Admixture is a component that is added to the concrete besides cement, fine aggregate, coarse aggregate, and water either before the mix or during the mix to modify the properties of the concrete. This modification can reduce the usage of special cement which comparatively costs high. Admixtures can either be a natural component or an artificial component. In this article, all we gonna discuss is artificial admixtures. Artificial admixtures are factory manufactured ones that provide more purity and are economical. There are around 15 types of admixtures classified based on their reaction with concrete such as Accelerator, retarder, Plasticizer, Superplasticizer, Air entraining admixture, integral waterproofer, Corrosion resistant inhibitors, Bonding inhibitors, etc.
Starting with Accelerators, which are used to accelerate the setting time of concrete so the curing period starts earlier, accelerators are mostly used when concreting is done in winter so that the concrete is prevented from the damage of freezing. The chemical components that can act as accelerators are Calcium nitrate (Ca(No3)2), Calcium Nitrate (Ca(No2)2), etc. There are a lot of commercial products available to make it easy to use such as Tec Mix 460, which has the required components are ingredients. They generally brisk hardening of concrete without altering the other properties of concrete. The factor that has to be considered before choosing an accelerator is the check for chloride ions in it. There is a greater chance for corrosion in the reinforcement if the presence of chloride ions is positive. The initial setting time of concrete can be modified up to 20 mins by using accelerators.
Retarder is the next type of admixture that is widely used in the field to delay the hydration of cement without affecting the mechanical properties of the cement i.e increasing the setting time of the concrete. This could help in situations where there is an unavoidable delay between the mixing and placing of concrete. retarders are available both in organic and inorganic form but the most used retarder component is ZnO (Zinc Oxide), which is commercially available in the market such as TEC MIX 240, which when added reduces the setting time of the concrete. you could wonder how the heat of hydration, a naturally occurring process is modified. let us take a situation where TEC MIX 240 is added to concrete, various studies prove that when it is added it forms a thin film in the concrete preventing the setting time of the concrete by preventing the reaction between the cement and the water in the concrete. The longer the delay is needed the thickness of the film is increased by modifying the retarder.
Superplasticizers is water-reducing admixture. The usage of plasticizers would mostly reduce the water consumption in the concrete, but superplasticizer usage can result in a reduction of water content by up to 30%. This reduction will not affect the workability of the concrete but increases the rheology(flow properties) of the concrete. Superplasticizers have synthesized polymer that is commercially available in the market in various brands such as Tec Mix 550, which will act as a dispersant to minimalize segregation.
Air Entraining admixtures form small stable air bubbles to the concrete mix which are less than the diameter of 1 mm. This is mostly used to resist freezing and thawing in the concrete. The volume of air entrained depends upon the mixing and mix design of the concrete. during transport and mixing, there is a lot of chance for the loss of air, to prevent this allowance should be made in advance so that the air formation is accurate on the hardened concrete. This concrete posses a greater disadvantage, for every 1 per cent of the admixture added 5 per cent of the concrete strength is affected, but that can be overcome to a certain extent by cohesion and compaction.
Integral Waterproofer is another important admixture used in the concrete. As the name suggests the waterproofing of the concrete starts within the concrete. This is economically efficient compared to the membrane waterproofing coating. In the market, there is the availability of a lot of material for easy usage such as TEC MIX 100. When TEC MIX 100 is added to the concrete, the mechanism happening in the concrete is very simple, the admixture forms crystal in the pores and voids of the concrete preventing the penetration of water. This will have improved effects on the self-healing of cracks, compressive strength, slump retention, etc as additional benefits.
Though there are a lot of admixtures, a few most important and most used admixtures are discussed and it is seen that usage of these admixtures have a lot of pros such as improved strength, workability, retardation of setting time, improving protection from weather conditions such as freezing and thawing.