15315 Silica Fume and Microengineering

Two admixtures are commonly used in high-strength concretes. One is superplasticizer, which is for improvement of workability. Another admixture that has been frequently used in high-strength concrete is silica fume. Silica fume is a waste product from the ferrosilicon manufacturing process. Due to its small size, sometimes it is called microsilica (MS). MS is a highly reactive amorphous silica that is highly pozzolanic. The reasons that concrete strength can be significantly increased by adding silica fume are

• The size of silica fume particles is smaller than that of cement particles (less than 1 mm) and thus through the so-called packing effect the silica fume can make a more dense concrete.

• The silica fume can react with calcium hydroxide (CH) and the products of this reaction are very efficient in filling up a large capillary space, thus improving the strength and permeability. It begins to react as soon as water is added. It has been shown that this reaction changes: (1) the amount of CH; (2) the composition of C-S-H; and (3) the pore size distribution.

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