Process and apparatus is disclosed for providing a chemical reaction between calcium oxide containing grit particles to produce calcium hydroxide and heat, capturing the heat of hydration and using it to preheat water initially at ambient temperature, to rise to an elevated temperature to increase the amount of lime present in the water to a supersaturated lime suspension level, with the chemical reaction running to completion, followed by cooling. Heat from a water jacket may be used to raise the temperature in the lime slaker.