Abstract: The invention provides a process of producing lightweight concrete units, which may or may not be reinforced, which comprises making a mix including cementitious material, fine and, optionally, coarse aggregate, water and material for aerating the mix, preferably aluminium powder and alkali; inserting the mix into a mould and allowing the mix to set into a block in the mould; stripping the mould, and autoclaving the block. In this process, in accordance with the invention, the mix is inserted into the mould before the aeration of the mix is completed and without completely filling the mould, the mould is provided with a closure which allows gas but not solids to escape, and the mix is allowed to expand to fill the mould and set under pressure produced by its own aeration.