The co-existence of thaumasite and ettringite in concrete exposed to magnesium sulfate, at room temperature and the influence of blast-furnace slag substitution on sulfate resistance

The effects of Type I Portland cement replacement by 45% or 72% blast-furnace slag on the sulfate resistance of laboratory concretes were analyzed by microstructural investigation. The concretes investigated were stored in water or in magnesium sulfate solutions for 23 years under laboratory conditions. For those stored in water only surface layers of carbonation and decalcification were observed. Concretes exposed to sulfate solutions formed brucite, ettringite and thaumasite. Thus, thaumasite was observed to form in concretes stored under laboratory conditions. In all cases both ettringite and thaumasite were found to co-exist in the damaged zones. However, the thaumasite appears to be moving in from the exterior after initial formation of ettringite, and has not resulted in the massive destruction of the hydrated matrix as has been found elsewhere at lower temperature exposures. Slag replacement was observed to be an effective means of conferring resistance to sulfate attack. Although the concretes studied were prepared at a W/cm (water-to-cementitious materials) ratio of 0.50, the depths of attack observed were comparable to those observed in concrete prepared at w/c = 0.45 using ASTM Type V (SRPC) cement alone. (C) 2003 Published by Elsevier Ltd.