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Chapter 1 - Understanding Silica Fume
Silica Fume Reactions

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Slide
Schematic of the transition zone. Note that in the area adjacent to the surface of an aggregate particle, the relatively uniformly sized cement grains are not packed as densely as they are in the bulk zone. This "wall effect" is one of the reasons stated for the relative weakness and permeability of cement paste in the transition zone.

When silica fume is added to the concrete, better particle packing occurs in this area. The silica fume in this area can further refine the transition zone through its pozzolanic reaction.

This sketch is taken from H. H. Bache, "Densified Cement/Ultra-Fine Particle- Based Materials," Presented at the Second International Conference on Superplasticizers in Concrete, 1981.