cement admixtures are added to concrete mixes to modify the workability and placement of the material. They can also enhance the strength and durability of concrete, reduce the water needed in a mix and improve finishability. The type of admixture used in a mix depends on the project requirements and the concrete’s intended end use. cement admixtures can be classified according to their main action in concrete as follows:
Water-reducing admixtures improve the workability of mixes with low w/c ratios without increasing permeability. This class of admixture can help contractors save time and money on jobs where pours must be completed within a tight schedule or when the job site is in a warm climate that hastens curing.
Accelerating admixtures speed up the rate at which concrete strengthens over time and shorten the setting time. They are usually formulated with chemical gas-forming agents such as aluminum powder, activated carbon and hydrogen peroxide, and are based on sodium silicate, calcium chloride or potassium hydroxide.
Shrinkage reducing admixtures reduce the amount of shrinkage cracking that occurs during the drying process, making them ideal for projects where shrinkage cracking could lead to structural problems. They are generally formulated with chemical compounds such as calcium sulfate, aluminum sulfate and zinc sulfate.
Plasticizers based on lignosulphonate can lubricate cement particles by reducing the electrostatic force between them, allowing for increased workability. This class of admixture is unpredictable in terms of its effects at different dosages.