A method for preparing a slag having hydraulically binding properties on the basis of a mixture comprising limestone, fly ash and any additives, which starting materials are mixed, melted and pulverised. It is an object of the present invention to provide a method for preparing a slag having binding properties by passing granules having a desired composition through a cylindrical vertical heat exchanger and supplying said granules to a melting furnace. The present invention is in particular suitable for use in the modification of shaft furnace of cement factories.