Autoclaved aerated concrete block (AAC), popularly called lightweight concrete, is an environmentally friendly building material that has been used widely in Europe since the 1920s. It uses no material larger than sand; the chemical reaction of aluminium and calcium hydroxide produces bubbles of hydrogen that double the volume of a block before the bubbles escape into the atmosphere. The lightweight blocks are then hardened under pressure at a temperature of 190 degrees Celsius. These blocks have numerous advantages over the heavier concrete blocks and a few disadvantages.
The initial cost of lightweight concrete blocks in the U.S. is higher than conventional blocks because American manufacturing facilities have historically produced the heavier blocks. Only a handful of companies produce AAC blocks, although that number is increasing and expected to continue to grow. As more AAC blocks are produced in the future, the cost will go down. The higher initial costs are offset to some extent by the cheaper costs of shipping and handling lighter blocks. The shipping costs increase as the distance from the production facilities increase.
Design review boards, planning commissions and building departments in the U.S. that review and pass building codes are not familiar with AAC blocks. They wrote their regulations for traditional concrete blocks and in some areas there may be resistance to change because of habit and political benefits of continuing arrangements with local suppliers of standard blocks. These entities need to be educated so that lightweight AAC blocks might be incorporated into local building codes.
Masons are used to applying cement-based mortar between heavy blocks. They have to adjust to applying thin-set mortar to lightweight blocks, a skill that requires more precision.
ACC blocks are acceptable for load bearing walls on low-rise buildings but are less suited to high-rise buildings.