PCA R&D Serial No. 2916a

Sulfate Resistance of Concrete Using Blended Cements or Supplementary

Cementitious Materials

by Javed I. Bhatty and Peter C. Taylor

©Portland Cement Association 2006 All rights reserved

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KEYWORDS Sulfate attack, mechanism, sulfate resistance, supplementary cementitious materials (SCMs), fly ash, ground granulated blast furnace slag (GGBFS), silica fume, blended cements, interground SCMs, cement chemistry, SCMs chemistry, reactivity, sulfate resistant models, sulfate attack prediction, R factor, calcium aluminate potential factor (CAP), oxide durability factor (ODF), sulfate optimization ABSTRACT This report briefly discusses the mechanism of sulfate attack and the role of selected supplementary cementitious materials (SCMs) in reducing this attack in concrete. The relationship between sulfate resistance and the chemical, physical, and mineralogical composition of SCMs has been elucidated. Based on a number of bench scale studies several models predicting sulfate resistance in fly ash-containing concretes have been cited and discussed. These include the derivations of 1) R factor related to active lime/iron oxide ratio, whereby low R factor values (< 3.0) favor sulfate resistance, 2) calcium aluminate potential factor (CAP) based on (lime + alumina + iron)/silica ratio and its direct relationship with calculated sulfate equivalent (CSE) to predict sulfate resistance, and 3) oxide durability factor (ODF) as a function of (lime • free lime)/(silica + alumina + iron) ratio, wherein a decrease in ODF corresponds to an increase in sulfate resistance.

Although low C3A cement (Type V) is preferred in producing sulfate resistant concrete, replacing cement with ground granulated blast furnace slag (GGBFS) in large amounts (> 50%) has shown good sulfate resistance even with medium C3A cements. Similarly, the use of 7% silica fume with Type I cement also exhibited improved sulfate resistance compared with plain Type I cement.

The report also discusses sulfate resistance of concrete that contained SCMs interground and optimized at the cement plant as compared to mixed at the concrete batch plant. From the very limited data available on the subject, use of interground SCMs with clinker has shown improved sulfate resistance for concrete, primarily attributed to finer and better particle size distribution that enhances the reactivity and reduces the permeability in concrete. The report also cites that the optimization of sulfate should be based on 3-day strength instead of 1-day strength as referred to by ASTM C 563. This may require a higher sulfate addition that can potentially improve sulfate resistance, as the porosity of the system would markedly reduce. REFERENCE Bhatty, Javed I. and Taylor, Peter C., Sulfate Resistance of Concrete Using Blended Cements or Supplementary Cementitious Materials, R&D Serial No. 2916a, Portland Cement Association, Skokie, Illinois, USA, 2006, 21 pages.

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Sulfate Resistance of Concrete Using Blended Cements or Supplementary

Cementitious Materials

By Javed I. Bhatty and Peter C. Taylor*

BACKGROUND The degradation of cementitious materials due to sulfate attack has been recognized since before the advent of portland cement (Bellport 1993). In the mid-18th century, Smeaton of Great Britain worked on the development of improved mortars for the construction of Eddystone Lighthouse. In the late 19th and early 20th centuries, Candlot (1890), Le Chatelier (1905), and Michaelis (1909) studied sulfate attack on concretes containing portland cement. In North America, sulfate attack on concrete was investigated in the early 20th century, notably by Bates, et al. (1913). Since then, extensive studies have been made to identify the cause and develop methods to prevent sulfate attack in concrete.

For new concrete infrastructures to be durable, guidelines on material selection, testing, and construction practice need to be developed (Ahn 2005). In order for a concrete to be resistant to sulfate attack in severe environments, cement with low C3A should be used in mixtures with low permeability. The literature has also shown that low permeability concrete can be produced by using supplementary cementitious materials (SCMs) such as fly ash, blast furnace slag, silica fume, metakaolin, or other natural pozzolans. This report discusses the mitigation effect on sulfate attack when concrete is produced with added SCMs.

Also discussed in this report is the effect on sulfate resistance of concrete using blended cements containing SCMs through intergrinding, versus addition at the concrete batch plant. The perception is that, because of sulfate optimization at the cement plant, concretes containing such cements are more resistant to sulfate attack than concrete batch plant additions. Furthermore, cements interground with SCMs may exhibit a better particle size distribution for enhancing reactivity and reducing permeability in concrete compared to those produced by blending at the concrete batch plant. SULFATE ATTACK Sulfates may be present in groundwater, and are often of natural origin, but can also come from fertilizers and industrial effluents (Neville 1997). Attack on concrete by such materials is a culmination of a series of reactions that occur in the presence of sulfate ions. Sulfate attack manifests itself in the form of loss in strength, expansion, surface spalling, mass loss, and eventually disintegration (Taylor 1997, Tikalsky and Carrasquillo 1989).

* Senior Scientist jbhattey@ctlgroup.com and Principal Engineer/Group Manager ptaylor@ctlgroup.com, CTLGroup, 5400 Old Orchard Road, Skokie, Illinois, USA, 847-965-7500.

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MECHANISM OF SULFATE ATTACK Sulfate attack is often discussed in terms of reactions between solid hydration products in hardened cement paste (such as calcium hydroxide, Ca(OH)2, and calcium aluminate hydrate, 4CaO·Al2O3·13H2O) and dissolved compounds such as sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), and calcium sulfate (CaSO4). Their reactions with the solid phases in hardened cement paste are as follows: Sodium Sulfate (Na2SO4) Sodium sulfate solution reacts with calcium hydroxide to form gypsum and Na(OH):

Na2SO4 + Ca(OH)2 + 2 H2O → CaSO4·2H2O + 2 Na(OH) Sodium sulfate also reacts with calcium aluminate hydrate (4CaO·Al2O3·13H2O) and results in the formation of ettringite:

6 Na2SO4 + 3 (4CaO·Al2O3·13H2O) + 34 H2O → 2 (3CaO·Al2O3·3CaSO4·32H2O) + 12 NaOH +2 Al(OH)3

Calcium Sulfate (CaSO4) In aqueous conditions, calcium sulfate reacts with calcium aluminate hydrate (4CaO·Al2O3·13H2O) to form ettringite (Bensted 1983):

3 CaSO4 + 4CaO·Al2O3·13H2O + 20 H2O → 3CaO·Al2O3·3CaSO4·32H2O + Ca(OH)2 When the supply of calcium sulfate becomes insufficient to form additional ettringite, calcium aluminate hydrate (4CaO·Al2O3·13H2O) reacts with ettringite already produced to form monosulfate (Bensted 1983):

3CaO·Al2O3·3CaSO4·32H2O + 2 (4CaO·Al2O3·13H2O) → 3 (3CaO·Al2O3·CaSO4·12H2O) + 2 Ca(OH)2 + 20 H2O

Magnesium Sulfate (MgSO4) Magnesium sulfate attacks calcium silicate hydrate and Ca(OH)2 to form gypsum:

MgSO4 + Ca(OH)2 + 2 H2O → CaSO4·2H2O + Mg(OH)2

3 MgSO4 + 3CaO·2SiO2·3H2O → 3 CaSO4·2H2O + 3 Mg(OH)2 + 2 SiO2·H2O

Magnesium sulfate also reacts with calcium aluminate hydrate to form ettringite: 3 MgSO4 + 4CaO·Al2O3·13H2O + 2 Ca(OH)2 + 20 H2O →

3CaO·Al2O3·3CaSO4·32H2O + 3 Mg(OH)2

These reactions (to form gypsum and ettringite) are expansive in nature; therefore they exert internal pressure in hardened concrete and eventually cause deterioration. It may also be noted that the severity of sulfate attack depends on the concentration of sulfate solution, and the

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rate at which the sulfate ions are replenished. If the concrete is exposed to sulfate bearing water that is flowing rather than stagnant, it will undergo a higher rate of attack. MITIGATION OF SULFATE ATTACK Both calcium hydroxide (CaO·H2O or most commonly Ca(OH)2) and calcium aluminate hydrate (4CaO·Al2O3·13H2O) involved in sulfate reaction are the major hydration products in cement hydration. Calcium hydroxide forms as a result of C3S and C2S hydration∗, and calcium aluminate hydrate is the principal product of C3A hydration. Therefore, any approach that prevents the formation of Ca(OH)2 and 4CaO·Al2O3·13H2O should improve resistance to sulfate attack.

As described by Neville (1997), three approaches to mitigate sulfate attack are usually recommended. One is to use cement with low C3A content, the source of calcium aluminate hydrates. ASTM C 150 addresses the need for sulfate resistance in cement by limiting C3A content. Another is to reduce the Ca(OH)2 in the hydrated cement paste by using cements that contain SCMs. The role of SCMs is to consume Ca(OH)2 in the pozzolanic reaction and to dilute the C3A content of the system. SCMs with low lime contents also help mitigate sulfate attack by reducing the alumina content of the mixture. In addition, concrete be made as dense as possible in order to prevent the ingress of sulfate solutions. A combination of SCMs and low water-cementitious materials ratio (to reduce permeability) is regarded as the most useful means of increasing resistance to sulfate attack. EFFECTS OF SCMs The positive effects of SCMs on the resistance of concrete to sulfate are reflected in the requirements of ACI 201, “Guide to Durable Concrete.” The use of fly ash and blast furnace slag in making sulfate-resisting concrete has frequently been reported. Slag and other SCMs with low lime contents reportedly mitigate sulfate attack by reducing alumina levels in the mixture. Conversely, a high lime Class C fly ash may decrease sulfate resistance (Mather 1981, Tikalsky and Carrasquillo 1992). However, the sulfate resistance of mixtures containing SCMs is dependent upon the degree to which concrete is cured, as these materials may hydrate more slowly and require extended curing to achieve sufficient impermeability. The majority of concrete placed in the United States now contains at least one SCM. Ternary concrete mixtures are often used to produce high performance concrete. Many SCMs result in greater later-age strength and lower permeability, and consequently, improve durability and resistance to sulfate attack.

Ahn (2005) has identified “permeability reduction” as a major parameter for controlling sulfate attack in concrete. This parameter may be achieved by:

1) Reducing the water-cementitious materials ratio 2) Selecting proper material and mix proportions 3) Replacing cement with mineral admixtures (SCMs) 4) Adequately consolidating concrete 5) Providing adequate curing

∗ C3S, C2S, C3A and C4AF are cement chemists' notation rather than the oxide notation used up to this point. Accordingly C=CaO, S=SiO2, A=Al2O3, F=Fe2O3; S¯ =SO3, and H=H2O. Also Ca(OH)2 or CaO·H2O in this paragraph could be CH for consistency.

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FLY ASH The role of fly ash in controlling sulfate attack in concrete has been widely examined. Of the chemical components, silica, lime, alumina, and iron oxide are considered the most critical. The effectiveness of different fly ashes may vary significantly because of the large variation in these compounds from source to source. Generally, low lime fly ashes show improved sulfate resistance because they consume the available Ca(OH)2 in the hydrated cement paste, while lime rich materials can hydrate independently and produce their own Ca(OH)2, thus increasing exposure to sulfate attack. In addition, low lime materials form proportionally more C-S-H than high lime materials, thus increasing the strength and reducing permeability, and increasing sulfate resistance.

Tikalsky and Carrasquillo (1989) referred to noteworthy studies by Dikeou (1970), Dunstan (1976, 1980, 1984, 1987), Kalousek (1972, 1976), Rosner (1982), Hartmann and Mangotich (1987), Mehta (1986), and Manz et al. (1987).

As a result of a 27-year-long study on the sulfate resistance of concrete containing fly ash, Dikeou (1970) concluded that fly ashes improved sulfate resistance of concrete regardless of fly ash type or the cement used. Generally, the degree of resistance to sulfate was in the following order (greatest to least), based on the cementitious material used:

Type V Cement + Fly Ash Type II Cement + Fly Ash Type V Cement Type II Cement Type I Cement + Fly Ash Type I Cement

According to Dunstan (1976, 1980, 1984, 1987), concretes containing low-calcium fly

ash are more resistant to sulfate attack than those containing high-calcium fly ash or no fly ash. Class C fly ashes produced from lignite or subbituminous coals typically contain calcium-rich glass and crystalline gehlenite phases. Low-calcium fly ashes produced from anthracite or bituminous coal are generally composed largely of mullite phases. Fly ashes having gehlenite-like composition are reactive by nature, and hence are more prone to sulfate attack than mullite containing fly ashes.

Based on his data, Dunstan (1976, 1980, 1984, 1987) proposed a method of predicting sulfate resistance of concrete containing fly ash. The method computes factor R by using CaO and Fe2O3 contents in the ash by the flowing equation:

(%)OFeCaOR

32

5(%) −=

Dunstan (1976) predicted the level of sulfate resistance for concrete containing fly ash by

the R factor as given in Table 1.

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Table 1. Relationship Between R Factor and Sulfate Resistance

Limits of R Factor Sulfate Resistance < 0.75 Greatly improved 0.75 to 1.5 Moderately improved 1.5 to 3.0 No significant change > 3.0 Resistance reduced

However, Dunstan (1987) reported that the limit 3.0 for R was a subjective value proposed in 1980 from sulfate expansion data of 25% fly ash-containing concrete subjected to the 10% sodium sulfate soak test. He suggested that a limit of 3.35 for R is more realistic and 3.70 may even be acceptable. Concrete with higher R factor values will have reduced sulfate resistance, whereas lower values will show improved sulfate resistance.

Rosner, et al. (1982) examined the usefulness of Dunstan’s R factor for four fly ashes with R factors of 0.18, 0.29, 2.71, and 5.30. Based on the expansion data, three Class F ashes with low R factors gave excellent sulfate resistance whereas Class C fly ash with R factor = 5.30 showed only marginal sulfate resistance when compared to the control cement. Contrary to Dunstan’s prediction, two fly ashes with the highest R factors also performed adequately.

Mehta (1986), however, countered that Dunstan’s R factor prediction was flawed because it did not take into account the crystalline form of reactive alumina in fly ash. Mehta’s studies included several Class C fly ashes replacing 25% and 40% Type I cement having 11% C3A. The data showed that despite having an R factor of 4.0, some ashes showed improved sulfate resistance, whereas a number of ashes with R between 1.2 and 4.5 reduced the sulfate resistance. Mehta (1986) concluded that sulfate resistance depends upon:

1) The stable hydrated aluminate phase when concrete is exposed to sulfate environments 2) The presence of calcium aluminosulfate or calcium aluminate hydrate makes concrete

susceptible to sulfate attack 3) The presence of reactive Al2O3/Fe2O3 is a true measure of aluminate reaction to sulfate

exposure

Manz et al. (1987) also concluded that with a given fly ash composition, R factor was inadequate to predict sulfate resistance. Their data was based on a four-year study on fly ash containing concretes with R factors ranging from 1.6 to 4.1, which showed minimal expansion when exposed to sulfate environments. Manz et al. proposed revised prediction parameters based on the reactive components in the fly ash. The first parameter is calcium aluminate potential (CAP), which regards the glassy calcium, aluminate, and iron as detrimental to sulfate resistance, and glassy silica as beneficial to sulfate resistance. CAP factor subtracts the inert crystalline compounds and the crystalline compounds that do not take part in sulfate expansion reaction. CAP is expressed as below:

CAP = (C* + A* + F*) / S* Where,

C* = Bulk CaO – Reactive crystalline CaO (lime, anhydrite, C2S) – inert crystalline CaO (melillite, merwinite)

A* = Bulk Al2O3 – inert Al2O3 (mullite) F* = Bulk Fe2O3 – inert crystalline Al2O3 (hematite, spinel) S* = Bulk SiO2 – inert crystalline SiO2 (quartz, mullite)

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The second parameter is a calculated sulfate equivalent (CSE). This factor assumes that S¯ (or SO3) in fly ash is desirable for forming early ettringite so that Ca(OH)2 in the cement paste is reduced. CSE is expressed as follows:

CSE = Anhydrite + 1.7 S¯

The factor 1.7 accounts for the formation of additional anhydrite from other sulfate sources in the fly ash. The proposed relationship between CAP and CSE in terms of sulfate resistance of concrete is shown in Figure 1.

In another study, Hartmann and Mangotich (1987) used Class C fly ashes to develop a more reliable method than Dunstan’s R factor to predict sulfate resistance in concrete. They pointed out the following limitations of the R factor approach:

1) It could not predict sulfate resistance of concrete containing cement alone 2) The factor did not account for high calcium fly ashes 3) The extent of sulfate resistance did not always match the R factor

Calculated Sulfate Equivalent, CSE

Cal

cium

Alu

min

ate

Pote

ntia

l, C

AP

Satisfactory

Unsatisfactory

Figure 1. Sulfate resistance based on CAP and CSE parameters (After Manz, et al. 1987).

Based on a two-year study on sulfate resistance of concrete containing fly ash and making use of data from Dunstan’s earlier work, Hartmann and Mangotich (1987) developed a new factor to predict sulfate resistance of concrete known as oxide durability factor (ODF):

ODF = C(%) • Free lime (%) / S(%) + A(%) + F(%) Where,

C = Bulk CaO in fly ash and cement A = Bulk Al2O3 in fly ash and cement S = Bulk SiO2 in fly ash and cement F = Bulk Fe2O3 in fly ash and cement

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The principle advantage of Hartmann and Mangotich’s (1987) approach over Dunstan’s

was that it accounted for both Al2O3 and SiO2 and recognized the importance of crystalline components of the cementitious materials (fly ash + cement) in concrete. It was claimed that the oxide durability factor (ODF) correlated well, not only with their own experimental data, but also with those from Dunstan’s studies. Typically, if there is a decrease in the value of ODF because of fly ash addition, there should be a corresponding increase in sulfate resistance of concrete.

Rasheeduzzafar, et al. (1990, 1994) also examined the resistance of Type I, Type V, and Type I cement + 20% Class F fly ash blends when subjected to various sulfate exposures such as Na2SO4 and MgSO4. They concluded that cement-fly ash blends performed better than Type I cement when exposed to Na2SO4 solution, but their performance became inferior when exposed to a combined Na2SO4–MgSO4 environment. They contended that the mechanism of sulfate attack in Na2SO4–MgSO4 exposure is intense on all cements but more so on blended cements, and the attack is predominantly controlled by the presence of magnesium.

Al-Dulaijan et al. (2003) concluded that use of fly ash with Type I cement improved sulfate resistance. Their testing included mortars made with Type I, Type V (sulfate-resistant cement), and blends with fly ash exposed to varying concentrations of sodium sulfate solutions. The sulfate resistance was evaluated by visual determination of specimen deterioration, loss in compressive strength, and a combination thereof.

Mortars made with Type I cement exhibited a significant level of physical deterioration, whereas Type I + 20% fly ash blends did not show any deterioration; Type V mortars showed only marginal deterioration (see Table 2). Increased strength reduction due to sulfate reduction was noted with mortars made with Type I cement. Strength reduction with Type V and Type I + fly ash was lowest. The strength reduction of mortars was calculated as the sulfate deterioration factor (SDF) as defined earlier by Rasheeduzzafar et al. (1994) by the following equation:

SDF (%) = (A – B) / A Where, A = Average compressive strength (MPa) of mortar cured in water B = Average compressive strength (MPa) of mortar cured in sulfate solution

Ground Granulated Blast Furnace Slag (GGBFS) Studies have shown that ground granulated blast furnace slag (GGBFS) also improves the sulfate resistance of concrete. GGBFS is glassy in nature and thus reacts with Ca(OH)2 in hydrated cement paste. Higher levels of cement replacement by slag (over 60%) appear to be more effective as, according to Wong and Poole (1988), the consumption of Ca(OH)2 becomes more pronounced.

The use of GGBFS in producing sulfate-resistant concrete has been recognized by both ACI (1991) and ASTM (1997), who reported blended cements with 60% to 65% slag as widely used in sulfate- and sea-water-resistant concretes.

Hogen and Meusel (1981) evaluated the use of slag in producing sulfate-resistant concrete. They used Type I/II, II, and V cement blends with slag at 40%, 50%, and 65% cement replacement. The sulfate resistance was significantly improved with Type II cement and to a lesser degree with Type I/II and V cements. The improvement increased with increasing slag

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addition. Also, the most improvement in sulfate resistance was recorded with Type II cement containing 6.3% C3A compared to Type I/II and Type V cements that contained 6.3% and 3.7% C3A respectively. In a separate study, Guyot et al. (1983) also confirmed that using cement with more than 70% slag imparted effective sulfate resistance to concrete. It was also noted that 15% to 20% addition of slag also provided protection from sulfate attack, provided the C3A contents of the cement were low.

Fearson (1986) examined a series of slag-blended cement mortars exposed to sodium sulfate environments. The slag contents ranged from 30% to 70% and the water-cementitious materials ratio varied between 0.45 and 0.60. It was noted that mortars made with 70% slag were superior to the sulfate resistant portland cements when specimens were compared at the same water-cementitious materials ratio. At 50% slag replacement, the mortars were similar to, or in certain cases better than, the sulfate resistant cements. Even 30% replacement of slag imparted some degree of sulfate resistance, though inferior to that of typical sulfate resistant cements. Selected specimens using 80% slag were tested at water-cementitious materials ratio = 0.60, and were found to be highly sulfate resistant.

Osborne (1991) studied several blends of cements with granulated and pelletized slags with respect to sulfate resistance and compressive strength development in concrete. The relative effects of Al2O3 in slag and C3A in cements were also investigated. It was noted that a 70% replacement by slag having low/medium Al2O3 (7.5% to 11.5%) in cement with medium C3A (9%) produced concrete with good sulfate resistant properties. In similar studies, Fearson and Higgins (1992) tested two slags with 11.5% and 15.5% Al2O3, respectively, and their composite with 13% total Al2O3. The slags were blended with a cement of 10.2% C3A. The tests showed that at a water-cement ratio of 0.60, a 70% replacement by all slags and 50% replacement by the composite slag produced sulfate resistant concretes. At a water-cement ratio of 0.45, all slags at 70% and 50% replacement, as well as 30% replacement with composite slag, produced sulfate resistant concretes.

Hemmings et al. (1995) tested North American sources of GGBF slags and confirmed their effectiveness in reducing sulfate related expansion of concrete. It was confirmed that with 50% GGBF addition to Type 10 cements (having medium to high C3A contents), sulfate expansion could be significantly reduced. In comparison, the beneficial effects were at least similar to or better than a Type 50 sulfate resistant cement. The one-year expansion data for concrete specimens made with Type 10, Type 50 sulfate resistant cements, and the cement-slag blends are shown in Figure 2. It is noteworthy that the expansion plots for cement-slag blends are well below those for Type 10 and Type 50 cements.

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Figure 2. Sulfate expansion data for mortars produced with sulfate resistant portland cements and cement blends containing slag (Hemmings, et al. 1995). Silica Fume Al-Dulaijan et al. (2003) used silica fume to enhance sulfate resistance of blended cement mortars. Again, the superior performance of the ternary system was attributed to:

1. The pozzolanic reaction by silica fume that consumes CH and forms secondary C-S-H, thus increasing overall C-S-H formation and reducing permeability

2. The dilution of C3A content due to the overall reduction of cement in concrete

Among others, cements evaluated for sulfate resistance were: Type I, Type V, and Type I + 7% silica fume. Fifty-mm cube mortar specimens were made using a water-cementitious materials ratio of 0.50. The mortars were subjected to sulfate exposures having varying concentrations of sodium sulfate for two years. Compressive strength and physical deterioration of mortars were observed to correlate with sulfate resistance.

The data suggested that both strength and deterioration was influenced by cement type, sulfate concentration, and the exposure time. Overall, Type V cement exhibited good resistance, in terms of low strength reduction and deterioration, followed by Type I + 7% silica fume, and plain Type I cement. (A Type I + 20% fly ash performed best when tested under identical conditions.) Selected data on specimen deterioration caused by sulfate exposure is given in Table 2.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 Age, weeks

Expansion, %

High C3A Type 10 cement

Low C3A sulfate resistant

Medium C3A Type 10 cement

50:50 Cement slags

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Table 2. Deterioration Rating of Mortar Specimens Caused by Sulfate Exposure

Deterioration rating in exposure time Cement

type Sulfate solution (%) 4 months 8 months 15 months 24 months

Type I 1 0 1 2 3 2 2 2 3 4 4 3 3 3 3 Type V 1 0 0 0 1 2 0 0 0 2 4 1 1 1 3 Type I + 7% silica fume 1 0 0 1 1 2 0 0 1 2 4 1 2 2 3 Type I + 20% fly ash 1 0 0 0 1 2 0 0 0 1 4 0 1 2 2

0 = no deterioration, 1, 2, 4 = increasing rate of deterioration

A ternary mix containing cement with fly ash and silica fume is relatively common in Canada. For instance, CSA Type 10E-F/SF cement is one with equivalent strength performance to Type 10 cement. This cement contains fly ash as the primary SCM and silica fume as the secondary SCM. Blending vs. Intergrinding The subject of intergrinding SCMs with clinker as opposed to separate batching to produce blended cements has been discussed only occasionally, although both processing and material advantages have been noted with intergrinding.

An early work by Davis et al. (1937) reported intergrinding of fly ash and clinker in producing cement blends. The mixtures were ground for the same length of time as required to produce portland cement of a given fineness. The effect of intergrinding was to increase the fineness of the blended cement. The interground cement containing 20% fly ash showed 8% increase in 7-day compressive strength compared with the blended cement; the one-year strength was identical however, suggesting that the effect of intergrinding was only beneficial at the early ages.

U.S. Army Corps of Engineers (1960) also reported that intergrinding fly ash, clinker, and gypsum could produce satisfactory blended cement. It was realized that the reactivity of cements containing fly ash decreased with increasing fly ash content but increased with the increasing fineness of fly ash. It was also reported that since the fly ash was harder to grind than the clinker, and since the difference in relative grindabilities appeared to increase as the percentage of fly ash decreased, it was economical to grind fly ash-blended cements in stages rather than as a single operation.

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Pandey et al. (2003) also studied the effects of SCMs on cement performance after intergrinding with clinker and gypsum. The materials tested were fly ash, limestone, calcined clay, and microsilica (silica fume). Two sets of blends were prepared. The first set used blended fly ash at 5% to 35% clinker replacement. The second set replaced between 5% and 25% of the clinker, using ternary mixtures of fly ash, calcined clay, microsilica, and limestone that were interground with the clinker in a laboratory ball mill. These blends were tested for strength and durability. Although the fineness of blends with 35% fly ash was high, their early-age mortar cube strength gain was lower than the plain cement; the strength gains at 90 days and beyond were noticeably higher. The early-age loss in strength could well be due to the dilution factor, while the strength gain at later ages could be a combination of particle fineness and the pozzolanic reactivity of the fly ash. Similar trends were observed when the cubes were cured in sulfated solutions. Pandey et al. (2003) reported that the presence of fly ash enhanced the strength when compared to plain mortar cubes cured in sulfate solution under identical conditions. The maximum gain in strength was noted with 25% fly ash, when mortars become more impervious due to the formation of additional C-S-H and the ensuing refinement of pores.

Nielsen (1980) and Osbǽck (1981) observed energy and strength advantages when fly ash was interground with clinker. Osbǽck (1981) also noted an increase in fineness, decrease in porosity and air content, and subsequent increase in density and strength when intergrinding was used to produce cements containing fly ash. The effect of intergrinding fly ash vs. separate batching on the reduction of alkali-silica reactivity has also been reported by Farbiarz et al. (1995). Their data showed that using interground blended cement with fly ash significantly reduced the alkali-silica reactivity of concrete. The improved effectiveness can be attributed to an increase in fineness of the fly ash particles, either by grinding the fly ash particles or by deflocculating large fly ash agglomerations, or a combination of both. Although the mechanisms of sulfate attack and deleterious expansion due to ASR are different, some similarities exist, such as a dependence on permeability. Therefore, this improvement in ASR-resistance might parallel an improvement in sulfate resistance.

Freeman and Carrasquillo (1995) studied the effect of intergrinding on sulfate resistance. Two concrete mixtures were produced using Type II cements and 35% Class C fly ash: one in which the fly ash was added at the time of batching and the other in which the fly ash was interground with the cement. The concrete mixes were spiked with varying amounts of soluble sodium sulfate added to the mix water, based on the hypothesis that increasing sulfate and alkali levels may improve the sulfate resistance of concrete.

Comparatively, the blended cements that contained interground fly ash were more resistant to sulfate attack. This effect is clearly evident with concrete samples using two fly ashes (see Figures 3 and 4).

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Figure 3. Sulfate susceptibility of concrete containing fly ash A added at batch plant compared with interground (Freeman and Carrasquillo 1995).

Figure 4. Sulfate susceptibility of concrete containing fly ash B added at batch plant compared with interground (Freeman and Carrasquillo 1995).

Intergrinding fly ash may have improved the ability of particles to react with sodium sulfate while the concrete was still fresh. The reactivity may also have improved due to a changed particle size distribution and improved degree of dispersion. It was also evident that optimum resistance to sulfate resistance was noted when the SO3 to C3A equivalent ratio of fly ash-cement mix was between 0.5 and 0.6.

Another important parameter that favors an improvement in sulfate resistance is the reported decrease in permeability of concrete when cements with interground fly ash are used with or without soluble sodium sulfate in the mix water (Freeman and Carrasquillo 1995). After

0

0.5

1

1.5

2

2.5

0 1 0.3 0.4 0.5 0.6 0.7 0.9 1.1

Sulfate/Aluminate Equivalent

Sulfa

te S

usce

ptib

ility

Rat

ing

Fly Ash as Admixture Fly Ash Interground

# = Na2O Equivalent

0.9 1.2

1.8

2.5

3.4

4.6

6.2Limit

Type II cement concrete

No Na2SO4 Admixture

0

0.5

1

1.5

2

2.5

0 1 0.3 0.4 0.5 0.6 0.7 0.9 1.1

Sulfate/Aluminate Equivalent

Sulfa

te S

usce

ptib

ility

Rat

ing

Fly Ash as Admixture Fly Ash Interground

# = Na2O Equivalent1.0

1.7 2.4

3.2

4.1

5.4

6.8Limit

Type II cement concrete

No Na2SO4

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two weeks of moist curing, the permeability of concrete made with interground fly ash cements without sodium sulfate admixture was equal to or higher than that of the Type II cement. However, after three months, the permeability of concrete containing cement with interground fly ash was significantly lower than that of Type II cement. Figure 5 shows ASTM C 1202 data for concrete containing interground fly ash cements. The addition of sodium sulfate admixture (4% to 5%) significantly decreased the chloride penetration (loosely related to permeability) of concrete after two weeks curing. However, after three months, the chloride penetration of concrete containing interground fly ash only was comparable to those containing sodium sulfate admixture.

Figure 5. Chloride permeability of concrete containing interground fly ash cements with or without sodium sulfate additive (Freeman and Carrasquillo 1995). Sulfate Optimization The ASTM C 563 sulfate optimization method describes how to adjust the SO3 level in cement to maximize its compressive strength. This method employs 1-day strength for mortar cubes made using 1:1 cement:sand proportions by mass. However, Hawkins (2002) has suggested that the SO3 optimization should be determined on the basis of 3-day compressive strength instead of 1-day strength. Using data from several mortars made with varying additions of SO3 levels and testing at 1, 3, and 7 days, Hawkins noted that optimum sulfate dosage increased with curing time (Figure 6).

0

2000

4000

6000

8000

10000

12000

0 20 40 60 80 100

Moist Curing, Days

Perm

eabi

lity,

Cou

lom

bs

Fly Ash A no additive

Fly Ash B no additiveType II cement

Fly Ash A w ith additive

Fly Ash B w ith additive

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Figure 6. Optimum sulfate as a function of curing time. (Hawkins 2002).

Hawkins (2002) demonstrated that SO3 optimization using 3-day strength not only improved the strength but also the sulfate resistance by reducing porosity; it also reduced the drying shrinkage. These properties are mostly attributed to the interaction of SO3 with aluminate phases in cement, which results in early-age modification of cement paste microstructure and affects expansion, shrinkage, and porosity of the system.

Hawkins (2002) emphasized the need to adopt a similar approach of SO3 optimization for cements using SCMs. Typically, ground granulated blast furnace slag also contains large proportions of alumina that reacts with SO3 to form ettringite and can directly interfere with the sulfate optimization. Using cement-slag mixtures in concrete containing 30% and 50% slag by mass and having Blaine finenesses of 430 m2/kg, 490 m2/kg, and 550 m2/kg, Hawkins (2002) again noted that for optimum SO3 determination, 3-day strength data (instead of 1-day strength) were more appropriate. Such optimization improved the compressive strength, drying shrinkage, and sulfate resistance in concrete when compared with those made with nonoptimized cement-slag mixtures. Mixtures prepared by using 3-day-optimum-sulfate contents with slag and having varying Blaine finenesses are shown in Table 3. Values of 1-day-optimum-sulfate cements are also shown for comparison.

Table 3: Test Data for Cements Prepared Using Different Optimization Ages and Slag Contents

30% Slag: Cement Mixture 50% Slag: Cement Mixture

Blaine, m2/kg 430 490 550 430 490 550

3-day Optimum SO3, % 3.98 4.08 4.08 4.39 4.55 4.85

1-day Optimum SO3, % 2.79 3.06 3.10 3.02 3.15 3.15

Hawkins (2002) concluded that if SCMs are interground with clinkers during finish milling, then optimization needs to be done on the interground cement. If SCMs are ground alone, gypsum should be interground with them at the optimum level as a mass percentage.

0

1000

2000

3000

4000

5000

0 1 2 3 4 5 6

Sulfate, %

Com

pres

sive

Str

engt

h, p

si 1-Day

3-Day

7-day

Optimum 2.75%

Optimum 3.40%

Optimum 3.80%

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SUMMARY This report briefly discusses the mechanism of sulfate attack and ways to mitigate it in concrete. The role of supplementary cementitious materials (SCMs) such as fly ash, silica fume, and ground granulated blast furnace slag in reducing sulfate attack in concrete has been addressed. The relationship between sulfate resistance and the chemical, physical, and mineralogical composition of SCMs and their proportioning have also been elucidated. Based on the bench-scale studies, computation of several factors for predicting sulfate resistance in concrete containing fly ash are discussed as a function of fly ash-cement chemistry. They are:

• R factor based on active lime/iron oxide ratio. It was noted that low R factors (< 3.0) favored sulfate resistance; therefore Class F fly ashes with low values of R factor gave better sulfate resistance compared with Class C fly ashes having high values of R factors.

• Calcium aluminate potential factor (CAP) based on (lime + alumina + iron)/silica ratio and its relation with calculated sulfate equivalent (CSE) to predict sulfate resistance. The CAP/CSE ratios lying below the correlation line show satisfactory sulfate resistance.

• Oxide durability factor (ODF) based on (lime • free lime)/(silica + alumina + iron) ratio. A decrease in ODF value corresponds to an increase in sulfate resistance.

Although the computation models are helpful in predicting sulfate attack based on the

composition of cement and SCMs present in the mix design, testing under field conditions is required to evaluate their acceptability.

The report also discusses the use of granulated ground blast furnace slag (GGBFS) in mitigating the sulfate attack. Although the use of low C3A cement (Type V) is generally preferred for sulfate resistance, replacing cement with ground granulated blast furnace slag (GGBFS) at larger amounts (> 50%) has shown good sulfate resistance even with cements of medium C3A contents. Use of 7% silica fume with Type I cement also exhibited sulfate resistance compared with plain Type I cement.

Also discussed in the report is the sulfate resistance of concrete produced by cements that contained SCMs interground and optimized at the cement plant, versus mixing at the concrete batch plant. Very limited data is available on the subject. However, interground SCMs with clinker have shown improved sulfate resistance. This could be because the interground SCMs acquire finer and better particle size distribution, which enhances the reactivity and reduces the permeability of concrete. Optimization of the sulfate content of the cementitious system is also likely to contribute to the improved performance.

The data also suggest that sulfate optimization of cement be based on 3-day strength instead of 1-day strength as used by ASTM C 563. This may require a higher dose of sulfate addition that can potentially reduce sulfate resistance, since the porosity of the system is markedly reduced.

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Dunstan, E.R., Jr., “Sulfate Resistance of Fly Ash Concrete – The R-Value,” Bryant and Katherine Mather Symposium on Concrete Durability, SP100, American Concrete Institute, pages 2027 to 2040, 1987. Farbiarz, J.; Carrasquillo, R.L.; and Snow, P.G., “Effect of Intergrinding Versus Separate Batching of Fly Ash on Alkali-Aggregate Reaction in Concrete,” in 37th IEEE Technical Conference, XXXVII Conference Record, San Juan, Puerto Rico, pages 179 to 196, 1995. Fearson, J.P.H., “Sulfate Resistance of Combinations of Portland Cement and Ground Granulated Blast Furnace Slag,” SP91-74, Madrid, American Concrete Institute, pages 1495 to 1524, 1986. Fearson J.P.H., and Higgins, D.D., “Sulfate Resistance of Mortars Containing Ground Granulated Blast Furnace Slag with Variable Alumina Content,” SP132-82, Istanbul, American Concrete Institute, pages 1105 to 1132, 1992. Freeman, R.B., and Carrasquillo, R.L., “Production of Sulfate-Resistant Concrete Containing High-Calcium Fly Ash and Sodium Sulfate Admixture,” Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Proc. 5th CANMET/ACI Int. Conf., pages 153 to 176, Milwaukee, Wisconsin, USA, 1995. Guyot, R.; Ranic, R., and Varizat, A., “Comparison of the Resistance to Sulfate Solutions and to Sea Water of Different Portland Cements With or Without Secondary Constituents,” ACI SP79-24, Proc. CANMET/1st Int. Conf. on Use of Fly Ash, Silica Fume, Slag, and Other Mineral By-Products in Concrete, Montebello, Quebec, Canada, pages 453 to 469, 1983. Hartmann, C., and Mangotich, E., “A Method of Predicting Sulfate Durability of Concrete,” presented at Bryant and Katherine Mather Symposium on Concrete Durability, SP 100, American Concrete Institute, 1987. Hawkins, P., SO3 Optimization for Ground Granulated Blast Furnace Slag, Bulk Materials International Slag Symposium, Hershey, Pennsylvania, May 2002. Hemmings, R.T.; Cornelius, B.J.; Mikols, W.J., and Luther, M.D., “New North American Sources of Ground Granulated Blast Furnace Slag for Improving the Sulfate Resistance of Concrete,” Proc. 5th CANMET/ACI Int. Conf. Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Supplementary Papers, pages 595 to 610, Milwaukee, Wisconsin, USA, 1995. Hogen, F.J. and Meusel, J.W., “Evaluation of Durability and Strength Development of Ground Granulated Blast Furnace Slag,” Cement, Concrete, and Aggregates, CCAGDP, Vol. 3, No. 1, pages 40 to 52, Summer 1981. Kalousek, G.L.; Porter, L.C., and Benton, E.G., “Concrete in Long-Term Service in Sulfate Environments,” Cement and Concrete Research, Vol. 2, pages 79 to 89, 1972.

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