Admixtures are extra ingredients added to concrete other than cement, sand and aggregate. They can modify the properties of hardened concrete to meet specific requirements.
They are used in the manufacturing of concrete to minimize construction costs, adjust the properties of hardened concrete, ensure the quality of the concrete during mixing, shipping, placing and curing, and solve certain problems that arise during concrete operations.
Basically, chemical admixtures are added to the mixture of cement, sand and aggregate in the concrete at the time of mixing. They are generally available in a liquid form.
Commonly, admixtures are classified into a number of different categories: Accelerators (A), Retarders (R), Water-reducing admixtures (W), Super plasticizers, Air entraining agents, and Corrosion inhibitors.
Accelerators – When added to concrete, mortar or grout, an accelerator accelerates the hydration of hydraulic cement and reduces the setting time. It can also accelerate the hardening or strength development of concrete and allow cold-weather concreting, early removal of formwork, surface finishing, and load application.
Retarders – When added to concrete, a retarding admixture slows the initial rate of hydration and extends the setting time. These admixtures are used to prevent the rapid stiffening of concrete that can result in difficulty in placement, consolidation and finishing.
Water-reducing admixtures – These admixtures are used to reduce the water demand in concrete, which can help the environment by lowering carbon dioxide emissions and energy consumption per unit volume of concrete. These admixtures are generally used in bridge decks, low-slump concrete coverings and repair applications.