Thermodynamic study on phase composition of hardened Portland cement paste exposed to CaCl2 solution: Effects of temperature, CaCl2 concentration, and type and dosage of supplementary cementitious materials

Calcium oxychloride formed in concrete exposed to a calcium chloride (CaCl2) solution may reduce the durability of cementitious materials. A thermodynamic modeling technique is employed to predict the phase assemblage of the hardened cement paste (HCP) at various temperatures, CaCl2 concentrations, and types and dosages of supplementary cementitious materials (SCMs) and to explore the degradation mechanism of HCP subjected to the CaCl2 attack. The results indicate that monocarbonate decomposes under CaCl2 attack to generate Kuzel's salt, and Kuzel's salt is then replaced by Friedel's salt at an even higher CaCl2 concentration, while calcium hydroxide (CH) transforms into 3-1-12. The strength loss in HCP is directly proportional to the volume ratio of the solid phases in the HCP before and after chloride attack. Increasing the replacement level of silica fume (SF), fly ash (FA), slag (SL), or metakaolin (MK) reduces the amounts of CH and 3-1-12 formed in HCP.