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

Abstract

Cubes of a sulfate-resisting Portland cement (SRPC) paste that had been stored for 6 months in solutions of Na 2SO 4 or MgSO 4 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 Al 2O 3 in the hydration products of the SRPC, and to the fact that some of the Al 2O 3 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.