Special cements are cements that serve some specific function such as altering the setting or hardening behavior of a concrete, producing different colors for architectural effects, imparting superior workability, imparting water retention and plasticity to mortars, resisting the penetration of water in walls or containment vessels or simply reducing the cost of the cementing agent. This article gives descriptions of the basic characteristics of certain special cements and their applications.
Those described include: blended hydraulic cements (Portland blast-furnace slag, Portland-pozzolan, and slag); cements with special setting and hardening properties (Type III, rapid-setting Portland, high alumina/calcium aluminate/aluminous, magnesia-phosphate, oil-well); cements with special colors (white Portland and buff-colored); water-repellant; masonry and expansive cements.
Expansive cements are used in shrinkage-compensating concretes. These concretes find application in concrete structures, especially floors and slabs, where normal drying shrinkage cracking is undesirable. When properly restrained by reinforcement or other means, shrinkage-compensating concretes will expand by an amount about equal to the expected drying shrinkage. Because of the restraint, a compressive stress is induced in the concrete and subsequent drying will reduce this stress rather than cause tensile stresses and cracking to develop. Usually, a resilient type of restraint of the kind provided by reinforcing bars is necessary to develop shrinkage compensation, although this is not always the case. Use of shrinkage-compensating concrete for post-tensioned slabs with no internal reinforcement has been advocated to eliminate two-step post-tensioning.