A high strength lightweight product formed from particulate inorganic materials such as a masonry building block is formed in a mould (1) from a flowable slurry of inorganic particularly materials and water. The interior of the mould is lined with perforate walls (2a, 3a, 4a, 3) fluidically coupled to a vacuum source (9). The fluid slurry contains entrained air and when pumped into the mound under superatmospheric pressure the gas bubbles contract in size. The fluidic mass is then subjected to subatmospheric pressure which enables the gas bubbles to expand the expel excess moisture from the mass.