Microstructural and microanalytical studies of sulfate attack III. Sulfate-resisting portland cement: Reactions with sodium and magnesium sulfate solutions

Cubes of a sulfate-resisting Portland cement (SRPC) 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 changes observed were broadly similar to those which we have found with a normal Portland cement (PC), but cracking and loss of material were less marked, less ettringite was formed, and decalcification of the CSH was much reduced. As with the PC, gypsum was formed, both in veins and mixed with the CSH. The differences are attributed to the low content of Al2O3 in the hydration products of the SRPC, and to the fact that some of the Al2O3 is already present as ettringite. The decreased formation of ettringite and decreased decalcification of the CSH in the SRPC together explain the superior resistance to sulfate attack.