EFFECT OF CONDENSED SILICA FUME ON THE MICROSTRUCTURE OF THE INTERFACIAL ZONE IN PORTLAND-CEMENT MORTARS

The development of the microstructure at the cement paste-sand grain interfacial zone was studied, and the effect of condensed silica fume was evaluated. In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 mu m from the sand grain surface, was significantly different from that of the bulk paste, and it was characterized by a massive Ca(OH)//2 layer engulfing the sand grain and by some channel-type gaps. When 15% condensed silica fume was added, the interfacial zone had a homogeneous and dense microstructure, similar to that of the bulk paste, without the presence of a massive Ca(OH)//2 layer or gaps. On the basis of observations at early ages, it is suggested that these characteristics were the result of differences in the nature of the fresh paste, which was more prone to bleeding in the portland cement mortar.