A concrete masonry unit (CMU) [US], concrete block, cement block or founation block [US] is a large rectangular brick used in construction. Concrete blocks are made from cast concrete, i.e. Portland cement and aggregate, usually sand and fine gravel for high-density blocks. Lower density blocks may be produced by using cinders as an aggregate, and are known as cinder blocks [US], breeze blocks [UK] or clinker blocks (if bottom ash or clinker is used as an aggregate). Concrete blocks that do not contain cinders are often mistakenly called cinder or breeze blocks in everyday speech. Lightweight blocks can also be produced using aerated concrete.