The Distribution, Structure, and Chemical Composition of Alkali-Silica Gels in Calcined Clay Concretes

This study investigates the effect of calcined clays (metakaolin, metasilt, metaclay) on the chemical composition, distribution, and structure of alkali–silica reaction (ASR) gels. Using 10 wt% of calcined clays reduced concrete expansion and minimized cracking but did not inhibit ASR gel formation. Micro X-ray fluorescence mapping revealed an average ASR gel content of 3 wt% in concrete, incorporating up to two-thirds of K₂O and nearly all Na₂O from the binder. Raman spectroscopy indicated structural similarities among gels in different concrete mixes, with an increased degree of polymerization in the metakaolin-containing concrete. Automated mineralogy identified four gel phases: Si gel, Ca-Si gel, Al-Ca-Si gel, and Al-Si gel. Ca-Si gels are formed at binder interfaces, while non-swellable Al-bearing gels are mainly formed in metakaolin-containing concrete located within aggregates. This study shows that aluminum can be incorporated into gels in calcined clay concretes, altering their structure and potentially affecting their expansion behavior in concrete.