The Function Of Silica Fume With Different Densities Inhigh Strength Concrete

Silica fume is nessecary in the cement-based material of high strength concrete. For the low density of silica fume, it cost more money on transportation and storage, while it has a difficult in pumping.Nowadays, most silica fume on market and in concrete industry is densified.By the contrast of densified silica fume and undensified silica fume, this paper covers the physical, chemical properties of silica fume, and its reaction mechanism. It dealswith the effect of silica fume on the compressive strength, microstructure in paste and concrete. The conclusions are reached as follows.1) The densified silica fume has lightly negative effect of the compressive strength of concrete, mortars and pastes in the early age.While the silica fume improve the strength of concretes, mortars and cement pastes on different levels at the age of 28 days. The improvement will become steadly after 28 days.2) The high temperature curing of early stage can excite the activity of the silica fume effecitively. It helps the silica fume mainly in the fisrt 72 hours of the hydration, while has a negative infect on compression strength of later stage.3) The silica fume agglomeration has been found inblended pastes, which cannot be broken down bynormal mixing.The core of agglomeration does not take part in hydration and the agglomeration may be the weakness of cement pastes. Both densified and undensified silica fume are well dispersed in concretes.4) Both densified and undensified silica fume can reduce the content of portlandite, refine the pore diameter and optimize the microstructure of pore. The densified silica fume is not as effecitive as undensified silica fume in pastes for the disperse condition.5) The silica fume can improve the interface bond strength between hardened cement paste and aggregate. The crystalline orientation degree, the crystalline size and the content of portlandite at the interfaceare obviously decreased by adding silica fume.