A green concrete mortar is prepared by the steps of preparing a first composition in which water is uniformly deposited about the particles of sand, preparing a second composition consisting of a powder of cement, mixing together the first and second compositions, thus forming stable shells about the particles of the sand, the shells having substantially a constant ratio of water to cement, and adding water to the shelled sand and then kneading the resulting mixture. The green concrete mortar can be poured into a moulding frame prepacked with a coarse aggregate i.e. gravel. After incorporating gravel into the mortar, the resulting green concrete can be moulded or blasted. The green concrete can be readily conveyed to a working station for constructing tunnels, buildings or the like.