Influence of Supplementary Cementitious Materials on Sulfate Resistance of Mortar Mixtures

Sulfate reaction is a damaging expansive process that deteriorates cement-based structures over time. Various methods have been proposed to mitigate the effects of sulfate attack in concrete. Using Type II or Type V portland cement is an appealing approach to control sulfate attack. Additionally, incorporating supplementary cementitious materials (SCMs) such as class F fly ash, a by-product of coal combustion for electricity generation, has proven effective at mitigating sulfate attack damage. However, future availability of fly ash is uncertain due to the energy industry transitioning towards more sustainable methods of energy production rather than relying on coal combustion. Consequently, there is an incentive to seek alternative SCMs that can effectively mitigate sulfate attack while being environmentally sustainable and economically feasible. In this study, ASTM C1012, a globally recognized standard test method for sulfate evaluation, was employed to assess sulfate resistance of mortar specimens. In total, 14 mortar mixtures containing various types and concentrations of alternative SCMs, including silica fume, metakaolin, and pumicite along with two types of portland cement (Type I and Type I/II) were produced. The results indicated that Type I/II portland cement had greater sulfate resistance compared to Type I cement in mortar mixtures, regardless of the type and concentration of SCMs used. Additionally, although metakaolin considerably improved sulfate resistance, silica fume and pumicite used in this study had only limited impact on sulfate resistance of the specimens. When evaluating ternary mixtures, using a combination of 22.5% metakaolin and 7.5% fly ash to replace 30% of Type I portland cement resulted in the greatest sulfate resistance among all mortar mixtures, with 0.054% expansion after nine months of testing. It is worth mentioning that when using Type I/II portland cement and only 15% metakaolin (as a cement replacement), sulfate resistance was comparable to the ternary mixture with 22.5% metakaolin, 7.5% fly ash, and with Type I portland cement. Overall, the results showed that metakaolin, fly ash, and pumicite can be considered effective SCMs for improving sulfate resistance.