Resistance of alkali-activated grouts to acid leaching

Abstract

At very low pH values materials based on hydraulic binders cannot be directly exposed to acid solutions due to leaching, hence the need for additional coatings. As these are generally characterized by relatively high cost and environmental impact, new solutions are being developed. In this study, grouts based on alkali-activated materials (activated slag and activated metakaolin with and without fly ash) have been considered as an alternative to be directly exposed to acid solutions at pH = 3.5, and compared with slag cement grout. The direct monitoring of mass and volumes was combined with the chemical analysis of the leachate and the observations of cross sections through SEM/EDS and X-ray tomography in order to understand the mechanisms involved in the leaching of studied materials. The cumulated amount of leached ions was converted into an equivalent mortar volume loss and compared with the direct measurement of the volume loss. A good correlation was found for slag cement and activated metakaolin, as leaching mainly affected the binding phase. For activated slag the volume loss was lower than expected due to the formation of new phases such as silica-rich gel. The higher volume loss of metakaolin-fly ash grout was attributed to the influence of partially reacted fly ash particles.