• Chemically, cement is a mixture of calcium silicates and small amounts of calcium aluminates that react with water and cause the cement to set.
• Calcium derives from limestone and clay, mudstone or shale as the source of the silica and alumina.
• The mix is completed with the addition of 5% gypsum to help retard the setting time of the cement.
‘Portland’ cement et al
• ‘Portland’ is the most widely produced cement. The name comes from its presumed resemblance to Portland stone.
• Other cements include: rapid-hardening, low-heat, sulfate-resisting and low-alkali.
• Increasingly cements are blended with ‘cement substitutes’ such as Pulverised Fuel Ash (PFA), aka ‘Fly ash’ and Ground Granulated Blast-furnace Slag (GGBS). The blends aim at reducing the overall environmental impact of using 100% cement. (see below)
The basic minerals used to make cement
• Cement producers usually locate their plants next to limestone deposits.
• Limestones of varying geological ages are distributed across the UK. They vary considerably in their chemistry and thickness and their suitability for cement manufacturing
• Carboniferous limestones are the major source of raw material in Britain. The other main limestones are Cretaceous (Chalk) and Jurassic.
• Chalk is porous and often has high moisture content that leads it to its use in the ‘semi-dry/wet’ manufacturing process of making cement. This particular process represents some 16% of total production.
• Shale is a pure sedimentary rock made of very fine silt, clay and quarz. Shale falls in the category of mudstones. Its grain size is less than 1/256mm. Shale is distinguished from other mudstones because it is fissile and laminated. Well into the 20th century, the words shale and slate could be interchangeable.
• Clays, mudstones and shales are very widely distributed in the UK. They occur in formations that may be several hundred metres thick. ( British Geological Survey CR/03/281N)
• Gypsum is a soft sulfate mineral composed of calcium sulfate dehydrate
• The largest and commercially most important deposits of gypsum and anhydrite occur as beds, which may persist over considerable areas with little change in quality or thickness. They are frequently interbedded with limestones, shales, mudstones, clays, dolomite, rock salt and locally sylvite.
• Although gypsum is widely distributed throughout England, and has historically been exploited in a large number of locations, mining now occurs in Cumbria, Staffordshire; Nottinghamshire (open cast), Leicestershire and East Sussex;
• About 20% of gypsum goes towards cement production.
And … Cement Substitutes
A number of by-products from other industries can be blended with Portland Cement (CEM I) which can improve performance but also increase the recycled content and reduce the ECO2 content of the concrete. The use of these secondary materials utilises material which might otherwise be disposed in landfill. Read MORE about cement substitutes.....
Cement Manufacturing Process
Materials are extracted / quarried / recovered and transported to the cement plant.
2 Crushing and milling
The raw materials, limestone, shale, silica and iron oxice are crushed and milled into fine powders.
3 Mixing and preheating
The powders are blended (the ‘raw meal’) and preheated to around 900° C using the hot gases from the kiln. The preheating burns off the impurities.
Next the material is burned in a large rotary kiln at 1500° C. Heating starts the de-carbonation where CO2 is driven from the limestone. The partially fused resulting is known as clinker. A modern kiln can produce around 6000 tons of clinker a day.
CaCO3 (limestone) + heat -> CaO (lime) + CO2
5 Cooling and final grinding
The clinker is then cooled and ground to a fine powder in a tube or ball mill. A ball mill is a rotating drum filled with steel balls of different sizes (depending on the desired fineness of the cement) that crush and grind the clinker. Gypsum is added during the grinding process to provide means for controlling the setting of the cement.
The cement is bagged transported for concrete production.
A decent and informative film if you can get past the first inebriated 20 seconds.