Portland cement is manufactured from calcium carbonate in the form of crushed limestone or chalk and an argillaceous material such as clay or marl.
Minor constituents such as iron oxide or sand may be added depending upon the composition of the raw materials and the exact product required.
In principle, the process involves the decarburization of calcium carbonate (chalk or limestone) by expulsion of the carbon dioxide, and sintering, at the point of initial fusion, the resulting calcium oxide (lime) with the clay and iron oxide. Depending upon the raw materials used and their water content at extraction, four key variations in the manufacturing process have been developed.
These are the wet, semi-wet, semi-dry and the dry processes.
(Sintering is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction).
Wet process
The wet process, which was the precursor to the other developments, is still used in some areas for processing chalk and marl clay. Clay is mixed with water to form slurry, when any excess sand is removed by settlement.
Equivalent slurry is prepared from the chalk which is then blended with the clay slurry, screened to remove any coarse material, and stored in large slurry tanks.
After final blending, the slurry is fed into the top of large slowly rotating kilns. The kilns, which are refractory brick-lined steel cylinders up to 200 m long, are fired to approximately 1450°C, usually with pulverized coal.
The slurry is dried, calcined and finally sintered to hard grey/black lumps of cement clinker.
(A refractory material is a material that retains its strength at high temperatures).
A major development in energy conservation has been the elimination or reduction in the slurry water content required in the manufacturing process, as this consumed large quantities of heat energy during its evaporation.
Semi-wet process
In the semi-wet process, chalk is broken down in water and blended into a marl clay slurry. The 40% water content within the slurry is reduced to 19% in a filter press; the resulting filter-cake is nodularised by extrusion onto a travelling preheater grate or reduced in a crusher/dryer to pellets. Heating to between 900°C and 1100°C in tower cyclones precalcines the chalk; the mix is then transferred to a short kiln at 1450°C for the clinkering process.
Semi-dry process
In the semi-dry process, dry shale and limestone powders are blended. About 12% water is added to nodularise the blend, which is then precalcined and clinkered as in the semi-wet process.
Dry process
In the dry process limestone, shale and sand (typically 80%, 17% and 3% respectively), are milled to fine powders, then blended to produce the dry meal, which is stored in silos. The meal is passed through a series of cyclones, initially using recovered kiln gases to preheat it to 750°C, then with added fuel to pre-calcine at 900°C, prior to passage into a fast rotating 60m kiln for clinkering at 1450°C. In all processes an intimately mixed feedstock to the kiln is essential for maintaining quality control of the product.
Most plants operate primarily with powdered coal, but additionally other fuels including petroleum coke, waste tyre chips, smokeless fuel plant residues, or reclaimed spoil heap coal are used when available. Oil, natural gas and landfill gas have also been used when economically viable. The grey/black clinker manufactured by all processes is cooled with full heat recovery and ground up with 5% added gypsum (calcium sulfate) retarder to prevent excessively rapid flash setting of the cement.
The Portland cement is stored in silos prior to transportation in bulk, by road or rail. The standard bag is 50 kg for reasons of health and safety.
COMPOSITION OF PORTLAND CEMENT
The starting materials for Portland cement are chalk or limestone and clay, which consist mainly of lime, silica, alumina and iron oxide. Table below illustrates a typical composition.
Minor constituents, including magnesium oxide, sulfur trioxide, sodium and potassium oxides amount to approximately 2%. During the clinkering process, these compounds react together to produce the four key components of Portland cement.
The relative proportions of these major components significantly affect the ultimate properties of the cements and are therefore adjusted in the manufacturing process to produce the required product range. Typical compositions of Portland cements are shown in Table below.
SETTING AND HARDENING OF PORTLAND CEMENTS
Portland cement is hydraulic; when mixed with water it forms a paste, which sets and hardens as a result of various chemical reactions between the cementitious compounds and water. Only a small proportion of the added water is actually required for the chemical hydration of the cementitious constituents to hydrated calcium silicates.
The additional water is needed to ensure the workability of the mix when aggregates are added. So that concrete, for example, can be successfully placed within formwork containing steel reinforcement. Water in excess of that required for hydration will ultimately evaporate leaving capillary pores in the concrete and mortar products. Typically, an increase in void space of 1% reduces crushing strength by 6%. It is therefore necessary to control carefully the water content of the mix by reference to the water/cement ratio.
A minimum water/cement ratio of 0.23 is required to hydrate all the cement, although as the cement powder is hydrated it expands, and thus a ratio of 0.36 represents the point at which cement gel fills all the water space. However, a water/cement ratio of 0.42 more realistically represents the minimum water content to achieve full hydration without the necessity for further water to be absorbed during the curing process.
The setting and hardening processes should be distinguished. Setting is the stiffening of the cement paste, which commences immediately the cement is mixed with water. Because the major cementitious constituents set at different rates it is convenient to refer to initial set and final set. Typically, initial set, or the formation of a plastic gel, occurs after one hour and final set, or the formation of a rigid gel, within 10 hours. The setting process is controlled by the quantity of gypsum added to the cement in the final stages of production.
Hardening is the gradual gain in strength of the set cement paste. It is a process which continues, although at a decreasing rate, over periods of days, months and years. The rate of hardening is governed partially by the particle-size
Distribution of the cement powder. Finely ground cement hydrates more rapidly, and therefore begins to set and harden more quickly. Furthermore, the relative proportions of tricalcium silicate and dicalcium silicate have a significant effect upon the rate of hardening.
TYPES OF CEMENT
Cements are classified primarily on the main constituents such as Portland cement or blast furnace cement. (In addition there may be minor constituents up to 5% and also additives up to 1% by weight.)
• CEM I Portland cement
•CEM II Portland-composite cement
• CEM III Blastfurnace cement
• CEM IV Pozzolanic cement
• CEM V Composite cement
Within these five main types of cement a wide range of permitted additional constituents, including silica fume, natural or industrial pozzolanas, calcareous or siliceous fly ash and burnt shale, may be incorporated.
Other types - Masonry cements
Portland cement mortar is unnecessarily strong and concentrates any differential movement within brickwork or block work into a few large cracks. These cracks are unsightly and may increase the risk of rain penetration.
Masonry cement produces a weaker mortar, which accommodates some differential movement and ensures a distribution of hairline cracks within joints. Masonry cements contain water-retaining mineral fillers, usually ground limestone, and air-entraining agents to give a higher workability than unblended Portland cement.
They should mix with building sand in ratios between 1:4 and 1:6½ depending upon the degree of exposure of the brick or block work. The air entrained during mixing increases the durability and frost resistance of the hardened mortar. Masonry cement is also appropriate for use in renderings but not for floor screeds or concreting. It is therefore generally used as an alternative to Portland cement with hydrated lime or plasticiser.
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