Effect of Aluminum on Synthesized and Naturally Formed Alkali-Silica Gels

Alkali-silica gels formed during alkali-silica reaction (ASR) can vary in composition and structure. This variation depends on the concrete composition, environmental conditions, gel maturation and the location of gel formation within the concrete matrix. An effective and well-known method to mitigate ASR is the use of aluminum-rich supplementary cementitious materials (SCMs). To investigate the effect of aluminum on ASR gel properties, concrete prisms with quartz sand, borosilicate glass, 90 wt.% Portland cement and 10 wt.% metakaolin were mixed. As an aluminum-free non-reactive “SCM”, limestone was used with the same replacement level. On these concretes, the expansion up to 140 days at 40 °C above water, the ASR gel composition and microstructure, as well as the spatial distribution of the gels, were investigated. Due to the small gel amounts formed, the structural investigations (²⁹Si,²⁷Al NMR and¹H NMR spectroscopy) were performed on synthetic ASR gels with similar compositions as in the concretes. The concrete expansion for the mix with limestone was approximately three times higher than with metakaolin (0.5 vs. 1.5 mm/m). However, the concrete with metakaolin contained larger amounts of an Al-ASR gel than the concrete with limestone. Analytical investigations on the synthetic ASR gels indicate increased connectivity for the Al-ASR gel compared to the Al-free ASR gel. This connectivity change could alter the swelling properties of the gels, thus limiting the expansion pressure in concrete.