Cement Hydration

Hydraulic cements, including portland cement, set and harden when mixed with water at room temperature. The reactions that cause setting and hardening are collectively described as exothermic hydration reactions.

The reaction of portland cement compounds with water is exothermic; that is, heat is generated from the reaction. The average is 120 calories per gram during complete hydration of the cement. In normal construction, structural members have relatively high surface-to-volume ratios such that the dissipation of the heat generated is not a problem. By insulating the forms, this heat can be used as an advantage during cold weather to maintain proper curing temperatures. However for dams, massive foundations, and other mass concrete structures, measures must be taken to reduce or remove heat by proper design and construction methods. This may involve circulating cold water in embedded pipe coils or other cooling means. Another method of controlling heat evolution is to reduce the percentage of compounds with high heat of hydration, such as C3A and C3S, and use a coarser fineness to produce a Type IV low heat of hydration cement. Since Type IV is generally no longer available in most locations, Type II and pozzolans or slag are used as a substitute. The use of large aggregate (nominal diameter greater than 150 millimeters) also helps to reduce the cement requirement and consequent heat by reducing the water demand, hence, less cement at the same water cement ratio.