Apart from cement, water and aggregates, chemical admixtures are another important ingredients of concrete that are used to improve concrete quality, manageability and acceleration or retardation of setting time. The properties that are commonly modified by using admixtures are the heat of hydration, accelerate or retard setting time, workability, water reduction, dispersion and air-entrainment, impermeability and durability factors. There are two types of admixtures, viz., chemical admixtures and mineral admixtures. Accelerators, Retarders, Water-reducing agents, Super plasticizers, Air entraining agents etc. are the commonly used chemical admixtures. Fly-ash Blast-furnace slag, Silica fume and Rice husk Ash are the examples of mineral admixtures.
The chemical admixtures are used for following purposes:
- Admixtures can be used to delay the chemical reaction that takes place when the concrete starts the setting process. These admixtures are used in concrete pavement construction which allows more time for finishing concrete pavements.
- Sometimes admixtures are used to improve concrete’s resistance to severe frost action or freeze/thaw cycles. Such admixtures also provide high resistance to cycles of wetting and drying.
- Some admixtures act as water reducers and when added to concrete can create a desired slump at a lower water-cement ratio than what it is normally designed. Such admixtures are used in bridge decks, low-slump concrete overlays, and patching concrete.
- Calcium chloride is often used as concrete accelerator which can help the rate of concrete strength development, or to reduce concrete setting time.
- Super plasticizers are special type of admixtures which are used to produce flowing concrete with a high slump in the range of 7-9 inches. They are used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved.
- There are also shrinkage reducing and corrosion inhibiting admixtures which are used for special purposes.
However, while using chemical admixtures one need to ensure the compatibility of chemical admixtures and super plasticizers with the cement being used. Also it is always better to conduct admixture specific tests like chloride ion permeability test, water permeability test and initial surface absorption test to avoid any future complications.