Enhancing Water Resistance of Cement and Gypsum-Cement Materials

In recent years silica fume, a by-product in the production of metallic silicon or ferrosilicon, has gained acceptance for use as a mineral admixture in cementitious materials. The principal areas of application of silica fume include producing high strength concretes and improving concrete resistance to aggressive chemicals. A new application of silica fume for enhancing water resistance of cement and gypsum-cement materials is reported. As a characteristic of water resistance the wet/dry strength ratio of the material was used. The values of strength and the wet/dry strength ratio for hardened 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 low value at 3 days, the wet/dry strength ratio of cement-silica fume paste rises significantly after 28 days of water curing, achieving the value of pure portland cement paste having the same water to cementitious material ratio, 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, compared with the ratio after 3 days of water immersion. The physical and chemical mechanisms of this improvement are discussed.