An improved system for slaking of lime which does not rely on the weighing of materials delivered to the vessel where the mixing takes place. A control system is utilized to control the volume of lime and water delivered to insure that a preselected temperature is obtained during the chemical reaction in the vessel. In a batch process the volume of lime and water is controlled based on the temperature sensed to maintain the temperature at the desired level. Once the reaction is complete the resultant slurry is discharged from the vessel for further processing. The vessel is then cleansed and prepared for the next batch.