Abstract: A process for the preparation in an aqueous medium of purified calcium sulfate, with high whiteness, characterized in that it comprises:a) the formation of an aqueous solution by dissolution of impure hemihydrated calcium sulfate, at a concentration at most of 13.0 grams per liter (expressed in dissolved CaSO.sub.4) and at a pH of at least 5.

Abstract: A process for manufacturing calcium silicate and/or aluminate based products, such as Portland cement or aluminous cements. The raw material subjected to a first heating step in which it is heated in a preheater-decarbonator and/or a rotating kiln to a temperature less than that at which impurities, such as alkalines, chlorine and sulfur, are volatized. The heated raw material is then subjected to a second heating step in the presence of a reducing gas, preferably in a plasma furnace, the raw material being heated to a temperature higher than the temperature at which the impurities are volatilized, as for example above 1950.degree. C., and more than 80% of the material is in fusion, whereupon the material is cooled to produce the clinkers of improved quality and/or with lower energy consumption. An important feature of the invention is the utilization of the gases generated during the second heating step and during cooling to implement heating of the raw material.