Agricultural and Biosystems Engineering
High temperatures accelerate the hardening of concrete and more water is generally required to maintain workable consistencies. If the water-cement ratio is not maintained by adding additional cement, strength and durability will be reduced. For example, if the temperature of concrete is increased from 50 degrees F to 100 degrees F, about 33 pounds (roughly four gallons) of additional water is needed per cubic yard of concrete to maintain a three-inch slump. If the water content of concrete is increased without increasing the cement content, the strength and other properties of concrete are adversely affected.
Pohl, Steve, "Hot Weather Precautions for Pouring Concrete" (2002). SDSU Extension Extra Archives. 6.