Durability of ternary blends containing high calcium fly ash and slag against sodium sulphate attack

• The addition of slag was found to enhance sodium sulphate resistance of cementing blends containing high-calcium fly ash.
• Blends with 60%Slag and high-calcium fly ash showed superior performance regardless of the individual level of slag or ash.
• Enhanced resistance to sulphate attack could not be explained based solely on the reduced ion diffusion through the mix.
• The resistance to sulphate increased with reduced calcium hydroxide and dilution of Portland cement.

Cementing blends containing high-calcium fly ash (HCFA) is known to have low resistance to sulphate attack. This paper presents the results of an experimental program that focuses on attempts to enhance the performance of HCFA through the incorporation of granulated blast furnace slag in the mix. The expansions of various blends were evaluated using the mortar bar test according to ASTM C1012. Mortar bars were made with 100% Portland cement (control mix), binary blends of either HCFA or slag, and ternary blends of HCFA/slag. The expansions were measured up to two years. The addition of slag was found to improve the resistance to sulphate. When the level of combined HCFA and slag reached 60% of the total cementing materials, there was a significant reduction in the expansion regardless of the level of each of the individual material. In addition to expansion results, the paper evaluates the role of chemical and mineralogical composition of the cementing blend, the ion diffusion of the mixtures, and the calcium hydroxide consumption on the sulphate resistance of the different blends.