Enhancing water resistance of cement and gypsum-cement materials

In recent years silica fume, a by-product in the production of metallic sil icon or ferrosilicon, has gained acceptance for use as a mineral admixture in cementitious materials. The principal areas of application of silica fum e include producing high strength concretes and improving concrete resistan ce to aggressive chemicals. A new application of silica fume for enhancing water resistance of cement and gypsum-cement materials is reported. As a ch aracteristic of water resistance the wet/dry strength ratio of the material was used. The values of strength and the wet/dry strength ratio for harden ed cement pastes of various ages, with water to cementitious material ratio varying from 0.33 to 0.50, were obtained. It is shown that, despite the lo w value at 3 days, the wet/dry strength ratio of cement-silica fume paste r ises significantly after 28 days of water curing, achieving the value of pu re portland cement paste having the same water to cementitious material rat io, and at 200 days exceeds it, approaching a value of 1. However, the most pronounced improvement of water resistance was fixed for the gypsum-cement materials with 5% addition of silica fume, where the value of the wet/dry strength ratio after 200 days of water immersion increased twice as much, c ompared with the ratio after 3 days of water immersion. The physical and ch emical mechanisms of this improvement are discussed.