Superplasticizer are water soluble, electrically charged organic polymers having molecular weights in the range 1000 – 100000 Daltons. The action exerted by these molecules in an aqueous slurry (paste) of cement particles may viewed as resulting from a superimposition of “physical” and “chemical” effects.

The “physical” effects are those which would be observed in any dense slurry of ‘inert’ (e.g limestone, titanium dioxide); in such slurries, superplasticizer molecules can exert a strong fluidizing effect at constant water content or a major water reduction at constant fluidity. The mode of action of superplasticizer molecules in these systems (and generally in any slurry of mineral particles), first relies on the adsorption of part of the molecules on the surface of the particles. The superplasticizer molecules convey an electrical charge to the particle surface and this favors a better dispersion (de-flocculation) of the particle via electrostatic repulsive forces. The adsorbed superplasticizer polymers also act as a physical barrier to keep particles apart (i.e. steric repulsion), further assisting the dispersion of the particles and fludification of the slurry. The molecular properties of the superplasticizer which optimize the “physical” contribution to the mode of action superplasticizer have now been largely elucidated.

The “chemical” role of superplasticizer in cement pastes may be qualitatively pictured as the way in which these polymer molecules can influence the course of cement hydration reactions and the nature of the hydration products. For example, superplasticizer molecules can interfere with the nucleation and growth of the hydration products, thus retarding the formation of connective get between the hydrating articles; this will help maintain fluidity (slump) for longer periods. Also depending on their chemical composition, superplasticizer molecules may become embedded, or intercalated, in the hydration products leading to new organo-mineral hydrates. In the case of these “chemical” mechanisms, the relationship between the molecular properties of the superplasticizer and the effects observed are only partially understood.

Flowability of the cement matrix over a long period of time depends on the fineness of the cement and chemical composition as well as water demand for a given flowability and hydration process. Dispersive action of polymeric admixtures depends on its affinity towards the particle surface, the dispersing medium and itself, as well as on dispersion mechanism via electrostatic and/or steric hindrance. These factors are greatly affected by changes in the cement pore solution and/or surface conditions. Therefore, extended time flowability of the cement paste is subjected to a state of competition between the cement pore solution and the surface conditions.

The mineral phases constituting Portland cement include alite, belite, aluminate phase, (C3 A) and ferrite phase, (C4AF) and gypsum. The formation of hydration products takes place where ionic species dissolve from the cement solids into the pore solution. Crystalline or amorphous solid hydration products begin to form in the pore solution or at the solid solution interphase. These hydrates grow in size via chemical reaction and physical process.

Generally, the dispersion mechanism of admixture on cement particles is classified as adsorbed and non-adsorbed polymers. Adsorbed polymers enhance dispersion via electrostatic and steric effects or both, whereas non-adsorbed polymers can induce depletion and tribology effects.

Action mode of Non-adsorbed portion of admixtures:


·        Depletion Effect

Depletion effect states that, polymers disperse or flocculate even if they are in a free state without adsorption. At excessive dosage, if the polymer concentration increased to above a critical value, the polymer coils loose their individual identity, where the chains interpenetrate and form continuous network. Therefore, surplus polymer remains in solution and fills the spaces between particles preventing cement particles flocculation.

·        Tribology Effect

Tribology is the science of friction, abrasion and lubrication. Low molecular weight compounds (Hydroxy-carboxylic acid, Glyconates, Ctrates, etc.); reduce the frictional forces between particles, enhancing flowability. In addition, the surface tension of the mixing water is also reduced.


It is interesting to study the effect of different chemical admixtures like Sodium lignosulphonate, Sulphonated naphthalene formaldehyde, Polyacrylate on the performance of different cement pastes such as normal Portland cement, sulphate resisting cement, high slag cement and fly ash cement. Also the production of High volume Fly Ash, that is with Fly Ash contents from 35 to 60%, the plasticizer used would need to different and compatible with the mix of two ingredients.