Abstract: A method for manufacturing a high impart strength carbonate building element is disclosed. Solids and water are mixed to produce a slurry in which the solids consist essentially of at least one alkali earth oxide or hydroxide and an amount of cellulose fibers in the range of 7-40% by weight of solids. The slurry is formed into a gas permeable shape having water-containing voids and a porosity in the range of 35-50%. Carbon dioxide is then caused to permeate into the shape through the voids to convert the hydroxide to a carbonate, thereby imparting high impact strength to the resulting element. This converting step is performed in a short time period, the duration of which depends on the percentage of carbon dioxide present in the processing atmosphere. This time period is about 30 minutes for an atmosphere of 100% carbon dioxide.