A MICROSTRUCTURAL STUDY OF THE EFFECT PRODUCED BY MAGNESIUM-SULFATE ON PLAIN AND SILICA FUME-BEARING PORTLAND-CEMENT MORTARS

A chemical and microstructural study has been carried out on Portland cement mortars with and without silica fume subjected to magnesium sulfate attack. The mortars appeared to be less durable than their counterpart pastes as indicated by greater surface deterioration greater depth of sulfate ingress, more extensive gypsum deposition, and a higher degree of decalcification of the CSH gel. The plain (non-silica fume) mortar was distinguished by massive deposition of gypsum. Preferred sites of deposition were around aggregates, in air voids and other cavities. In the silica fume bearing mortar less gypsum was deposited. Nevertheless, the CSH of the silica fume bearing mortar experienced a higher degree of decalcification than the CSH of the plain mortar. Based on occurrence of surface deterioration and cracking pattern, it appears that silica fume addition did not improve durability. It is concluded therefore that the decalcification of the CSH plays a major role in the deterioration process.