Admixtures are liquids that can be added to concrete either at the jobsite or at a concrete plant. They are divided into five classes, depending on their function: air-entraining admixtures; water reducers; retarders; plasticizers (superplasticizers); and accelerating or hardening agents.
Decreased hydration time
Superplasticizers decrease hydration time in concrete by reducing the degree of hydration, which is due to a reduction of Ca(OH)2 and calcium silicate hydrates. This also reduces the amount of mixing water that is needed to achieve a specific hydration duration, thereby improving the reactivity of the cement paste.
Decreased rheological stress
Superplasticizer-cement interactions in the early stages of cement hydration may be affected by the surface composition of the concrete particles. This has been attributed to the fact that the surface mineralogical composition of the concrete particles is often very different from that of the bulk material, as shown in Figure 3.
Increased air content
In many cases, superplasticizers may cause increased air entrainment in the concrete mix. This can be undesirable in some situations, especially for structures where air voids are not present, such as certain types of footings and walls.
This may result in the need for a special formulation of the superplasticizer admixture to ensure proper air entrainment and workability. It is important to understand that all admixtures have their own unique properties and thus need to be formulated correctly in order to produce the desired effect.