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ICH 556 : CHEMISTRY AND TECHNOLOGY OF CEMENT, CERAMICS AND
GLASS
Chemistry and Technology Of Cement 1.1 Introduction Cement is a finely ground, non-metallic, inorganic powder when mixed with water forms a paste that sets and hardens. Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to be used in the presence of water. Non-hydraulic cement will not set in wet conditions or underwater, rather it sets as it dries and reacts with carbon dioxide in the air. Some aggressive chemicals can attack it after setting. Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, pozzolanas, such as fly ash. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., Portland cement). This hydraulic hardening is primarily due to the formation of calcium silicate hydrates as a result of the reaction between mixing water and the constituents of the cement. In the case of aluminous cements hydraulic hardening involves the formation of calcium aluminate hydrates. Cement is a basic material for building and civil engineering construction. In Europe the use of cement and concrete (a mixture of cement, aggregates, and water) in large civic works can be traced back to antiquity. Portland cement, the most widely used cement in concrete construction, was patented in 1824 (it is called "Portland Cement", because when it hardened it produced a material similar to stones from the quarries near Portland Island in UK). 1.2 Chemical Composition of Cement The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding. Table 1.0 shows the approximate oxide composition limits of ordinary Portland cement. Table 1.0 Approximate Oxide Composition Limits of Ordinary Portland Cement Table 1.1 A typical chemical analysis of ordinary Portland cement
The oxides account for over 90% of the cement. The oxide composition of (ordinary) Portland cement may be expressed as follows: SO 3 (sulfur trioxide): Comes from gypsum. The amount of gypsum (CaSO4˙2H2O) can be approximated by multiplying the amount of SO 3 by 2.15. MgO (magnesia): To control the detrimental expansion, MgO is limited to 5% (expansion due to the hydration of free MgO in hardened concrete). Free CaO: Same as free MgO, free CaO is undesirable. Because these oxides hydrate much later than other compounds of cement. Besides, they show a large volume expansion after hydration resulting in disintegration of hardened concrete. Na 2 O & K 2 O (alkali oxides): A limiting value of alkali oxides is often specified for cements which are used with reactive aggregates to prevent alkali-aggregate reaction which results in disruptive expansion. L.O.I. (loss on ignition): Indicates "prehydration or carbonation" due to prolonged or improper storage. LOI is the loss of the weight of a cement sample when heated at 1000oC. i.e. LOI ≤ 3% (ASTM) (American Society for Testing and Materials) I.R. (insoluble residue): Fraction of cement which is insoluble in HCl acid. It comes mainly from the silica which has not reacted to form silicate compounds in the rotary kiln. It is a measure of the completeness of reactions in the kiln. i.e IR ≤ 0.75% (ASTM). 1.3 Main Chemical Compounds of Portland Cement Oxides interact with each other and form more complex compounds. The measurement of the amount of these major compounds by conventional chemical methods is not possible.
Hydration of cement is not a sudden process, but it occurs in stages. The rate and extent of hydration depend on several factors, such as the type and amount of cement, the water-cement ratio, the presence of additives, and the temperature23. The main phases of hydration are: Phase 1: Initial mixing reaction. This is a rapid reaction that occurs when water comes in contact with cement. The main compounds involved are tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF), which react with water to form calcium aluminate hydrates. This phase lasts for a few minutes and generates a lot of heat. Phase 2: Dormancy. This is a period of slow or no reaction that follows the initial mixing reaction. The main compound involved is tricalcium silicate (C3S), which reacts slowly with water to form C-S-H and CH. This phase lasts for several hours and allows the cement paste to be workable and pumpable. Phase 3: Acceleration. This is a period of rapid reaction that resumes after the dormancy period. The main compound involved is again C3S, which reacts faster with water to form more C-S-H and CH. This phase lasts for a few hours and causes the cement paste to harden and gain strength. Phase 4: Deceleration. This is a period of slow reaction that follows the acceleration period. The main compound involved is dicalcium silicate (C2S), which reacts slowly with water to form C-S-H and CH. This phase lasts for several days and causes the cement paste to gain more strength Phase 5: Steady state. This is a period of very slow reaction that continues indefinitely at a decreasing rate. The main compounds involved are C2S and C3A, which react with water to form more C-S-H, CH, and calcium aluminate hydrates. This phase causes the cement paste to gain strength over time
- Strength: The strength of hardened cement is its most important property. The rate of hardening of cement depends on the chemical and physical properties of the cement, the curing conditions and the water/cement ratio. Strength in cement is a measure of how well it can resist breaking or cracking under external forces. It depends on several factors, such as the water- cement ratio, the type and amount of additives, the mixing, placement and curing methods, and the fineness of the cement. The cement strength is determined by the varied tests done on the mortar and concrete cubes at specified intervals of 2 days, 7 days, and 28 days of hardening. There are three main strength classes for cement: 32,5, 42,5 and 52,5 followed by a R or N. The R refers to rapid or early strength development and the N to normal or standard strength development. While 32,5 is the low strength, 42,5 is the middle strength, and 52,5 is the highest strength. When choosing the cement right for your project you will want to keep in mind that the compressive strength reached by the 42,5N and the 42,5R will be the same once completely cured. However, the 42,5R will reach a higher initial compressive strength. As it is for all the classes.
- Soundness: Soundness is a physical property of cement paste, which determines the ability of the cement paste to retain its volume after setting is completed. If the cement is unsound, it may undergo delayed expansion and cause cracking or disruption of the concrete structure. The unsoundness is due to the presence of free CaO (lime) and free MgO (magnesia) in cement. These constituents hydrate very slowly after setting of cement. Since Ca(OH) 2 and Mg(OH) 2 occupy larger volume, expansion takes place.
2.0 CEMENT MANUFACTURING PROCESS
2.1 Manufacturing Process The cement manufacturing process consist of the following multi stages as explain in the following flow chart. Figure 1.0 Flow Diagram of Cement Manufacturing Process Before detailing the technological processes a brief description is given of the nature and requirements of the raw materials, fuels and mineral additions which are needed to manufacture the final products, i.e. clinker and cement. The choice of the technological process applied is to a certain extent governed by the physical nature – especially the moisture content – of the raw materials available. Finally, information is given on resources consumption and emissions typically found with cement kiln systems in operation. 2.1 Main Processes, Sub-Processes and System Boundaries There are four main process routes in the manufacturing of cement – the dry, semi-dry, semi-wet and wet process. Common to all these processes are the following sub-processes: Quarrying. Raw materials preparation. Fuels preparation. Clinker burning. Mineral additions preparation. Cement grinding. Cement dispatch
Conventional (fossil) fuels used in cement industry are mainly coal (lignitic and hard coal), petcoke (a product from crude oil refining), and heavy oil (“bunker C”). Natural gas is rarely used due to its higher cost. “Alternative” fuels – i.e. non-fossil fuels derived from industrial (“waste”) sources – are widely used today to substitute in part for the traditional fossil fuels. Fuels preparation – i.e. crushing, drying, grinding, and homogenising – usually takes place on site. Specific installations are required such as coal mills, silos and storage halls for solid fuels, tanks for liquid fuels, and the corresponding transport and feeding systems to the kilns. The thermal fuel consumption is largely dependent on the basic process design applied in the burning of clinker. 2.1.3 Clinker Burning: The prepared raw material (“kiln feed”) is fed to the kiln system where it is subjected to a thermal treatment process consisting of the consecutive steps of drying/preheating, calcination (e.g. release of CO 2 from limestone), and sintering (or “clinkerisation”, e.g. formation of clinker minerals at temperatures up to 1450° C). The burnt product “clinker” is cooled down with air to 100-200°C and is transported to intermediate storage. The kiln systems commonly applied are rotary kilns with or without so-called “suspension preheaters” (and, in more advanced systems, “precalciners”) depending on the main process design selected). The rotary kiln itself is an inclined steel tube with a length to diameter ratio between 10 and 40. The slight inclination (2.5 to 4.5%) together with the slow rotation (0.5 – 4.5 revolutions per minute) allow for a material transport sufficiently long to achieve the thermal conversion processes required. Exhaust heat from the kiln system is utilised to dry raw materials, solid fuels or mineral additions in the mills. Exhaust gases are dedusted using either electrostatic precipitators or bag filter systems before being released to the atmosphere. 2.1.4 Cement Grinding: Portland cement is produced by intergrinding cement clinker with a few percent of natural or industrial gypsum (or anhydrite) in a cement mill. Blended cements (or “composite” cements) contain other constituents in addition such as granulated blast-furnace slag, natural or industrial pozzolana (for example, volcanic tuffs or fly ash from thermal power plants), or inert fillers such as limestone. Mineral additions in blended cements may either be interground with clinker or ground separately or mixed with Portland cement. Grinding plants may be located remotely from the clinker production facility. The different cement types have to be stored separately in cement silos prior to bagging and dispatch. 2.1.5 Mineral Additions Preparation: Mineral additions from natural or industrial sources intended to be used in blended cements may need to be dried, crushed or ground in separate installations on site. Separate “grinding plants” where mineral additions and blended cements only are produced may also be located remote from the clinker production facility. 2.1.6 Cement Dispatch: Cement may be shipped as bulk cement or – usually to a lesser extent – packed into bags and palletised for dispatch. Transport methods used (i.e. road, railway, waterways) depend on local conditions and requirements.
2.2 Material Technological Characteristics In Cement Production 2.2.1 Portland Cement Clinker Portland cement clinker is produced from a mixture of raw materials containing calcium, silicon, aluminium, and iron as the main elements. When mixed in the correct proportions, new minerals with hydraulic properties – the so-called clinker phases – are formed upon heating up to the sintering (or clinkerisation) temperature as high as 1450° C. The main mineral components in clinker are silicates, aluminates and ferrites of the element calcium. Table 1.3 Main Clinker Phases: The clinker formation process can be divided into 4 steps. i.e.: Drying and preheating (20 – 900° C): release of free and chemically bound water Calcination (600 – 900° C): release of CO 2 : initial reactions with formation of clinker minerals and intermediate phases. Sintering or clinkerisation (1250 – 1450° C): formation of calcium silicates and liquid phase. Kiln internal cooling (1350 – 1200° C): crystallization of calcium aluminate and calcium ferrite Minor mineral constituents in cement clinker include uncombined calcium oxide (“free lime”) and magnesium oxide, as well as alkali sulphates. Additional chemical elements present in the raw materials such as manganese, phosphorus, titanium or heavy metals are mainly incorporated in the mineral structure of the major clinker phases. The properties of clinker (and thus, of the cement produced from it) are mainly determined by its mineral composition and its structure. The chemical and mineralogical composition of Portland cement clinker is given in Table 1.4.
Prior to entering the rotary kiln, the raw meal is heated up to a temperature of approximately 810-830° C where the calcination (i.e. the release of CO 2 from the carbonates) is already about 30% complete. The exhaust gases leave the preheater at a temperature of 300-360° C and are further utilised for raw material drying in the raw mill. A bypass system extracts a portion (typically 5-15%) of the kiln gases from the riser pipe between the kiln and preheater. This gas has a high dust burden. It is cooled with air, volatile compounds are condensed onto the particulates and the gas then passes through a dust filter. Modern suspension preheater kilns usually have 4 cyclone stages with a maximum capacity limited to approximately 4000 t/d. In some cases, 2-stage cyclone preheaters or 1-stage preheaters supported by internal chain heat exchangers are still in operation. A considerable capacity increase can be obtained with precalciner kilns with a second combustion device between the rotary kiln and the preheater section. In the precalciner, up to 60% of the total fuel of the kiln system can be burnt. At an exit temperature of about 880° C, the hot meal is calcined to a degree of around 90% when entering the rotary kiln. Kiln systems with 5 to 6 stage cyclone preheater and precalciner are considered standard technology for new plants today, as the extra cyclone stages improve thermal efficiency. In some cases, the raw meal is fed directly to a long dry kiln without external preheater. A system of chains in the inlet part of the rotary kiln provides the heat exchange between the hot combustion gases from the hot zone of the kiln and the kiln feed. Long dry kilns have a high heat consumption and high dust cycles requiring separate dedusting cyclones. 2.3.2 The Semi-Dry Process In the semi-dry process, dry raw meal is pelletised with 10-12% of water on an inclined rotating table (“granulating disc”) and fed to a horizontal travelling grate preheater in front of the rotary kiln (“Lepol” system). The pelletised material is dried, pre-heated and partly calcined on the two-chamber travelling grate making use of the hot exhaust gases from the kiln. A higher degree of calcination can be achieved by burning part of the fuel in the hot chamber of the grate preheater. The hot exhaust gases from the kiln first pass through a layer of preheated pellets in the hot chamber. After intermediate dedusting in cyclones, the gases are drawn once again through a layer of moist pellets in the drying chamber of the grate. As much of the residual dust is precipitated on the moist pellet bed, the total dust load of the exhaust gases at the preheater outlet is low. As a drawback of the semi-dry process, kiln exhaust gases cannot be utilised in the raw meal drying and grinding system due to the low temperature level. The maintenance costs of grate preheaters are high. Modern installations rarely use the semi-dry process. 2.3.3 The Semi-Wet Process In the semi-wet process the raw slurry is dewatered in filter presses. Typically, modern chamber filtration systems produce filter cakes with a residual moisture content of 16-21%. In the past, filter cakes were further processed in extruders to form pellets which were then fed to grate preheater kilns with three chambers. With modern cement plants, slurry filtration is applied only where raw materials have a very high natural moisture content, i.e. chalk. Filter cake coming from the filter presses is kept in intermediate storage bins before it is fed to heated crushers or dryers where a dry raw meal is produced which is fed to a modern preheater or precalciner kiln.
With the dryers/crushers operating full time in parallel with the kiln (compound operation), these systems have a very good energy recovery by making full use of the kiln exhaust gases and the cooler exhaust air. 2.3.4 The Wet Process Conventional wet process kilns are the oldest type of rotary kilns to produce clinker. Wet kiln feed (raw slurry) typically contains 28 to 43% of water which is added to the raw mill (slurry drums, wash mills and/or tube mills). Batch blending and homogenisation is achieved in special slurry silos or slurry basins where compressed air is introduced and the slurry is continuously stirred. The slurry is pumped into the rotary kiln where the water has to be evaporated in the drying zone at the kiln inlet. The drying zone is designed with chains and crosses to facilitate the heat exchange between the kiln feed and the combustion gases. After having passed the drying zone, the raw material moves down the kiln to be calcined and burnt to clinker in the sintering zone. Conventional wet kiln technology has a high heat consumption and produces large volumes of combustion gases and water vapour. Wet rotary kilns may reach a total length of up to 240 m compared to short dry kilns of 55 to 65 m length (without the preheater section). In modern wet kiln systems, the raw slurry is fed to a slurry drier where the water is evaporated prior to the dried raw meal entering a cyclone preheater/precalciner kiln. Modern wet kiln systems have a far lower specific heat consumption compared to conventional wet kilns.
