Limestone (primary source of calcium carbonate CaCO3) and clay (primary source of silica SiO2, alumina Al2O3 and iron oxide Fe2O3) are typically mined in company-owned quarries and pre-blended to a target chemical material composition. Corrective materials like sand, iron ore, bauxite or industrial waste materials (alternative raw materials) are then used to fine-tune and correct the chemical composition of this pre-blend material in the raw mill. Fine ground raw meal is then stored and further homogenized in the raw meal silo.
The heart of the cement manufacturing process is the thermal process in the kiln system with clinker capacities of up to 12,000 tonnes per day nowadays.
A kiln system consists of a static preheating system with typically five cyclone stages and a pre-calciner. The raw meal (so-called “kiln feed”) is heated up to 1,000°C and the calcium carbonate portion is calcined: CaCO3 -> CaO + CO2 (g). In the attached rotary kiln, the material is then heated up to 1,450°C and Portland cement clinker is formed.
Clinker is an assembly of four artificial minerals (alite, belite, tricalcium aluminate and ferrite) which have hydraulic properties, i.e. they harden when mixed with water.
Fast cooling and freezing of these artificial minerals then happens in the clinker cooler, the third element of a cement kiln system. A crucial ingredient to make this mineral transformation happen is the fuel. Historically oil and gas were used, then coal and petcoke, but since the 1980s more and more alternative fuels are employed for heating. Here it must be highlighted that the fuel ash is incorporated fully in the clinker so the final clinker composition is always the sum of the calcined raw meal and the remaining fuel ash after burn-out of the fuel. Clinker is the important intermediate product stage in cement manufacturing and is also traded worldwide as a commodity between cement producers.
Cement is produced by mixing and fine-grinding clinker, gypsum and mineral components (e.g. limestone, pozzolana, fly ash or granulated blast furnace slag, silica fume) in cement mills.
Gypsum is added to control cement setting, i.e. the hardening process. Mineral components are added to reduce the clinker portion in cements and introduce special product properties. Pure Ordinary Portland Cements (OPC) contain only clinker and gypsum; blended cements are the ones with mineral components.