Abstract: Box-like units having at least one open side for use, for example, in the construction of prefabricated buildings are cast from a composition of gypsum and water optionally including other material such as Portland or Aluminous cement. The mixture is cast between an external mould defining the external faces of the unit and an internal core defining the internal faces of the unit and the core is formed from, or covered with, material which causes its faces which come into contact with the mixture to have a higher co-efficient of heat reflection than that of the faces of the external mould which come into contact with the mixture. The mixture is allowed to set and the external mould is removed, after which the mixture is allowed to harden further and during this hardening becomes heated by the exothermic reaction between the gypsum and the water to cause the hardened mixture to expand away from the core. This allows the core to be subsequently removed without it being necessary to make the core collapsible.