admixtures

Admixtures are materials added to cementitious systems, such as concrete or mortar, other than cement, aggregates, and water. They are used to modify fresh properties like workability and setting time, as well as hardened properties such as strength, durability, and permeability. By adjusting the mix, admixtures help meet project requirements, improve performance, and sometimes reduce cost and environmental impact.

Admixtures are typically categorized into chemical admixtures and mineral admixtures. Chemical admixtures include water-reducing agents and plasticizers, which improve workability; air-entraining agents, which create microscopic air voids to enhance freeze–thaw resistance; set retarders and set accelerators, which slow or hasten early strength development; and viscosity-modifying, waterproofing, and corrosion-inhibitor formulations. Mineral admixtures, such as fly ash, ground granulated blast-furnace slag, and silica fume, participate in pozzolanic or hydraulic reactions and are often classified as supplementary cementitious materials. Polymer-based admixtures and epoxy or acrylic modifiers are used in repair mortars and overlays to improve adhesion and flexibility.

Application and compatibility are critical. Admixtures are usually added at batching or during mixing according to manufacturer prescriptions and applicable standards. They must be compatible with cement, aggregates, and each other to avoid problems such as segregation, premature setting, excessive air, or loss of strength. Certain admixtures, such as chloride-based accelerators, are avoided in reinforced concrete due to corrosion risk.

Benefits of admixtures include improved workability, reduced water demand, enhanced strength and durability, reduced permeability, and more controlled set times. They can also enable lower cement contents, contributing to reduced environmental impact. Standards and guidelines from organizations such as ACI and EN govern selection, dosage, and compatibility.