Secondary fuel can be a very inhomogeneous sample, containing all types of polymers, paper, wood and low fractions of metal. Also recycled wood or parts of automobile tires might be used as secondary fuel. Before using it as fuel in the production of cement, several chemical analyses are necessary as quality control.
Cement is made by heating limestone (calcium carbonate), with small quantities of other materials (such as clay) to 1450°C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement', the most commonly used type of cement.