Sulfate attack on hardened Portland cement pastes with different porosities in presence of water-repelling admixtures

Different cement pastes were prepared using ordinary Portland cement with various water/cement (w/c) ratios in both the presence and absence of calcium stearate as a water-repelling admixture. The various w/c ratios used were (0·32–0·20). Superplasticiser (sodium salt of naphthalene formaldehyde sulfonic acid polycondensate) was used for mixtures with w/c = 0·20 to facilitate the mixing process. The different cement pastes were cured in a humid atmosphere (approximately 95% relative humidity) for the first 24 h, then under water for the remaining time up to 28 days. After 28 days of hydration, the different specimens were immersed in 7% sodium sulfate solution for up to 16 months. The degree of sulfate attack on different specimens was investigated using: X-ray diffraction analysis and scanning electron microscopy. The results of the compressive strength, percentage of expansion, bulk density and mass change at various curing ages showed an increase in the sulfate resistance of the hardened cement pastes prepared with w/c ratios of 0·24 and 0·20. The presence of 0·5% calcium stearate as an admixture in the cement pastes improved the sulfate resistance in the pastes with different w/c ratios used in this study and that related to its waterproofing effects. This conclusion was confirmed by both X-ray analysis and scanning electron microscopy.