Made from crushed limestone (or chalk) and clay then mixed with water to produce a slurry. The slurry is pumped into a long rotary kiln and heated to 1400 deg.C. which first dries it then burns it to a white hot clinker which, after cooling, is ground in a ball mill. Ordinary Portland Cement (O.P.C.) has a final set of 12 hours. and reaches its working strength after 28 days. Rapid-hardening Portland Cement is made by a finer grinding of the clinker and has an advantage in its earlier gain of working strength.