Building Materials, Third Edition

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™—X The addition of chemicals such as zinc or aluminium powder releases gases. This
method is generally not adopted since it requires high control.

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X are surface active chemicals imparting electrostatic charges on the cement
particles. This causes cement particles to repel each other and thus prevent coagulation. A
small amount of air is also entrained in the concrete and workability is increased. The dispersing
agents reduce the strength. The most commonly used dispersing agent is calcium
lignosulphonate.

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 2 are classified as follows:

1. Rust generating (expandite) —consisting of finely divided iron and NH 4 Cl. Iron rusts
   and expands, producing non-shrink grouts.


2. Methocel—obtained from Dow chemicals.


3. Hydrogen generating (Nevander)—checks bleeding but does not expand.


4. Chemically expansive (sulphoaluminate)—Denka Tascan.

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™3X are organic or a combination of organic and inorganic substances, which allow a
water reduction for a given workability, or give higher workability at the same water content.
Plasticizers are principally surface active (surfactants). They induce a negative charge on the
individual cement particles such that the fine cement particles are dispersed due to inter
particle repulsion.
Fine cement particles being very small clump together and flocculate when water is added
to concrete. This ionic attraction between the particles trap considerable volume of water and
hence water required for workability of concrete mix is not fully utilised. Negative charges are
induced on the fine cement particles causing flocs to disperse and release the entrapped water.
Water reducing admixtures or plasticizers therefore help to increase the flow of the concrete
mix considerably. Fig. 10.39 illustrates this disperse mechanism due to ionic repulsion.

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