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Hoe Does Silica Fume Work in Concrete? |
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In cementitious compounds, silica fume works on two levels, the first one described here is a chemical reaction called the "pozzolanic" reaction. The hydration (mixing with water) of Portland cement produces many compounds, including calcium silicate hydrates (CSH) and calcium hydroxide (CH). The CSH gel is known to be the source of strength in concrete. When silica fume is added to fresh concrete it chemically reacts with the CH to produces additional CSH. The benefit of this reaction is twofold; increased compressive strength and chemical resistance. The bond between the concrete paste and the coarse aggregate, in the crucial interfacial zone, is greatly increased, resulting in compressive strengths that can exceed 15,000 psi. The additional CSH produced by silica fume is more resistant to attack from aggressive chemicals then the weaker CH.
The second function silica fume performs in cementitious compounds is a physical one. Because silica fume is 100 to 150 times smaller than a cement particle it can fill the voids created by free water in the matrix. This function, called particle packing, refines the microstructure of concrete, creating a much denser pore structure. Impermeability is dramatically increased, because silica fume reduces the number and size of capillaries that would normally enable contaminants to infiltrate the concrete. Thus silica fume modified concrete is not only stronger, it lasts longer, because it's more resistant to aggressive environments. As a filler and pozzolan, silica fume's dual actions in cementitious compounds are evident throughout the entire hydration process. |
Four Seasons Hotel & Tower Extremely dense reinforcing steel requried, self consolidating high performance silica-fume concrete. |
The word pozzolanic is derived from the name of a town in Italy,
Pozzuoli. It was there that the ancient Romans combined ground
lime stone with volcanic ash to produce mortars to bind large stones
together. The durability of the resulting structures can still
be witnessed today, two thousand years later.