Mitigating effect of chloride ions on sulfate attack of cement mortars with or without silica fume

Abstract

This paper presents a detailed experimental study on the sulfate attack of mortar specimens with or without silica fume exposed to sulfate and sulfate–chloride solutions (with the same concentration of SO42– ions) up to 510 d. The overall aim of the study is to investigate the beneficial effect of chloride ions on sulfate attack. In addition, the role of silica fume and water–binder ratio (w/b) in resisting sulfate attack is also reported. To qualitatively assess the performance of mortar specimens exposed to test solutions, visual examination and compressive strength and expansion tests were carried out. X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) techniques were also used to evaluate the products formed by hydration and chemical reaction and the change of porosity for paste samples. Results indicated that the presence of chloride ions in sulfate environments mitigated the deterioration of ordinary Portland cement mortar specimens, especially with a higher w/b, due to sulfate attack. It seems that the mitigating effect of chloride ions on sulfate attack is attributable to the increased solubility of sulfate products in the chloride-bearing sulfate solution, and the chemical binding of the ions to form Friedel’s salt.