Cracks in the surface

THE cement industry is one of the world’s most polluting: it accounts for 5% of man-made carbon-dioxide emissions each year. Making this most useful of glues requires vast quantities of energy and water. Calcium carbonate (generally in the form of limestone), silica, iron oxide and alumina are partially melted by heating them to 1450°C in a special kiln. The result, clinker, is mixed with gypsum and ground to make cement, a basic ingredient of concrete. Breaking down the limestone produces about half of the emissions; almost all the rest come from the burning of fossil fuels to heat the kiln.