Mechanisms of Alkali-Silica Reaction in Alkali-Activated High-Volume Fly Ash Mortars

Alkali-activation of high-volume fly ash (HVFA) is a viable approach to produce durable cementitious binders with faster and stronger strength development than its water-activated counterparts. However, the use of alkaline activator increases the risk of alkali-silica reaction (ASR) in these systems. In this work, the compressive strength and ASR susceptibility of alkali-activated fly ash-OPC mortars containing reactive aggregate are studied. The results show that in comparison to plain water-activated fly ash-OPC mixture, the alkali incorporation at a low concentration improves strength development only when the fly ash replacement ratio is higher than about 80%; however, excessive alkali has an adverse influence. Regardless of activator type and dosage, alkali-activated fly ash-OPC mortars are ASR innocent as assessed in the accelerated mortar bar test and scanning electron microscopic analysis, provided that the fly ash percentage higher than about 40%. The mechanism for the insignificant ASR expansion and damage in alkali-activated HVFA mortars is likely attributed to the low calcium content that prevents gelation of deleterious and expansive ASR products.