Calcium chloride (CaCl2), which is commonly used as a deicing salt, can react with calcium hydroxide (Ca(OH)2) in cement-based materials to form calcium oxychloride. This reaction causes damage that typically manifests itself as flaking of concrete pavements at the joints and leads to expensive repairs and a reduction of the service life. In this paper, cement pastes with different fly ash replacement levels were prepared to provide pastes with differing amounts of Ca (OH)2. Thermogravimetric analysis was used to quantify the Ca(OH)2 content in these pastes. Low-temperature differential scanning calorimetry (LT-DSC) was used to quantify the amount of calcium oxychloride formed when these pastes were exposed to CaCl2 solutions. The reduction in the flexural strength of these pastes saturated with different CaCl2 solutions was also measured. As the concentration of CaCl2 increases, the reduction in flexural strength increases. There is a lower flexural strength reduction in pastes with fly ash, because these pastes have lower Ca(OH)2 and form lower amounts of calcium oxychloride. The strength reduction is directly correlated to the amount of formed calcium oxychloride.