Cement is a complex mix of many chemical compounds. Of these, the principal four are:
Alite is the compound that gives cement its immediate strengthening property, whereas belite provides material strength over longer periods of time.
These compounds are obtained by a process known as clinkerization, which involves heating a mix of calcium oxide (CaO), silicon oxide (Si2O2), aluminum oxide (Al2O3) and ferric oxide (Fe2O3) at temperatures over 1400 Celsius to produce clinker, the main ingredient of cement.
The four oxides required to supply the clinkerization process can be obtained by raw materials like limestone, schist, clay, dolomite, silica sand, bauxite, wet fly ash and iron ore. These are obtained by quarrying. Silica oxide, aluminum oxide and ferric oxide are readily found in their respective raw materials, but calcium oxide must be extracted from calcium carbonate (CaCO3) found in limestone. This is achieved by a process known as calcination, which absorbs heat energy and releases calcium carbonate and carbon dioxide (CO2). Calcination occurs in a production stage before clinkerization, where the raw materials, having being mixed an milled into powder are being preheated to gradually reach kiln temperatures.
Clinker, which is produced in the form of sintered lumps, is then mixed with a small proportion of gypsum and milled into powder to produce cement. The addition of gypsum slows down cement’s hydration process to make it workable in construction, but does not alter cement’s base chemical properties. In some occasions the mix is diluted with clinker substitutes such as pozzolana to produce cement types with inferior chemical characteristics.