Walls built of Blocks as opposed to Bricks; may be used externally, with a rendered finish, but more widely for internal walls. Normal 450mm x 225mm concrete blocks can vary in thickness from 50mm to 150mm (hollow blocks up to 225mm thick) but these have been mainly replaced by lightweight blocks to reduce weight; improve thermal insulation, and reduce costs. Lightweight Blocks may be made from fly-ash mixed with cement; a foamed aerated concrete or concrete expanded by added aluminium powder. Also, Hollow Terra-Cotta Blocks which may be smooth faced or keyed for plastering.