Microstructural and microanalytical studies of sulfate attack .4. Reactions of a slag cement paste with sodium and magnesium sulfate solutions

Cubes of a blended cement paste that had been stored for 6 months in solutions of Na2SO4 or MgSO4 were examined by scanning electron microscopy using backscattered electron imaging and X-ray microanalysis. The blend contained 31% of Portland cement (PC) and 69% of blastfurnace slag (ggbs). The changes observed were broadly similar to those which we have found with a normal PC but less gypsum was formed. The veins of this phase formed near to the faces and edges of the cube stored in Na2SO4, and those of gypsum and of brucite in the paste stored in MgSO,, were microstructurally different from those in the PC paste and were less continuous. Decalcification of the C-S-H was marked with MgSO4 but also significant with Na2SO4. Matters discussed include the part played by the sulfide released from the slag, the role of CH in external sulfate attack and the reported tendency for materials made with slag cements to soften and disintegrate as a result of sulfate attack rather than to expand.