Slag is a by-product of steel. It contains about 30-35 per cent calcium oxide, 35-38 per cent silica, 10-18 per cent aluminium oxide and 7-10 per cent magnesium oxide. Slag, generated from manufacture of pig iron, is granulated and called blast furnace slag. It is non-biodegradable. One tonne of slag saves around 3.30 GJ (giga-joule) thermal energy, or energy obtained from 99 litres of petrol, and 55.5 kWh electrical energy, or energy obtained from 6 litres of petrol.