AAC is a lightweight cellular material which is formed by a chemical reaction between finely divided calcareous and siliceous materials. The calcareous component is usually lime and/or cement. The siliceous component usually consists of natural or ground sand and/or industrial by-products such as slag and pulverized fuel ash (PFA), also known by fly ash. The cellular structure is achieved by either a chemical process causing aeration, or by introducing air voids by mechanical means into a slurry which contains no coarse material The aeration is caused by the addition of a small quantity of acidic aluminium paste that reacts with the alkaline lime &; cement to expand the mixture.