<P> A concrete masonry unit (CMU) is a standard size rectangular block used in building construction . </P> <P> Those that use cinders (fly ash or bottom ash) are called cinder blocks in North America, breeze blocks (breeze is a synonym of ash) in the United Kingdom, and hollow blocks in the Philippines . In New Zealand they are known as concrete blocks or more commonly as just blocks . In Australia they are also known as Besser blocks or Besser bricks, because the American - based Besser Company was a major supplier of concrete - block - making machinery . Clinker blocks use clinker as aggregate . In non-technical usage, the terms cinder block and breeze block are often generalized to cover all of these varieties . </P> <P> Concrete blocks are made from cast concrete (e.g. Portland cement and aggregate, usually sand and fine gravel, for high - density blocks). Lower density blocks may use industrial wastes, such as fly ash or bottom ash, as an aggregate . Lightweight blocks can also be produced using autoclaved aerated concrete . </P>

Large concrete or cement masonry units for building