Quicklime (an impure calcium oxide) is obtained by calcining limestone containing very little or no clay. It combines very rapidly with water, giving off considerable heat and producing slaked lime (calcium hydroxide). Slaked lime is usually employed for soil improvement or in the sugar industry. Hydraulic lime is obtained by low temperature calcination of limestone containing sufficient clay (although usually less than 20 %) to ensure that the product sets under water. Hydraulic lime differs from natural cement in that it still contains appreciable amounts of uncombined quicklime, which may be slaked with water. The heading excludes purified calcium oxide and calcium hydroxide (heading 28.25).
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