There are essentially two types of concrete block: aggregate blocks, which as the name suggests contain aggregates; and aircrete blocks, made from a type of concrete that contains air pores and has no aggregates larger than ground sand.
Blocks offer a number of useful attributes, such as good acoustic performance, fire and flood resistance, and thermal mass, which helps provide a stable year-round temperature. From an environmental perspective, they are locally produced and can have a high recycled content, giving them low embodied CO2.
Members of the Concrete Block Association and Aircrete Product Association are signatures of the British Precast sustainability charter. This includes a requirement for annual company third party sustainability audits. They are also part of the Concrete Industry Sustainable Construction Strategy.
Aggregate blocks come in a wide range of densities and compressive strengths, which are grouped into the three categories of ultra-lightweight, lightweight and dense. Ultra-lightweight and lightweight blocks are manufactured from cement together with one of a variety of natural or man-made expanded aggregates. These include: granulated or foamed blast-furnace slag, expanded clay, pumice, or shale, furnace bottom ash and fly ash from power stations.
Invented in the mid-1920s, aircrete blocks are made from cement, lime, sand, fly ash and water, which are mixed to form slurry. A small amount of aluminium powder is then added, which reacts with the lime to form tiny bubbles, causing the mixture to expand into a ‘cake’. When the mixture is partially set, it is cut to form blocks and then cured in a steam autoclave.