The main raw materials used in cement manufacture are limestone, shale, clay, sand and iron ore. These natural materials are extracted from mines and transported to the cement plant, where they are blended to manufacture the required cement quality. Environmental issues arising from the handling of raw materials, including dust and stormwater runoff, need to be managed at both the mine and cement plant. The industry is uniquely positioned to responsibly use waste products as an alternative to natural raw materials. This reduces the environmental impacts of mining and landfill.
Precisely combining the elements calcium, silica, aluminium and iron (found in the raw materials) is essential to ensure the target chemistry for clinker is achieved. Once the correct balance of raw materials has been achieved, the ‘raw mix’ is very .finely ground into ‘raw meal’. Raw meal is heated to 1000ºc extremely quickly, ‘calcining’ the calcium carbonate in limestone to calcium oxide before being fed into a rotary kiln. This process is heated by a precalciner burner.
In the rotary kiln tube, the feed moves slowly as the tube rotates, becoming hotter as it approaches the kiln flame, reaching a temperature of 1450ºc. At this temperature a calcium silicate matrix called ‘clinker’ is formed. The clinker is then cooled rapidly and stored. No ash is produced. Materials fed into the process become part of the clinker, ultimately forming an intrinsic matrix in which the materials are fixed and immobile. High temperature, long retention times and complete combustion are strengths of this process.
The continuous process and strict, highly technical control ensures the conversion of the materials into a new product. This is vital to produce cement clinker with a mineralogical structure that is highly reactive with water. To conserve natural resources and reduce CO2 emissions, alternative fuels such as waste tyres, timber, oils and other materials are used.
The combustion of fuels, and the conversion of limestone to clinker releases emissions such as CO2, metals, oxides of nitrogen (NOx) and dust which are monitored and regularly reported as part of each site’s license to operate. Manufacturing sites in Australia monitor emissions as key environmental indicators.
Once cooled, clinker is ground with gypsum and other minerals, such as limestone, to produce the grey powder commonly recognised as cement. Changing the chemical composition of the raw mix enables cements with different properties to be produced for different uses. Cement can also be blended with supplementary cementitious materials (SCMs), such as flyash and slag, to make blended cements.
Cement is then pumped to silos for storage and dispatch. Mineral additions and SCMs are added to reduce the amount of clinker in cement. This in turn reduces the fuel, power, process emissions and raw materials associated with cement production. At the same time it creates a use for material considered to be a waste product of other industries.
Cement and SCM are distributed by road, rail or sea.