Effectiveness of addition of silica fume as a waste material on durability of cement composites

The papers presents the results of the comparative study of resistance of cement composites with and without addition of silica fume (SF) exposed to the sulphate environment. The concrete samples were exposed to the various liquid media: fresh water, sulphuric acid with pH 3 and 4.2 and magnesium sulphate solution with the concentrations of 3g/L and 10 g/L, respectively. The laboratory experiment proceeded during 270 days under model conditions. A laboratory study was conducted to compare the performance of concrete samples in terms of the concrete deterioration influenced by the leaching of calcium and silicon compounds from the cement matrix. The changes in the elemental concentrations of calcium and silicon in both solid samples and liquid leachates were measured by using X-ray fluorescence method. Experimental studies confirmed that silica fume based concrete samples were found to have better performance in terms of both silicon and calcium ions leaching. The calcium ions in leachates of concrete samples without silica fume were 1.04 to 1.62 times higher than in leachates of concrete samples with silica fume. Released amount of silicon ions from samples without silica fume addition was 1.03 to 1.47 times higher. The most significant difference between released silicon ions from concrete samples without/with SF was observed for samples exposed to magnesium sulphate solution in opposite to calcium ions where the most significant difference was observed for samples exposed to sulphuric acid. The calculated Si/Ca ratios in leachates varied from 0.404 to 16.267 in dependence on the medium.