The distributions of bound sulfates and chlorides in concrete subjected to mixed NaCl, MgSO4, Na2SO4 attack

This article describes phenomena typical of those observed in concrete flatwork exposed to sulfate soils, which had been undergoing sulfate attack. In particular, the spatial distribution of sulfate-containing phases as a result of the ingress of ground water is discussed. Intrusion of sulfate from ground water resulted in the formation of zones of ettringite and gypsum. These zones formed in response to the sulfate gradient. Because the high permeabilities are typical of the concretes examined, significant amounts of the intruding sulfate accumulated near the top surfaces of the concretes where they were available to form gypsum. As a consequence, the microstructures of these concretes are characterized by zones of gypsum near the top and bottom surfaces, zones of ettringite adjacent to these, and central zones of monosulfate and Friedel's salt. The latter formed due to the ingress of chloride. Because of their high permeabilities, the use of sulfate-resisting cement alone was insufficient to protect these concretes from sulfate attack. (C) 2000 Elsevier Science Ltd. All rights reserved.