Cement additives are added to the cement to improve the properties of cementing slurry. They are mainly divided into many kinds according to the associated effect. The following are the main four additives.
A) Accelerators are used to increase the early strengthen time. They are useful at shallow depths with temperatures less than 100F to decrease the time into 4 hours. ( e.g. sodium metasilicate & sodium chloride)
B) Retards are used to increase the pumping time of slurry. They are mainly affect the viscosity. (e.g. lignosulfonates)
C) Extenders are used to decrease the induced hydrostatic pressures on the weak formations by allowing the addition of more water to the slurry to lighten the mixture and keep the solids from separating. (e.g. bentonite)
D) Pozzolans are added to portland cement to decrease the cement slurry’s density and viscosity. They can be either a volcanic ash or a high silica clay. (e.g. Diatomaceous & Fly ashes )