If concrete will be exposed to aggressive chemicals in a natural or an industrial environment, what role can admixtures play in improving concrete performance? Admixtures significantly improve performance for some, but not all, exposures.


Production of high-quality, low-permeability concrete is the first line of defense against chemical attack. Admixtures reduce concrete permeability in several ways. They can: permit the use of a lower water-cement ratio; make concrete easier to consolidate; convert soluble hydration products to insoluble ones; fill voids within the cement-paste matrix; and reduce shrinkage, thus reducing cracking potential.


Leaching is a mild form of distress that occurs when water dissolves components in the concrete. Admixtures can help control leaching through two mechanisms: reducing permeability and converting the soluble calcium hydroxide into insoluble calcium silicate hydrate (CSH).


Concrete that's exposed to sulfates, usually in soil or groundwater, can disintegrate in only a few years due to physical or chemical reaction, or both. Silica fume is very effective in improving sulfate resistance by converting calcium hydroxide to CSH. Air-entraining admixtures improve sulfate resistance, primarily because air entrainment permits a reduced water-cement ratio that decreases permeability.


Because no portland-cement concrete is totally immune to acid attack, admixtures can be used only to slow the rate of deterioration. Water-reducing admixtures, including superplasticizers, reduce the water-cement ratio and thus permeability.