Effect of temperature on pore solution composition in plain cements

Cement pastes with a water-cement ratio of 0.6 were prepared using three ordinary portland cements with C₃A contents of 2.43, 7.59 and 14%. Three levels of chlorides 0.3, 0.6 and 1.2% by weight of cement, derived from sodium chloride, were added through mix water. The pastes were allowed to cure in sealed containers at 20 and 70°C for 180 days and then subjected to pore solution extraction. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. Results show that increase in temperature from 20 to 70°C increased unbound chlorides and decreased hydroxyl ion concentration of pore solutions for all the three cements. The simultaneous increase in unbound chlorides and decrease in hydroxyl ion concentration drastically increased Cl⁻/OH⁻ ratio of the pore solution, thereby indicating an increase in corrosion risk. This adverse effect of increase in the Cl⁻/OH⁻ ratio of the pore solution with increase in temperature is higher in the high 14% C₃A cement than in the low C₃A cements, and is also higher for the low 0.3% chloride treatment level than the higher chloride inductions. Increase in temperature is also expected to cause an increase in ionic diffusion to steel embedded in concrete as well as in the rate of corrosion reaction. All these factors tend to increase corrosion risk of steel reinforcement in concrete with an increase in temperature.