Concrete Technology

Admixtures in Cement Concrete

Admixtures are materials other than cement, water and aggregates added to a concrete mix, either before or during mixing, to modify the properties of fresh or hardened concrete. They are added in small quantities (generally less than 5% by weight of cement). The main types and their uses are:

1. Accelerators These speed up the setting and early strength gain of concrete. The most common is calcium chloride (CaCl₂). Used in cold-weather concreting, for rapid repair works, and where early removal of formwork is required. Chloride-based accelerators are avoided in reinforced concrete due to corrosion risk.

2. Retarders These delay the setting time of concrete, keeping it workable for a longer period. Examples: sugar, tartaric acid, lignosulphonates. Used in hot-weather concreting, for long hauls, and in large pours to avoid cold joints.

3. Plasticizers (Water-reducing admixtures) These increase the workability of concrete without adding extra water, or reduce water content for the same workability (giving higher strength). Examples: lignosulphonates, sulphonated naphthalene formaldehyde. Used in pumpable concrete, thin sections and heavily reinforced members.

4. Superplasticizers (High-range water reducers) These give a very large reduction in water (up to 30%) or greatly increased slump. Used in high-strength concrete, self-compacting concrete, and where placement is difficult.

5. Air-entraining admixtures These entrain tiny air bubbles in concrete. They greatly improve resistance to freezing and thawing and to sulphate attack. Used in cold climates and in concrete exposed to frost action. Examples: resin soaps, fatty acids.

6. Waterproofing / Damp-proofing admixtures These reduce the permeability of concrete. Examples: calcium stearate, aluminium stearate, butyl stearate. Used in water-retaining structures, basements and roofs.

7. Pozzolanic / Mineral admixtures Materials like fly ash, silica fume, rice husk ash and GGBS react with lime to form extra cementing compounds. They improve strength, durability, reduce heat of hydration and lower cost. Used in mass concrete and high-performance concrete.

8. Gas-forming and foaming agents Aluminium powder or hydrogen peroxide generates gas bubbles to produce lightweight (aerated) concrete. Foaming agents produce cellular concrete for insulation and non-load-bearing partitions.

9. Colouring admixtures Pigments such as iron oxide are added to give colour to concrete for architectural and decorative purposes.

10. Corrosion inhibitors Chemicals like calcium nitrite are added to protect steel reinforcement from corrosion in chloride-rich environments.

In practice, admixtures are selected based on the requirement — workability, setting time, strength, durability or economy — and must be used within the manufacturer's recommended dosage, as excess use can harm concrete properties.