Phase Diagram and Volume Change of the Ca(OH)2─ CaCl2─ H2O System for Varying Ca(OH)2/CaCl2 Molar Ratios

Abstract

Calcium chloride (CaCl2) from deicing salts can react chemically with calcium hydroxide [Ca(OH)2] from hydrated cement paste to form an expansive product, calcium oxychloride, that can damage concrete. This paper uses two experimental techniques, volume change measurement and low temperature differential scanning calorimetry (LT-DSC), to characterize the phase transitions associated with the formation of calcium oxychloride in the Ca(OH)2─ CaCl2─ H2O system. The temperature at which calcium oxychloride begins to form increases as the CaCl2 concentration increases, and it is not influenced by the [Ca(OH)2/CaCl2] molar ratio. Within the testing temperature range, the extent of reaction [the mass percentage of Ca(OH)2 consumed to form calcium oxychloride] increases as the CaCl2 concentration increases; the extent of reaction decreases as the [Ca(OH)2/CaCl2] molar ratio increases. The temperatures associated with the phase transitions are used to develop phase isopleths for the Ca(OH)2─ CaCl2─ H2O system for varying concentrations and molar ratios. These phase isopleths are integrated to form a ternary phase diagram of the Ca(OH)2─ CaCl2─ H2O system. The phase diagram provides a powerful tool to determine the phase transitions in the Ca(OH)2─ CaCl2─ H2O system as the [Ca(OH)2/CaCl2] molar ratio varies.