Limestone kiln. This is a vertical-shaft kiln with a conveyor belt (top left) to add raw materials to the kiln. Smoke (top) forms as limestone (calcium carbonate) is heated until it dissociates to form lime (calcium oxide) and carbon dioxide. Lime is used to make cement in the construction industry. It is also converted to slaked lime (calcium hydroxide), an alkaline material used in farming to neutralise acidic soils. Photographed in Seilles, Belgium.