Limestone rock products. Clockwise from lower left these are: limestone rocks; lime; hydrated lime powder; finely crushed limestone. Limestone is calcium carbonate. Lime is calcium oxide. Lime is formed by heating limestone until it dissociates into calcium oxide and carbon dioxide. Limestone can be ground up (milled) for other uses, or to improve the lime-manufacturing process. Water will react with lime to produce hydrated lime (calcium hydroxide), which is also called slaked lime. This alkaline material is used in farming to neutralise acidic soils. Lime is also used to make cement for the construction industry. Limestone can also be used as a building material.