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Chapter 1 - Understanding Silica Fume
Silica Fume Reactions

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Socket where a sand grain has been pulled away from cement paste in 1-day old mortar. The sand grain was originally at the top of the picture. Note the open structure and the presence of crystals of calcium hydroxide in this region. "In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 śm from the sand grain surface, is significantly different from that of the bulk paste matrix away from the sand grain. It is characterized by a massive CH layer engulfing the sand grain and by some channel type gaps." "The formation of this zone may be the result of the presence of some water-filled gaps around the sand grains in the fresh mortar. These gaps may be the result of bleeding and inefficient filling with cement particles of the 20-śm space around the grain surface."

Arnon Bentur and M. D. Cohen, "Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars," Journal of the American Ceramic Society, Vol. 70, No. 10, pp. 738-743, 1987.