Alkali-silica reaction products and their development

Abstract The work described was undertaken to investigate whether the NBRI accelerated test in fact mimics the natural alkali-silica reaction. The test specimens showed all the characteristics of the alkali-silica reaction and the reaction products proved very similar to those found in structures affected by the reaction in the field. These products can be grouped into (a) exuded and surface gels (amorphous material and a tobermorite gel) and (b) white reaction products generated within the concrete and mortar specimen. The NBRI test, therefore, appears to accelerate the reaction and not to modify the naturally occuring process. This in turn indicates that the test can be used to investigate the mechanism of the alkali-silica reaction. The alkalis Na and K in most of the reaction products that form during the test can be removed by soaking the concrete and mortar specimens in water. In some cases the Na and K are replaced by Ca. This suggests that when sufficient water is available, the reaction products are able to interact with a Ca-bearing phase in the cement matrix. If this phase is Ca(OH) 2 , as expected, then it is very likely that alkali hydroxides are by-products of the interaction, and consequently that the alkali-silica reaction is self-perpetuating, since the alkali hydroxide would be continuously regenerated.