Microstructural and microanalytical studies of sulfate attack. I. Ordinary portland cement paste

Portland cement pastes that had been stored for 6 months in solutions of sodium or magnesium sulfate were examined by scanning electron microscopy using backscattered electron imaging and X-ray microanalysis. For a paste stored in Na 2SO 4 solution, successive changes were observed on passing from the unaltered material in the interior towards the surface. These were (1), replacement of monosulfate by ettringite, which was closely mixed with the C-S-H gel, (2), disappearance of calcium hydroxide, partial decalcification of C-S-H and precipitation of gypsum and (3), further decalcification and leaching. Much of the gypsum occurred in veins sub-parallel to the surface, with which were associated cracks. A paste stored in MgSO 4 solution showed broadly similar effects, as well as a largely continuous surface layer of gypsum and brucite, except at the cube edges, where a gel high in magnesia and silica was formed. This was probably cryptocrystalline serpentine. Neither specimen contained massive deposits of ettringite.