Contractors occasionally speak of getting a "hot load" of concrete, meaning the concrete stiffens quickly and is difficult to place, consolidate, and finish. Though hot loads can be caused by high concrete and air temperatures, certain cement or admixture properties can also lead to early stiffening.
If inadequate amounts of gypsum are added to the cement, flash set can occur--a rapid development of rigidity in freshly mixed portland cement paste, mortar, or concrete. Further mixing can't dispel this rigidity, and a large amount of heat is produced in the process. Rapid development of rigidity can also be caused by false set. But in this case, little heat is generated. False set occurs because some of the gypsum dehydrates as a result of contacting hot clinker or high temperatures in the grinding mill. This creates plaster that rehydrates to form gypsum and stiffen the concrete.