Increasing the workability of concrete (w/cm)

The water: cement ratio is a major factor in many important characteristics of concrete. This includes compressive and flexural strength, permeability, resistance to weathering, bond of concrete and reinforcement, volume change from drying and wetting, shrinkage cracking tendency and many others.

Reducing the water content of concrete w/cm)

This can be achieved by incorporating superplasticizers into concrete mixtures. These admixtures are usually in the form of a free-flowing, spray dried powder or as a liquid solution.

Adding superplasticizer to concrete mixtures requires changes in the normal procedures of mixing and placing concrete. In particular, the proportions of cement, sand and aggregate must be adjusted to avoid segregation. The dosage of superplasticizer varies and is determined by the Marsh Cone Test.

Effects of surface mineralogy on superplasticizer-cement interactions

The surface mineralogical composition of a cement particle can have significant effects on the interaction between the polymer and the cement particle during hydration. It can also affect the amount of adsorption of the superplasticizer onto the cement particle, and this in turn may influence the strength or plastic viscosity.

Figure 20.5 shows the adsorption of PCE superplasticizer, starch-modified VMAs and welan gum VMAs in the range of concentrations normally used in concrete (see ACI 312 & ASTM C417). It is noted that there may be an increase in the yield stress caused by bridging flocculation, although this is only observed in cases where both welan gum and superplasticizer are present.