Deterioration in Sulfuric Acid of Cement Pastes incorporating High CaO and Low CaO Fly Ashes

Abstract

This research studies the deterioration in sulfuric acid solution (pH 1) of cement pastes with fly ash. Effects of three major factors affecting the acid attack behavior are described. First, for the water to binder ratio, it was found that cement pastes with a higher water to binder ratio (w/b of 0.40) have a lower mass loss in sulfuric acid solution than those with a lower water to binder ratio (w/b of 0.25). In addition to the known mechanism of higher porosity in higher w/b pastes which makes the higher w/b pastes to be able to accommodate more gypsum, another mechanism was described based on the different abilities of calcium ion and sulfate ion to diffuse out of and into the pastes, respectively. Second, the type and content of binder provide a great influence on the degradation of cement pastes. The test of cement pastes with 30% and 50% fly ash replacement demonstrated that the fly ashes decrease the deterioration of pastes in sulfuric acid solution. In addition, it was observed that fly ash with low calcium oxide provided better resistance to sulfuric acid attack than fly ash with high calcium oxide did. Finally, the characteristics of the deterioration of paste specimens due to sulfuric acid attack was found to correlate well with CaO/SiO₂ ratio.