Chemical and mineral admixtures; plasticizers

Chemical admixtures (IS 9103 / ASTM C494)

1. Water reducers / plasticisers (Type A): lignosulphonates, hydroxylated carboxylic acids.
2. Retarders (Type B): sugar, gluconate, phosphates.
3. Accelerators (Type C): calcium chloride, calcium nitrate, triethanolamine.
4. Water-reducing retarders / accelerators (Type D and E).
5. Superplasticisers (Type F and G): SNF, SMF, PCE.
6. Air-entraining agents: Vinsol resin, fatty-acid soaps.
7. Water-proofing, corrosion inhibitor, shrinkage-reducing, viscosity modifying admixtures.

Mineral admixtures

Finely divided pozzolanic or cementitious materials added in large dosage: fly ash, silica fume, ground granulated blast furnace slag, rice husk ash, metakaolin, natural pozzolanas (volcanic ash), and inert fillers (limestone powder).

How plasticisers reduce the water content

Cement particles in water carry opposite electric charges at their surfaces and attract each other, forming flocs that trap part of the mixing water inside. This trapped water does not help workability.

1. The plasticiser molecule has a hydrophilic and hydrophobic end; it adsorbs on the cement surface.
2. It gives all the particles the same negative charge, so they repel each other (dispersion), and a lubricating water film forms around each grain.
3. The flocs break up and the trapped water is released to lubricate the mix, so the paste becomes more fluid.
4. Therefore, for the same slump, mixing water can be reduced by 5-15 per cent (up to 30 per cent with superplasticisers), giving a lower w/c, higher strength, lower permeability, or for the same w/c a higher slump.