Varying fly ash and slag contents in Portland limestone cement mortars exposed to external sulfates

• Addition of SCMs improved mortar’s resistance to external sulfate attack.
• The beneficial effects of SCMs are enhanced at higher temperatures.
• With SCMs moderate and high-C3A cements were used to create sulfate-resisting mortar.
• Interground limestone content had little to no effect on sulfate resistance.
• Ettringite, gypsum, and thaumasite were found in all mortar bars in 5 °C Na2SO4.

The partial replacement of Portland cement with supplementary cementing materials (SCMs) is known to improve concrete’s resistance to external sulfate attack. However, an increase in thaumasite formation has been observed in Portland cements with interground limestone. The effects of varying SCM and limestone replacement levels on the severity of external sulfate attack in moderate-C3A and high-C3A Portland cements in 5 °C and 23 °C Na2SO4 solution are studied. The partial replacement of Portland cement with SCMs greatly enhances the resistance to external sulfate attack, including thaumasite sulfate attack. The rate of deterioration is strongly correlated to SCM content, C3A content, and temperature.