Mitigation of Calcium Oxychloride Formation in Cement Pastes Using Undensified Silica Fume

Abstract

Certain chloride-based deicing salts can react with calcium hydroxide in cement paste to form calcium oxychloride. Calcium oxychloride formation results in expansive pressures that damage concrete. This study examines the use of undensified silica fume as a potential method to mitigate calcium oxychloride formation in cement paste. Cement pastes were prepared with four different volume replacement levels of silica fume and three different water–to–cementitious material ratios. Thermogravimetric analysis and low-temperature differential scanning calorimetry were performed at various ages to quantify the amounts of calcium hydroxide and calcium oxychloride. Thermodynamic modeling was used to compute the phase distribution of products formed. In pastes containing silica fume, the amounts of calcium hydroxide and calcium oxychloride generally decrease as the amount of silica fume increases and as the sample ages. However, even when 10% of the cement volume was replaced with silica fume, calcium oxychloride formed in relatively large amounts. Therefore, the use of undensified silica fume to mitigate calcium oxychloride damage appears to require greater silica fume replacement levels than those that are currently used (3–10%). This conclusion is valid in spite of calculations from thermodynamic modeling based on calcium hydroxide amounts showing that the silica fume degree of reaction is moderate to high (30–85%).