Lightweight Concrete Blocks

Lightweight concrete blocks have been available for as long as dense concrete blocks and were initially made to be lighter and easier to handle than their dense counterparts. They were manufactured using a wide range of lighter aggregate materials instead of fine and coarse natural aggregates. The aggregate materials – fuel ash, furnace clinker, coke (for ‘breeze blocks’), and slag from iron and steel blast furnaces – were by-products of industrial processes, so local availability would determine how the blocks were made in different areas of the UK. A number of these aggregates, such as blast-furnace slag, contained significant quantities of hydraulic lime meaning that smaller amounts of cement were required to make lightweight blocks. Depending on the aggregate being used and the strength of block required, the aggregate:cement ratio would typically range from 6:1 to 8:1. With the reduced cement content and the use of industrial by-products, lightweight blocks are much cheaper to produce.

Depending on their density and strength, lightweight blocks are used for load-bearing and non-load-bearing walls, partitions and cavity walls. Despite being referred to as ‘lightweight’, they are still reasonably heavy (around 10kg).

Since the late 1960s, much greater emphasis has been placed on the thermal insulation qualities of the lighter-weight blocks than on their ease of handling. As a result, many of the older-style blocks have been superseded by blocks made of aerated concrete (‘aircrete’).

Aerated concrete blocks are classified as a lightweight block but the unique manufacturing process, the properties of the finished block and their extensive use make them worthy of separate consideration.

Lightweight Concrete Blocks

Fig. 37 Aerated concrete block.

Aerated concrete blocks have a very low density of


475kg/m, making individual blocks very lightweight indeed (a standard solid 100mm block weighs around 4.5kg).