Excess water produces voids that reduce density and hence the strength of the cement paste. Rate of hydration and therefore strength gain increase with increasing temperature, as for most chemical reactions. A set-retarding admixture is often used to modify setting time, especially in hot weather. An accelerator admixture, besides affecting setting time of concrete, is used primarily to increase rate of strength gain through increased rate of hydration. Certain problems experienced with portland cement may or may not be readily prevented.
False set is an abnormally early stiffening that can be worked out by subsequent mixing.
It is, therefore, not a difficult problem in ready-mix operations where prolonged agitation in transit ensures a workable mix at the job site. It can be serious where the concrete is mixed on the job site. False set can usually be prevented by implant formulations and procedures. The phenomenon of pack-set, where the dry cement powder densifies in silos and bins and becomes hard and difficult to discharge, can be prevented by the use of certain additives during the grinding of clinker. The rapid increase in sophistication of design and construction techniques and the greater attention to variations in regional and job conditions have created demand for modifications of certain properties of concrete.