Mitigating alkali-silica reaction induced concrete degradation through cement substitution by metakaolin and bentonite

Abstract

This paper reports the results of an investigation on the efficiency of cement modification with clay minerals in preventing deteriorations of concrete due to alkali-aggregate reactions. Coupled substitution of metakaolin and bentonite in Portland cement was studied for a synergistic effect to mitigate alkali-silica reactivity (ASR). The expansion and cracking behavior of mortar bars were investigated. Hydration products of cement blends in two environments, immersion in saturated lime water and “mortar bar test” environment (i.e., in 1 N sodium hydroxide solution at 80 °C), were characterized. The role of the clay-based minerals in mitigating ASR was also evaluated by monitoring the strength development and characterizing the microstructural defects of the cement blends. The results indicate that cement modification with a combination of metakaolin and bentonite can effectively mitigate ASR-induced concrete deteriorations. By replacing 10% and 30% cement, expansion of mortar was decreased from deleterious to innocuous level. With 50% cement substitution, no ASR expansion was observed, and accordingly, no crack could be detected. The incorporated aluminum originating from the minerals slowed down the SiO2 dissolution, and formation of gel and its swelling potential. In addition, the decreased alkalinity and consumption of calcium in the modified cement matrix also contributed to the suppressed ASR. This paper demonstrates a low-cost approach to mitigating ASR; and metakaolin, bentonite, or both can effectively replace other supplementary cementitious materials used for the same purpose (e.g., fly ash) which may not be available or cost effective.