Heat of hydration and the importance of controlling it

Q. What is meant by heat of hydration and why is it sometimes important to control the heat generated in concrete?

A. The reactions that bring about the hardening of hydraulic cement pastes are exothermic, that is, they are accompanied by the liberation of heat. This heat of hydration is an important factor in mass concrete (ACI 207.1R). Mass concrete as any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and resulting volume change to minimize cracking. In mass concrete, the temperature of the interior concrete may be much higher than that of the exterior, particularly when there is subsequent cooling. Cracks often result if the thermal gradient is too great. Some concrete elements do not need to be massive to require steps to be taken to cope with the heat generated by chemical reactions of the cementitious material. Such elements include base-restrained retaining walls, pavements, and other slabs on grade. High temperature can be controlled by placing concrete with low initial temperature and replacing a portion of portland cement with slower reacting cementitious materials or reducing the cement content by use of chemical admixtures, or both.

References: SP-1(02); ACI 301-20; ACI 207.1R-05; ACI 207.2R-07; ACI 224R-01; ACI PRC-207.4-20

Topics in Concrete: cracking; curing; durability; mass concrete; placing