Cement is used, most commonly, to permanently shut off water penetration to a given area. The first step in cementation is preparing the cement. It is crucial to know how much cement you will need for the job. It is also important to identify the proper set cement according to how dense you need the cement and the viscosity of the cement required. There are special mixers such as, hydraulic jet mixers, re-circulating mixers or batch mixers – these are all used to combine dry cement with water and create wet cement, otherwise known as slurry. Accelerators or retardants can be added to the cement to either decrease or increase the time it takes to set the cement. Cementation is possible according to a chemical reaction that ultimately yields the dry cement. First calcium oxide is produced by lime calcination which is possible at temperatures above 825 degrees Celsius at atmospheric pressure for ten hours. Then the calcium oxide is then mixed with water to make slaked lime. Slaked lime is an inorganic compound and appears as a colorless crystal or white powder. The excess water in the slaked lime is evaporated and the setting process takes place. There are forms of cement that harden underwater when constantly exposed to wet weather and this tends to be the hardest forms of cement. However when cement is placed above water the cement must set. The entire setting process is the chemical reaction that produces the cement, this reaction takes a significant amount of time due to the partial pressure of carbon dioxide. Typically the setting process requires a minimum time of 24 hours to complete. Once this is exposed long enough in the air the substance then becomes extremely hard and dense. Cementation is a chemical reaction that is used widely throughout the world for a variety of construction purposes because cement is a durable substance.