Abstract: A process for the manufacture of a non-asbestos corrugated sheet comprising forming into corrugated form an aqueous slurry of a mixture comprising, on a dry weight basis, 40-60% cement, 30-40% silica and 5-15% cellulose fibres, compressing said corrugated form so produced in a press to reduce its thickness and increase its density, removing said form from said press and submitting said form to autoclaving to cause reaction to occur between said cement and said silica to form a binder matrix. The aqueous slurry may include a moldability aid such as finely divided silica, and the corrugated forms are suitably compressed individually and then passed through a steam tunnel partially to harden them before autoclaving. In corrugated sheets formed are suitable for external cladding and roofing.