Abstract: The yield of cement clinker recovered from a kiln assembly is enhanced by feeding a particulate material comprising silica and an oxide of at least one of calcium and aluminum, for example, fly ash into contact with hot cement clinker; the fly ash melts to a partially fused material which chemically reacts with the hot clinker to produce a pyroprocessed cement clinker of crystalline hydraulic silicates; the extruder is added to the hot cement clinker downstream of the formation of the cement clinker, and suitably is added to the hot cement clinker at the upstream end of the cooler of the kiln assembly, or within the kiln, downstream of the burning zone, and more especially in the initial cooling zone at the discharge end of the kiln.