Factors Affecting Kinetics and Gel Composition of Alkali–Silica Reaction in Alkali-Activated Slag Mortars

Abstract

Alkali activation of metallurgical slags produces low-carbon cementitious binders for sustainable concrete production. However, the potential deleterious reaction of internal alkalis in alkali-activated slag (AAS) with siliceous aggregates is a serious durability concern. In this work, the alkali–silica reaction (ASR) in alkali-activated ground granulated blast furnace slag mortars with various activator dosages and types, aggregate reactivity, as well as incorporation of pulverized fly ash or silica fume is studied. The ASR-induced length expansion and microstructural damage in AAS mortars are studied by a modified accelerated mortar bar test and scanning electron microscopy with X-ray energy-dispersive spectroscopy. The results show that the kinetics of ASR-induced damage, as well as the gel composition and spatial distribution, in AAS mortars is highly dependent on the alkali dosage, activator type, and aggregate reactivity. The ASR-induced length expansion is approximately proportional to the volumetric fraction of dissolved glassy aggregates. However, AAS mortars show noticeable autogenous shrinkage at the initial stage of immersion, making the conventional judgment of ASR risks using length expansion as the sole indicator invalid. The dissolution kinetics of aggregate, which is influenced by the level of alkalinity and availability of reactive silica, seems to be the rate-limiting reaction of ASR-induced damage in AAS mortars under accelerated testing conditions.