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Water cement system

Normal water cement system constitutes a system of particle suspended in highelectrolyte concentration bringing the particle close enough to stick. This process is

termed as flocculationi.

In normal cement pastes where particles come into close contact with each other

there is a tendency for cement pastes to form large flocs due to the attractive vander Waals forces holding particles together ii.

Portland cement in contact with water has a tendency to flocculate due to van der

Waals forces, electrostatic interactions between the opposite charges and surface

chemical interactions between the hydrating particles iii.

This will result in the formation of an agglomeration of particles with open structurewith spaces that entrain water molecules. These water molecules are not

immediately available for hydration and do not have a lubricating effect.

Effect of Superplasticiser

In the presence of a superplasticizer, deflocculation or dispersion of cement particles

occurs due to adsorption and electrostatic repulsion due to zeta potential or sterichindrance according to the type of super plasticizer.

This process does not allow the formation of entrapped water and discouragessurface interaction of the particles. Some steric hindrance is possible especially whenhigh molecular weight superplasticizers are used. Such a phenomenon would also

prevent particle-particle interactions.

The superplasticiser interact with cement particle, introducing a membrane of

absorbed charged molecule around each particle.

The apparent viscosity of cement pastes containing superplasticizers depends also onthe fineness of cement. There is an apparent increase in viscosity at higher Blaine

fineness values. A linear correlation exists between the fine fraction lower than 11micrometer and viscosity

Effect of air entrain admixture

Air entraining admixtures introduce spherical air bubbles 10-250um in diameter bymodifying the surface tension of aqueous phase in the mix.

This is accomplished by compound having negatively charged head group which is

hydrophilic and a non polar tail which is hydrophobic.

The generated air bubbles with apparent negative charges form bridges betweencement particles giving an increased yield value. The bubble acts like ball bearings to

allow larger paticles to flow past each other, thus decreasing plastic viscosity.

The air-entraining action is known to involve adsorption at the air water and solid-water interfaces. The soluble surfactant ions are adsorbed on cement particles

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making them hydrophobic, so that as bubbles are generated during mixing, they

adhere to cement.

iConcrete technology. ML Gambhir, 3

rdedition. Tata McGrahill, page 145.

ii

Chemical admixture for concrete. Roger Nixom and Noel Mailvaganam, 3rd

Edition,E & FN Spon, page 44

iiiConcrete Admixture hand book, VS Ramachandran, 2

ndedition, Noyes Publication. page