Role of superplasticizer

Superplasticisers (high-range water reducers, ASTM C494 Types F and G, IS 9103) are chemical admixtures that disperse the cement particles so effectively that they permit a large reduction in water (15-30%) at the same workability, or produce very high workability with the same water.

Types: sulphonated naphthalene formaldehyde (SNF), sulphonated melamine formaldehyde (SMF), modified lignosulphonates, and polycarboxylate ethers (PCE, latest and most efficient).

Mechanism: cement particles in water attract each other and form flocs that trap water. The long polymer molecules adsorb on the cement surface and give a negative charge (electrostatic repulsion) or a steric barrier (PCE), so flocs break up, trapped water is released and the particles disperse.

Role in concrete

1. High workability (slump 150-250 mm, flowing concrete) without bleeding and segregation: used for congested reinforcement and pumping.
2. Water reduction of 15-30 per cent, giving lower w/c ratio, so higher strength (for the same cement content).
3. High-strength and high-performance concrete (M60 and above) with w/c as low as 0.25-0.35.
4. Cement saving: reducing both water and cement at the same w/c.
5. Lower permeability, better durability; self-compacting concrete is made with it.
6. Better surface finish, faster placing.

Precautions: the effect lasts 30-60 minutes (slump loss), so it is added at site just before placing; correct dosage (0.5-2% of cement); compatibility with cement; may cause segregation at overdose.