A method and apparatus for the production of a hydraulic binder of the type used for plaster and masonry mortar within a cement producing installation including a preheating zone and a clinker calcining zone. In accordance with the invention, the cement producing plant is modified to the extent of providing for the removal of a predetermined portion of the material being processed from at least one point between the preheating zone and the clinker calcining zone wherein the withdrawn material has a predetermined condition of activation so that it is useful alone or in a mixture with other ingredients in the manufacture of the binder.