Method and apparatus for moulding hydraulic cement or the like material

A method of moulding a hydraulic substance comprising aggregate and a cement and water mixture, utilizing an injection zone, a moulding zone and an overflow zone, said overflow zone being separate from the moulding zone, which comprises filling the moulding zone with aggregate, reducing the pressure in the injection zone, in the moulding zone and in the overflow zone to a vacuum, introducing said cement and water mixture into the injection zone, transferring said mixture from the injection zone into the moulding zone in sufficient excess quantity to cause said mixture to overflow from the moulding zone into the overflow zone, returning the overflow zone to at least atmospheric pressure whereby at least a portion of said excess quantity of said mixture in the overflow zone is returned to the moulding zone to eliminate the voids between the aggregate in the moulding zone created by removing the excess water and entrained air under reduced pressure, and replacing said voids with said cement and water mixture to produce a substantially void-free product, said mixture being maintained in sufficient quantity in said overflow zone to avoid the introduction of air into the moulding zone.