Lightweight blocks

These incorporate a variety of lightweight aggregates and are generally used for the internal skins of cavity walls. Although slightly more expensive than ordinary dense blocks, they have much better levels of insulation and are light and easy to handle. The level of thermal insulation varies depending on the type of aggregate used. However, because of their low mass they do not provide very good sound insulation and are not always suitable for party walls.

In modern construction a particular type of lightweight block made from autoclaved aerated concrete (AAC), generally known as aerated blocks, have become very common. Aerated blocks are made from cement, lime, sand, pulverised fuel ash and aluminium powder. Once these materials are mixed with hot water the aluminium powder reacts with the lime to form millions of tiny pockets of hydrogen. These subsequently diffuse from the material to be replaced by air.

Aerated blocks are easy to handle and provide good thermal insulation. They are very easy to cut and shape and will directly take the fixings of screws as well as nails. Some aerated blocks are not suitable for party walls because of their low density, although in recent years a number of manufacturers have produced aerated blocks specifically designed for this purpose. In modern cavity walls, with their emphasis on high levels of thermal insulation, aerated blocks are by far the most common method of forming the internal leaf.