Admixtures have been used to improve the qualities of concrete block for years. They can impart a more finished appearance to concrete block as well as facilitate drying of wetted units and make units less absorptive.
There are three basic types of water-repellent admixtures that are integral to most block mixes: calcium stearate water repellants create a waxy barrier to moisture absorption in the block. Fatty acid admixtures react with hydration by-products to produce a material similar to soap scum. This acts as a barrier to absorption. Polymeric water repellents are the most costly of the three, but also the most effective, as they are not soluble in water.
Water repellents are also useful in controlling efflorescence. Fatty-acid admixtures neutralize salts, minimizing efflorescence, but lack long-term effectiveness. Polymeric admixtures, however, provide an integral barrier to salt migration and also help prevent absorption.
Some builders specify lighter block units for easier handling at the jobsite. Others value a denser and, therefore, stronger unit. Densifiers produce block that is less porous, heavier per unit size, and more resistant to freeze-thaw damage. A similar effect can be achieved with accelerators, which cause the mix to set more rapidly.
Multiproperty admixtures combine the benefits of a plasticizer, densifier, and efflorescence controller. They are economical, costing approximately one to two cents per block. These admixtures make masonry easier to paint, and create less construction downtime due to wet blocks.