Packaged cement, for example, requires tightly sealed moisture-proof bags. These aggregates naturally reduce the cost of the final building material, but they are also necessary in reducing the proportion of pure cement paste, which has a high shrinkage characteristic. If drying periods intervene early in the hydration process, the reaction is retarded seriously and later wetting does not compensate in full for early loss in strength. Curing is therefore very important to good quality concrete. The lowest possible water-cement ratio consistent with workability is required for maximum strength and durability.
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.