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.
LOW PERMEABILITY KEY TO CHEMICAL RESISTANCE
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.