 34 MPa for M25 grade

concretes. However in case of PPCC, in spite of using the maximum permissible water-cement ratio

of 0.55 as per IS 456-2000, 28-d strength of 42 MPa was obtained and this strength could be taken as

corresponding to M35 grade concrete.

The test data shows that it is possible to achieve the required target strength at 28 day in case

of M25 grade of concrete containing 25% fly ash. However, it is seen that for a given grade of

concrete, the lower water-binder ratio should be used for FACC. Thus, at similar strength levels,

OPCC would have comparatively higher water-cementitious material ratio than the corresponding

FACC. In case of PPCC, the water-cement ratio to be adopted for given grade of concrete depends

upon the nature of the PPC being considered.

The compressive strengths at 360 day were marginally more than that at 180 day. The

increase in compressive strength at 180 day relative to that 28 day was in the range of 10 % to 45%

and this range increased to 24% to 65% at 360 days, depending upon the type of mix

3.2 Accelerated sulphate attack on concrete specimens

In the present study, a combination of Na2SO4 and MgSO4 (each 5% solution) was used for

investigating the response of test specimens. Since, there is no accepted duration of wetting and

drying cycles, it was decided to expose the specimens continuously to sulphate solution. After 56



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days of curing, the specimens were immersed in combined sulphate solution. The pH of solution was

maintained in the range of 8.1 to 8.5. All the concrete test specimens were exposed continuously to

sodium sulphate and magnesium sulphate solution (concentration of each salt was 5% by mass) after

56 days of curing. The physical deterioration of the specimens, pH of the solution and possible

changes in visual nature of specimens were monitored at regular intervals of time. The concentration

of the solutions was adjusted once in a month. Photo1 shows a view of specimens exposed to

sulphate attack. The compressive strengths of the test specimens after desired period exposure were

determined to know the deteriorating effects of sulphates on concrete (Table 4).

Photo 1: A View of specimens exposed to sulphate attack

Up to 180 days of exposure to sulphate solution, there was no perceptible (visible) physical

deterioration. During combined sulphate attack, the changes in mass of test specimens were less than

about 6% for all the mixtures investigated indicating only minor effect of sulphate exposure on

external appearance of test specimens. With regards to compressive strength, following observations

can be made:

After 180 days of exposure, the OPCC specimen (mix OA) had about 20% reduction in

compressive strength and in case of concrete mixes containing fly ash (both PPCC and FACC), the

change of compressive strength was in the range of –2% to +5% (Table 4, Figure 2). The changes in

concretes containing fly ash were almost negligible as this variation are well within practically

acceptable limits for any concrete.

Figure 2: %Change in Compressive Strength after Sulphate Attack



324

After 360 days of exposure, the following points were observed:

• The OPCC specimen (mix OA) had about 23% reduction in compressive strength and in case

of concrete mixes containing fly ash (both PPCC and FACC), the change of compressive

strength was in the range of –7% to -34% (Table 4, Figure 2).

• The changes in concrete containing fly ash are comparable to those of OPCC. Up to 25% fly

ash content, the loss in compressive strength is less that of OPCC. The PPCC (P1A) has

shown higher compressive strength loss.

In case of sodium sulphate attack, ettringite may form due to sulphate and C3A present in the

cement. This leads to expansion, and cracking which enables easy penetration of solution into the

concrete and leads to further deterioration. In the present case, negligible expansion and cracking

were observed indicating that all the concretes considered had high resistance to ettringite formation.

C3A + sulphate + water � C3A 3CS·H32 (Ettringite)

However, sulphate attack on CH and CSH portions of cement, involves a different mechanism which

may cause expansion and spalling. However, its most important feature is the loss of strength and

adhesion of the cement paste due to decalcification of C-S-H which is primarily responsible for the

binding capacity of the cement paste.

Concrete forming gypsum under sulphate attack shows whitish patches on the surface and

perceptible loss of strength (Photo 2). However, fly ash concretes could resist gypsum formation (or

gypsum corrosion) better due reduction in free lime content of hydrated matrix.

OA P1 A

Photo 2: View of specimens deteriorated due to sulphate attack

The attack on C-S-H by magnesium sulphate (Mg SO4) is not directly related to ettringite

formation. In this type of attack, without the ettringite formation, there is loss of strength and

adhesion of the cement paste due to decalcification of C-S-H The formation of brucite, which is not

readily soluble worsens the situation. This process is suspected to have set in the present case as the

sulphate penetration and build up within the concrete mass would have reached the level required

(more than 1000 ppm) for the process of silica gel formation to initiate. Since fly ash concretes with

higher fly ash content, which have lower free lime content, have shown higher loss in strength, it is

evident that the deterioration should have been more by reaction of magnesium sulphate within C-S-

H rather than with lime and calcium aluminate fractions.

From the test results, it is seen that, performance of PPCC could be lower than that of FACC

and the fly ash content of PPC seem to be critical in determining its resistance to sulphate attack.



325

Table 4: Compressive Strength after Exposure to Sulphate Solution of Magnesium and Sodium

* Strength before start of exposure to sulphate solutions

Based on the test results available, the following observations are made:

I Compressive strength of concrete mixtures recorded only about 0 to 2 MPa growths between

the ages 90 and 180 days. During this period, the split tensile and flexural tensile strengths

showed marginal increase. Thus, there was only insignificant increase in strengths after 90

days.

II Exposure of specimens to accelerated sulphate environment has shown that at 180 days, OPCC

specimens had shown marginal reduction in compressive strength while FACC and PPCC

specimens have not shown any apparent signs of deterioration. But, after 360 days of exposure,

the fly ash based concretes seem to loose higher percentage of strength. Thus, it would become

necessary to determine or know the content of fly ash in manufacturing PPC to know its level

of sulphate resistance, since, a minimum amount Portland cement portion of PPC or any fly ash

based concrete is important from sulphate resistance considerations.

III The test data show that the measurable significant deterioration generating effect of sulphate

exposure can be obtained very long period of accelerated test duration, say about a year and

more.

IV Even though parameters such as low w/b ratio, low C3A content, blending of cement with

pozzolana (such as fly ash), are generally useful to reduce the sulphate attack, their explicit

individual parameter alone cannot ensure sulphate resistance. In the present study, between 180

days and 360 days of exposure,

a) At w/c ratio of 0.55, OPCC performed better than PPCC (presence of fly ash in PPCC

could not contribute towards improved performance)

b) In spite of lower w/b ratio of 0.50 in FAC, OPCC with higher w/c ratio of 0.55 was

better

c) Fly ash containing mixes, PPCC and FAC, had different resistances to sulphate attack

V Literature study shows that as the type and degree of sulphate attack depends upon the type of

cation in the sulphate (such as calcium, sodium, magnesium, ammonium, etc), it is necessary to

identify different strategies for reducing ill-effects of sulphate and low C3A alone not ensure

sulphate resistance of concrete.

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Mix IDs

Strength after exposure % Change in strength

0* 90d 180d 360d 90d 180d 360d 0d&90d 90d &180d 180d & 360d

OA 41 36 33 31 -12 -20 -23 -12 -7 -5

OAF25 41 45 43 35 10 5 -15 10 -5 -10

P1A 49 51 48 32 4 -2 -34 4 -6 -33



326

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Volume 4, Issue 1, 2013, pp. 165 - 181, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

21. Behrouz Mohebimoghaddam and S.Hossein Dianat, “Evaluation of the Corrosion and Strength of

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(IJCIET), Volume 3, Issue 2, 2012, pp. 198 - 206, ISSN Print: 0976 – 6308, ISSN Online:

0976 – 6316.