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Common admixtures; accelerating admixture

Short answer

Common admixtures include water reducers, superplasticizers, retarders, accelerators, and air-entraining agents. Accelerators like calcium chloride speed up setting and early strength gain by enhancing cement hydration; they are used in cold-weather concreting and repairs but chloride-based ones are avoided in reinforced concrete due to corrosion risk.

List out common admixtures available in the market. Elaborate in brief the accelerating admixture.

Common admixtures available in the market

  • Chemical: plasticisers (water reducers), superplasticisers (SNF, PCE), accelerators, retarders, air-entraining agents, water-proofing compounds, corrosion inhibitors, shrinkage reducers, viscosity modifiers, bonding agents.
  • Mineral: fly ash, silica fume, ground granulated blast furnace slag (GGBS), rice husk ash, metakaolin.
  • Others: pigments, fibres, expansive agents, pumping aids.

Accelerating admixture (ASTM C494 Type C, IS 9103)

An accelerator is an admixture that speeds up the setting and early strength gain of concrete by speeding up the hydration of C3_3S and C3_3A.

Types

  • Soluble inorganic salts: calcium chloride (most common and cheap), calcium nitrate, calcium nitrite, sodium thiocyanate, sodium carbonate.
  • Organic: triethanolamine, calcium formate.
  • Rapid-setting shotcrete accelerators: aluminates, silicates.

Dosage: calcium chloride up to 2% of cement mass (IS 456: chlorides in the mix limited, so it is not used in RCC and prestressed concrete).

Effects

  • Initial and final setting times are shortened; 1-3 day strengths rise, with lower or similar 28-day strength.
  • Heat of hydration is liberated earlier.
  • Chlorides cause corrosion of steel; also shrinkage, creep and sulphate attack may increase.

Uses: cold weather concreting, early removal of formwork, emergency repair, precast production, shotcrete, concreting under water or against flowing water, plugging leaks.

Precautions: avoid in RCC when chloride-based; trial mixes and accurate dosage; quick placing; sufficient curing.

✓ Answer checked by Claude Opus 5.5 (high effort)

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