Before discussing the Process of manufacture of Cement, let’s first understand what Cement itself is. Cement is a binder material or a substance used for construction that sets, hardens, and adheres to other materials to bind them together.
Cement is seldom used on its own, but rather to bind sand (fine aggregate) and gravel (coarse aggregate) together.
Cement mixed with fine aggregate produces mortar for masonry, or sand and gravel produce Concrete. Concrete is the most widely used material in existence and is behind only water as the planet’s most-consumed resource.
The following are the stages of the Process of manufacturing Cement, i.e Portland Cement:
- crushing and grinding the raw materials,
- blending the materials in the correct proportions,
- burning the prepared mix in a kiln, and
- grinding the burned product, known as “clinker,” together with some 5 per cent of gypsum (to control the setting time of the cement).
In the Manufacture of Cement, the three manufacturing processes are known as the wet, dry, and semi-dry processes. They are so termed when the raw materials are ground wet and fed to the kiln as a slurry, ground dry and fed as a dry powder, or ground dry and then moistened to form nodules that are fed to the kiln.
It is estimated that around 4–8 per cent of the world’s carbon dioxide (CO2) emissions come from the manufacture of cement, making it a major contributor to global warming.
In the Manufacture of Cement, some of the solutions to reducing greenhouse gas emissions are common to other sectors, such as increasing the energy efficiency of cement plants, replacing fossil fuels with renewable energy sources, and capturing and storing the CO2 that is emitted.
In addition, given that a significant portion of the emissions is an intrinsic part of clinker production, novel cements and alternative formulations that reduce the need for clinker are an important area of focus.
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Table of Contents
Crushing and grinding
All except soft materials are first crushed, often in two stages, and then ground, usually in a rotating, cylindrical ball or tube mill containing a charge of steel grinding balls.
This grinding is done wet or dry, depending on the process in use. For dry grinding, the raw materials may first need to be dried in cylindrical, rotary dryers.
Soft materials are broken down through vigorous stirring with water in wash mills, producing a fine slurry that is then passed through screens to remove oversize particles.
Blending and Proportioning
A first approximation of the chemical composition required for a particular cement is obtained by selective quarrying and control of the raw material fed to the crushing and grinding plant.
Finer control is obtained by drawing material from two or more batches containing raw mixes of slightly different compositions. In the dry process, these mixes are stored in silos; slurry tanks are used in the wet process.
Thorough mixing of the dry materials in the silos is ensured by agitation and vigorous circulation induced by compressed air. In the wet process, the slurry tanks are stirred by mechanical means or compressed air, or both. The slurry, which contains 35 to 45 per cent water, is sometimes filtered, reducing the water content to 20 to 30 per cent, and the filter cake is then fed to the kiln. This reduces the fuel consumption for burning.
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Burning or Calcination of the Dry Mix
The earliest kilns in which cement was burned in batches were bottle kilns, followed by chamber kilns, and then continuous shaft kilns.
The shaft kiln, in a modernised form, is still used in some countries, but the dominant means of burning is the rotary kiln. These kilns—up to 200 meters (660 feet) long and six meters in diameter in wet process plants, but shorter for the dry process—consist of a steel, cylindrical shell lined with refractory materials.
They rotate slowly on an axis that is inclined a few degrees to the horizontal. The raw material feed, introduced at the upper end, moves gradually down the kiln to the lower, or firing, end.
The fuel for firing may be pulverised coal, oil, or natural gas injected through a pipe. The temperature at the firing end ranges from about 1,350 to 1,550 °C (2,460 to 2,820 °F), depending on the raw materials being burned.
Some form of heat exchanger is commonly incorporated at the back end of the kiln to increase heat transfer to the incoming raw materials and so reduce the heat lost in the waste gases.
The burned product emerges from the kiln as small nodules of clinker. These pass into coolers, where the heat is transferred to incoming air and the product is cooled. The clinker can be ground into cement immediately or stored in stockpiles for later use.
In the semidry process, the raw materials, in the form of nodules containing 10 to 15 per cent water, are fed onto a travelling chain grate before passing to the shorter rotary kiln. Hot gases coming from the kiln are sucked through the raw nodules on the grate, preheating the nodules.
Dust emissions from cement kilns can be a serious nuisance. In populated areas, it is common and often required to install cyclone arrestors, bag-filter systems, or electrostatic dust precipitators between the kiln exit and the chimney stack.
More than 50 per cent of the emissions from cement production are intrinsically linked to the production of clinker and are a by-product of the chemical reaction that drives the current process.
There is potential to blend clinker with alternative materials to reduce the need for clinker itself, thereby helping to mitigate the climate impacts of the cement-making process.
Modern cement plants are equipped with sophisticated instrumentation for controlling the burning process. In some plants, raw materials are sampled automatically, and a computer calculates and controls the composition of the raw mix. The largest rotary kilns have outputs exceeding 5,000 tons per day.
Grinding
In the Manufacture of Cement, the clinker and the required amount of gypsum are ground to a fine powder in horizontal mills similar to those used for grinding the raw materials.
The material may pass straight through the mill (open-circuit grinding), or coarser material may be separated from the ground product and returned to the mill for further grinding (closed-circuit grinding).
Sometimes a small amount of a grinding aid is added to the feed material. For air-entraining cement (discussed in the following section), the addition of an air-entraining agent is similarly made.
Finished cement is pumped pneumatically to storage silos from which it is drawn for packing in paper bags or for dispatch in bulk containers.
The significant types of cement: composition and properties
Portland cement
Chemical composition
In the Manufacture of Cement, Portland cement is made up of four main compounds: tricalcium silicate (3CaO · SiO2), dicalcium silicate (2CaO · SiO2), tricalcium aluminate (3CaO · Al2O3), and a tetra-calcium aluminoferrite (4CaO · Al2O3Fe2O3).
In an abbreviated notation differing from the standard atomic symbols, these compounds are designated as C3S, C2S, C3A, and C4AF, where C stands for calcium oxide (lime), S for silica, A for alumina, and F for iron oxide. Small amounts of uncombined lime and magnesia are also present, along with alkalies and minor quantities of other elements.
Hydration
In the Manufacture of Cement, the most critical hydraulic constituents are the calcium silicates, C2S and C3S. Upon mixing with water, the calcium silicates react with water molecules to form calcium silicate hydrate (3CaO · 2SiO2 · 3H2O) and calcium hydroxide (Ca[OH]2).
These compounds are given the shorthand notations C–S–H (represented by the average formula C3S2H3) and CH, and the hydration reaction can be crudely described by the following responses: 2C3S + 6H2O = C3S2H3 + 3CH2S + 4H2O = C3S2H3 + CH2S.
During the initial stage of hydration, the parent compounds dissolve, and the dissolution of their chemical bonds generates a significant amount of heat.
Then, for reasons that are not fully understood, hydration comes to a stop. This quiescent, or dormant, period is crucial in the placement of Concrete. Without a dormant period, there would be no cement trucks; pouring would have to be done immediately upon mixing.
Following the dormant period (which can last several hours), the cement begins to harden, as CH and C–S–H are produced.
This is the cementitious material that binds cement and Concrete together. As hydration proceeds, water and cement are continuously consumed. Fortunately, the C–S–H and CH products occupy almost the same volume as the original cement and water, with volume approximately conserved, making shrinkage manageable.
Although the formulas above treat C–S–H as a specific stoichiometry, the formula C3S2H3 does not form an ordered structure of the uniform composition. C–S–H is actually an amorphous gel with highly variable stoichiometry. The ratio of C to S, for example, can range from 1:1 to 2:1, depending on mix design and curing conditions.
Structural properties
In the Manufacture of Cement, the strength developed by Portland cement depends on its composition and the fineness to which it is ground.
The C3S is primarily responsible for the strength developed during the first week of hardening, while the C2S is responsible for the subsequent increase in strength.
The alumina and iron compounds, which are present in lesser amounts, make only a minor direct contribution to strength.
Cement and Concrete can suffer deterioration from attack by some natural or artificial chemical agents. The alumina compound is the most vulnerable to chemical attack in soils containing sulfate salts or in seawater, while the iron compound and the two calcium silicates are more resistant.
Calcium hydroxide released during the hydration of the calcium silicates is also vulnerable to attack. Because cement liberates heat when it hydrates, Concrete placed in large masses, as in dams, can cause the temperature inside the mass to rise as much as 40 °C (70 °F) above the outside temperature.
Subsequent cooling can be a cause of cracking. C3A shows the highest heat of hydration, followed in descending order by C3S, C4AF, and C2S.
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Types of Portland cement
Five types of portland cement are standardised in the United States by the American Society for Testing and Materials (ASTM): ordinary (Type I), modified (Type II), high-early-strength (Type III), low-heat (Type IV), and sulfate-resistant (Type V). In other countries, Type II is omitted, and Type III is referred to as rapid hardening. Type V is known in some European countries as Ferrari cement.
There are also various other special types of Portland cement. Colored cement is made by grinding 5 to 10 per cent of suitable pigments with white or ordinary grey portland cement.
Air-entraining cement is produced by the additional grinding of a small amount, about 0.05 per cent, of an organic agent that causes the entrainment of fantastic air bubbles in Concrete.
This increases the Concrete’s resistance to freeze-thaw damage in cold climates. The air-entraining agent can alternatively be added as a separate ingredient to the mix when making the Concrete.
Low-alkali cement is a type of portland cement with a total alkaline content not exceeding 0.6 per cent. These are used in Concrete made with certain types of aggregates that contain a form of silica that reacts with alkalies to cause an expansion that can disrupt the Concrete.
Masonry cement is used primarily for mortar. They consist of a mixture of portland cement and ground limestone or other filler together with an air-entraining agent or a water-repellent additive. Waterproof cement is a type of portland cement to which a water-repellent agent has been added.
Hydrophobic cement is obtained by grinding portland cement clinker with a film-forming substance, such as oleic acid, to reduce the rate of deterioration when the cement is stored under unfavourable conditions.
Oil-well cement is used for cementing work in the drilling of oil wells, where they are subject to high temperatures and pressures. They usually consist of Portland or pozzolanic cement (see below) with special organic retarders to prevent the cement from setting too quickly.
Slag cement
The granulated slag, formed by the rapid chilling of suitable molten slags from blast furnaces, serves as the basis for another group of construction cements.
A mixture of portland cement and granulated slag, containing up to 65 per cent slag, is known in English-speaking countries as portland blast-furnace (slag) cement.
The German Eisenportlandzement and Hochofenzement contain up to 40 per cent and 85 per cent slag, respectively. Mixtures in other proportions are found in French-speaking countries under various names, including ciment portland de fer, ciment métallurgique mixte, ciment de haut fourneau, and ciment de liatier au clinker.
The properties of these slag cements are broadly similar to those of portland cement, but they have a lower lime content and a higher silica and alumina content. Those with a higher slag content have an increased resistance to chemical attacks.
Another type of slag-containing cement is a super-sulfated cement consisting of granulated slag mixed with 10 to 15 per cent hard-burned gypsum or anhydrite (natural anhydrous calcium sulfate) and a few per cent of portland cement. The strength properties of super-sulfated cement are similar to those of portland cement, but it exhibits increased resistance to various forms of chemical attack.
Pozzolanic cement is a mixture of portland cement and a pozzolanic material that may be either natural or artificial. The natural pozzolanas are the primary materials of volcanic origin, but include some diatomaceous earth. Synthetic materials include fly ash, burned clays, and shales.
Pozzolanas are materials that, though not cementitious in themselves, contain silica (and alumina) in a reactive form able to combine with lime in the presence of water to form compounds with cementitious properties.
Mixtures of lime and pozzolana still find some application, but largely have been superseded by modern pozzolanic cement. Hydration of the Portland cement fraction releases the lime required to combine with the pozzolana.
High-alumina cement
In the Manufacture of Cement, High-alumina cement is a rapid-hardening cement made by fusing a mixture of bauxite and limestone at 1,500 to 1,600 °C (2,730 to 2,910 °F) in a reverberatory or electric furnace, or in a rotary kiln.
It can also be made by sintering at about 1,250 °C (2,280 °F). Suitable bauxites contain 50 to 60 per cent alumina, up to 25 per cent iron oxide, not more than 5 per cent silica, and 10 to 30 per cent water of hydration.
The limestone must contain only small amounts of silica and magnesia. The cement contains 35 to 40 per cent lime, 40 to 50 per cent alumina, up to 15 per cent iron oxides, and preferably not more than about 6 per cent silica. The primary cementing compound is calcium aluminate (CaO·Al2O3).
High-alumina cement achieves a high proportion of its ultimate strength within 24 hours and exhibits high resistance to chemical attack. It can also be used in refractory linings for furnaces.
In the Manufacture of Cement, a white form of cement, containing minimal proportions of iron oxide and silica, has outstanding refractory properties.
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Expanding and non-shrinking cement
Expanding and non-shrinking cement expands slightly on hydration, thus offsetting the slight contraction that occurs when fresh Concrete dries for the first time.
Expanding cement was first produced in France in about 1945. The American type is a mixture of Portland cement and an expansive agent, made by mixing a combination of chalk, bauxite, and gypsum.
Gypsum plasters
In the Manufacture of Cement, Gypsum plasters are used for plastering, the manufacture of plasterboards and slabs, and in one form of floor-surfacing material.
These gypsum cements are mainly produced by heating natural gypsum (calcium sulfate dihydrate, CaSO4 · 2H2O) and dehydrating it to give calcium sulfate hemihydrate (CaSO4 · 1/2H2O) or anhydrous (water-free) calcium sulfate. Gypsum and anhydrite obtained as by-products in chemical manufacture are also used as raw materials.
In the Manufacture of Cement, the hemihydrate, known as plaster of Paris, sets within a few minutes of mixing with water. For building purposes, a retarding agent, typically keratin —a protein —is added. The anhydrous calcium sulfate plasters are slower-setting, and often another sulfate salt is added in small amounts as an accelerator. Flooring plaster, initially known by its German title of Estrich Gips, is of the anhydrous type.
Cement testing
In the Manufacture of Cement, Various tests to which cement must conform are laid down in national cement specifications to control the fineness, soundness, setting time, and strength of the cement. These tests are described in turn below.
Fineness
Sieve tests long controlled the fineness, but more sophisticated methods are now primarily used. The most common method, used both for controlling the grinding process and for testing the finished cement, measures the surface area per unit weight of the cement by determining the rate of air passage through a bed of cement.
Other methods depend on measuring the particle size distribution by the rate of sedimentation of the cement in kerosene or by elutriation (separation) in an airstream.
Soundness
After it has set, cement must not undergo any appreciable expansion, which could disrupt a mortar or Concrete. This property of soundness is tested by subjecting the set cement to boiling in water or to high-pressure steam. Unsoundness can arise from the presence in the cement of too much free magnesia or hard-burned free lime.
Setting time
In the Manufacture of Cement, the setting and hardening of cement are continuous processes, but two distinct points are identified for testing purposes.
The initial setting time is the interval between the mixing of the cement with water and the time when the mix has lost plasticity, stiffening to a certain degree. It marks roughly the end of the period when the wet mix can be moulded into shape.
The final setting time is the point at which the set cement has acquired sufficient firmness to resist a certain defined pressure. Most specifications require an initial minimum setting time at ordinary temperatures of about 45 minutes and a final setting time of no more than 10 to 12 hours.
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Strength
In the Manufacture of Cement, tests that measure the rate at which a cement develops strength are typically conducted on a mortar commonly composed of one part cement to three parts sand, by weight, mixed with a defined quantity of water.
Tensile tests on briquettes, shaped like a figure eight thickened at the centre, were formerly used but have been replaced or supplemented by compressive tests on cubical specimens or transverse tests on prisms.
The American Society for Testing and Materials (ASTM) specification requires tensile tests on a 1:3 cement-sand mortar and compressive tests on a 1:2.75 mortar.
The British Standards Institution (BSI) provides an alternative, recommending a compressive test on a 1:3 mortar or on a Concrete specimen.
An international method issued by the International Organisation for Standardisation (ISO) requires a transverse test on a 1:3 cement-sand mortar prism, followed by a compressive test on the two halves of the prism that remain after it has been broken in bending.
Many European countries have adopted this method. In all these tests, the size grading of the sand and its usual source are specified.
In the testing of most cements, a minimum strength at 3, 7, and sometimes 28 days is specified, but for rapid-hardening portland cement, a test at 1 day is also sometimes required. For high-alumina cement, tests are required at 1 and 3 days.
Strength requirements established in different countries are not directly comparable due to differences in test methods.
In actual construction, to check the strength of Concrete, compressive tests are made on cylinders or cubes made from the Concrete being placed.
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