The mechanism of magnesium sulfate attack in portland cement and portland cement with silica fume paste was studied by investigating the microstructure and composition of specimens immersed for one year in magnesium sulfate solution. Results are presented in two parts. Part 1, presents the microstructure of the pastes. Part III, presents the chemical and mineralogical variations across the specimen together with a proposed mechanism of magnesium sulfate attack. The magnesium sulfate attack resulted in the formation of the "surface double-layer" which was composed of brucite and gypsum layers. In addition, there was a sequential formation of inner gypsum layers resembling the occurrence of lisegang bands found in rocks. The surface double-layer was composed of brucite, about 40 to 120-mu-m thick, and followed by a contiguous layer of gypsum, about 20 to 70-mu-m thick. Subsequently, up to four internal gypsum layers were deposited at depths of up to 1200-mu-m from the surface. These layers were parallel to the beam surfaces ranging in thicknesses from 15 to 60-mu-m. In portland cement with silica fume paste there were almost no inner gypsum layers, instead, massive deposition of dispersed gypsum crystals appeared up to at a distance of about 800-mu-m from the surface. The attack had apparently influenced the local degree of hydration of the cement constituents; near the surface only residual ferrite and belite crystals were observed, but in the middle, unreacted alite and belite were predominant.
