Effect of calcium hydroxide on the alkali-silica reaction of alkali-activated slag mortars activated by sodium hydroxide

Alkali-silica reaction (ASR) is one of the major durability issues in ordinary Portland cement (OPC) concrete. Previous literature suggested that the alkali-activated slag (AAS) concrete is less susceptible to ASR compared to OPC concrete. This study investigated the effect of calcium hydroxide on the ASR expansion of AAS mortars with the aim to understand the mechanism governing the ASR resistance of the AAS mortars. The ASR expansions of AAS mortars containing different amount of calcium hydroxide were compared with OPC mortars. The investigations on the ASR products, pore solution and the binder were conducted. The results show that the control AAS mortar exhibited innocuous ASR expansion despite of its high pore solution alkalinity. Addition of calcium hydroxide substantially increased the ASR expansion of the AAS mortars and the expansion increment was almost linearly proportional to the amount of calcium hydroxide addition. The results suggest that the deficiency of calcium hydroxide is one controlling factor for the low ASR expansion of the AAS mortars. No obvious difference was observed in the ASR products in all the mortars in terms of morphology and composition, implying that the additional calcium hydroxide may increase the amount of the ASR products in the AAS mortars, as the presence of calcium could facilitate the gelation and formation of the ASR products.