Cement chemistry, 2nd edition. Authors: Since , his main research interest has been the chemistry of cement. First Page Preview | PDF ( KB). 3. CEMENTS. Introduction. • Finely ground powders and all have the important property that when mixed with water a chemical reaction (hydration) takes place. The raw materials used for the manufacture of cement consist mainly of lime, silica, Approximate Oxide Composition Limits of Ordinary Portland Cement.
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Session III—Chemistry of Hydration of Cement Compounds; Tuesday morning, Session V—Properties of Cement Paste and Concrete; Thursday morning. Cement Chemistry and. Michel Michaux. Erik Nelson. Benoit Vidick. Saint- Etienne, France. Conductor casings are cemented to prevent drilling fluids circulating. composition of a hydrated paste of that cement can be calculated. in heat of hydration, and in related physical and chemical properties of the hydration prod-.
His early work laid the foundations of our understanding of the structure at the nanometre level of C-S-H, the principal product formed when cement is mixed with water, and the one mainly reponsible for its hardening. Subsequent studies took him into many additional aspects of the chemistry and materials science of cement and concrete.
Lea's Chemistry of Cement and Concrete
His work has been recognized by Fellowships and by other honours and awards. Back to Book Listing. download this book in print. View Chapters. Select All. For selected items: Only 5. Of this, China accounted for 1. These include emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries , and damage to countryside from quarrying.
Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them. CO2 emissions[ edit ] Global carbon emission by type to This reduction in primary energy requirements is equivalent to approximately 11 million tonnes of coal per year with corresponding benefits in reduction of CO2 emissions.
These emissions may be reduced by lowering the clinker content of cement.
They can also be reduced by alternative fabrication methods such as the intergrinding cement with sand or with slag or other pozzolan type minerals to a very fine powder. To reduce the transport of heavier raw materials and to minimize the associated costs, it is more economical to build cement plants closer to the limestone quarries rather than to the consumer centers. Heavy metals Tl, Cd, Hg, Environmental regulations exist in many countries to limit these emissions.
As of in the United States, cement kilns are "legally allowed to pump more toxins into the air than are hazardous-waste incinerators. The high pH prevailing in the cement porewater Nickel , zinc and lead are commonly found in cement in non-negligible concentrations.
Chromium may also directly arise as natural impurity from the raw materials or as secondary contamination from the abrasion of hard chromium steel alloys used in the ball mills when the clinker is ground. Use of alternative fuels and by-products materials[ edit ] A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil.
Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln referred to as co-processing , replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications.
Cement Chemistry Handbook - Fuller
Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale , and limestone. Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value, and burns to give ash containing minerals useful in the clinker matrix.
These characteristics of a clinker kiln offer numerous benefits and they ensure a complete destruction of organic compounds, a total neutralization of acid gases, sulphur oxides and hydrogen chloride. Furthermore, heavy metal traces are embedded in the clinker structure and no by-products, such as ash of residues, are produced.
Although the choice for this so-called alternative fuels AF is typically cost driven, other factors are becoming more important. Use of alternative fuels provides benefits for both society and the company: CO2-emissions are lower than with fossil fuels, waste can be co-processed in an efficient and sustainable manner and the demand for certain virgin materials can be reduced.
Yet there are large differences in the share of alternative fuels used between the European Union EU member states. The societal benefits could be improved if more member states increase their alternative fuels share. The Ecofys study  assessed the barriers and opportunities for further uptake of alternative fuels in 14 EU member states.
The Ecofys study found that local factors constrain the market potential to a much larger extent than the technical and economic feasibility of the cement industry itself.
Green cement[ edit ] Green cement is a cementitious material that meets or exceeds the functional performance capabilities of ordinary Portland cement by incorporating and optimizing recycled materials, thereby reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable construction material.
New manufacturing processes for producing green cement are being researched with the goal to reduce, or even eliminate, the production and release of damaging pollutants and greenhouse gasses, particularly CO2. Of prime importance is the state of the gypsum, as di-, hemi-, or anhydrite, in the cement as it first reacts with water. Depending upon that state, the gypsum and 3.
False set is the premature stiffening of the cement paste due to most of the gypsum being either hemi-hydrate or soluble anhydrite due to overheating.
Upon mixing with water, crystallization of reformed gypsum causes stiffening. This stiffening can be broken upon remixing without additional water. False set can happen either by fast set of gypsum hemi-hydrate or because of a slow reaction. False set can be prevented by lowering the mill exit temperature, thereby, reducing the degree of gypsum dehydration, the amount of gypsum added to the mix, or replacing part of the gypsum with a natural insoluble anhydrite.
On the other hand, flash set occurs if the C A is more reactive than gypsum with water. The setting is characterized by a high evolution of heat and short setting time. Flash set can be prevented by adding more gypsum to the cement or by dehydrating the gypsum to a more reactive form, i. Therefore, as the cement is being ground, the mill material temperature must carefully be controlled. Between C, the gypsum changes into calcium sulfate hemihydrate plaster of paris by releasing 1.
The cement mill material temperature is controlled primarily by cooling the mill with an internal water spray in the second compartment. Additional cooling is accomplished with air in the separator. The cement mill exit temperature should not reach C and is usually targeted at C.
Finally, if the cement material temperature has not been controlled in the mill system, the cement might enter the storage silo at too high a temperature causing dehydration of the gypsum. The rate of the transformation increases with temperature and with falling dew point. The change after equation a is rapid at a temperature of C.
The water molecule released can give rise to formation of lumps in the cement and to scaling on the storage silo wall by 2 4. The description here will concentrate on the process for ordinary grey cement with some comments on the other types of cement.
The dry process is used to make the majority of the cement produced in the world. The wet process, however, is still used where fuel cost has allowed it. The wet process can furthermore be justified where the raw materials are very wet such as chalk, a soft limestone, and clay.
An intermediate solution is the semi-dry process where the raw mix is prepared as slurry. The slurry can then be filtered to remove a portion of the water before the burning or the slurry may be pumped directly into a dryer crusher working in unison with the kiln. The difference in the composition of the raw mix and the clinker is threefold.
First, is the change in each of the materials as they are heated up in the kiln. The changes are due to a loss in weight mainly from the release of carbon dioxide and water. Second, is a change due to absorption of ash from coal used as fuel. There is also a change due to absorption of sulfur in the fuel.
Finally, there is a change due to the small dust loss in the exhaust gas. Some of this dust is returned to the process but some might be wasted as in a bypass. In a wet process, the dust may be discarded in order to reduce the alkali content in the clinker. The raw mix must therefore compensate for these changes and losses; otherwise, the clinker will not have the correct chemical and mineralogical composition. The way in which this is done will be explained below. Many raw materials are suitable for the production of cement.
In principle, as long as they can be mixed to give the right composition of the clinker, they can be used. There are some restrictions naturally. They must be available in large quantities and be economically feasible. In addition, there might also be restrictions on use due to minor components in the raw materials.
Limestone is the largest component used in producing cement. It is available as CaCO3 in marble, limestone, chalk and marl. Limestone is sometimes found together with magnesium carbonate. Only small amounts of MgO can be tolerated in cement due to the risk of the expansion reaction in the concrete.
Limestone containing a large amount of magnesium carbonate is called dolomite. In some cases a type of limestone is found that is quite close in chemical composition to the cement composition.
When the limestone is of a higher purity than the requirement for the raw mix,. The amount of limestone is calculated using the formula below:. It is a very hard and abrasive mineral. It is used when the mix is insufficient in silica. It will increase the MS or the silica ratio:.
Iron can be used in the form of iron ore, usually an iron oxide, or as a waste product from the fertilizer industry, such as pyrite ash. It is used to regulate or reduce the alumina modulus, the ratio of alumina to iron oxide:. Bauxite an alumina mineral rich in Al2O3 and can be used to increase the MA.
Fly ash, one of the waste materials from the power generation industry is also used as a raw material. This is known as pulverized fly ash, PFA. Typically, this is higher in SiO2 content. The number of components used in the raw mix is typically raw materials depending upon the need for correction of the three main modules: The physical nature of a raw material is also important. Very wet materials can be the reason for choosing the wet or semi-dry process.
Very abrasive materials like sand and basalt are costly to crush and grind to the fine state needed in the raw mix. The chemical variation in the raw material is also important.
Table of Contents
If there are great variations, more homogenization will be required. The chemical composition of the raw mix has to be prepared correctly to yield a good clinker. Also, the variation in the raw mix going to the kiln has to be small to obtain good burning conditions for the kiln and preheater. The first step in the mix design is the determination of the chemical composition of the raw materials.
It is common to have an approximate analysis of each raw material and use this for the calculation of the mix ratios. A sample after the raw mill is easily obtained for analysis. An analysis can be performed quickly using X-ray fluorescent analysis, XRF. Timely adjustments can then be made to the raw mill weighfeeders. The calculations made in the table show that 5 raw materials have been available at this plant. This has made it possible to satisfy 4 conditions, one less than the number of raw materials.
The four conditions here are: The alkalies often have to be restricted to satisfy a requirement for low alkali cement.
Low alkali cement is needed when the aggregate contains reactive silica, which can give an expansion in concrete. The chloride content is also shown. Chloride is a volatile component and can form coatings together with alkalies in the preheater. The chloride content has to be restricted in preheater kiln systems to 0. The raw mix must contain the proper fineness and be homogenized before Chemistry Bible Rev.
In the dry process, the kiln feed is a dry powder with a typical moisture content of 0. The fineness of the raw mix is measured on sieves. The normal sieves with respective residues used are: Also, the composition of the residue is important.
Free silica quartz will for instance result in poor reactivity or burnability of the material in the burning zone. When the coarse particles have a similar composition to the kiln feed less quartz then a greater amount of residue can be tolerated. In the wet process, the slurry moisture should be as low as possible but still be transportable via slurry pumps. The reactivity of the kiln feed for slurry or raw meal is checked by the burnability test in the laboratory.
The procedures can vary from different kiln suppliers but in principle a few small nodules are made of the raw mix ground to a fixed sieve residue.
The clinker formed is then crushed and ground determining the amount of free CaO. The free CaO is compared to the free CaO content expected or found on a standard raw mix and classified as hard, normal or easy burnability. If the raw mix is hard to burn, then the raw mix may have to be ground finer or the composition might have to be altered.
The Chemistry Bible Rev.
The reaction zones that occur as the raw mix is fed to the pyro system are: The CO2 leaves the kiln with the exhaust gas. CaO and MgO are formed. Cooling Zone: The advantages of a modern dry kiln with a preheater compared to a long wet kiln are: The clinker formation is very important for plant operation and for cement quality. Fine dusty clinker will be difficult to handle in the grate cooler and a large dust circulation may start between the cooler and the burning zone. The coating in the burning zone can become quite porous and unstable.
Grinding of fine clinker calls for a higher power consumption in the cement mill. The two factors determining the clinker formation and the clinker size are: The nodulization depends on the liquid to bind the fine particles together.
The formation is a function of particle size and the amount of liquid. In the rotary kiln, o. Formation of C S starts, the rate increases and the size of the crystals increase. The formation and growth of crystals in the burning zone eventually stops the agglomeration.
Four main characteristics of clinker formation are: Alite size: Belite size: Belite color: Rapid cooling is desired resulting in clear crystals. Slower cooling gives yellow to amber colored crystals.
Alite Birefringence: Figure 1. The raw mix chemistry has a major influence on the process. A high MS will result in less liquid phase formation and require a higher burning temperature. Lowering the MS will give better burning and nodulzation. A lower MA results in a 3. Depending upon the level of LSF, a higher LSF will result in a higher burning zone temperature and above a certain level the nodulization is impeded and the clinker gets more dusty.
The dry process is used predominantly today because of the lower heat consumption and the better process control compared to the wet process. The wet process is only used when fuel is very cheap or the raw materials are very wet not making it economically feasible to replace it. The wet kiln was for many years the standard equipment in the industry. Fuel was cheap and the process of slurry preparation was easy. Homogenization in silos and large slurry basins blended the slurry perfectly. The wet kiln had to perform drying, preheating, calcination, burning and often clinker cooling in one piece of equipment.
However, the wet kiln has some limitations:. The table is included because there is a connection between cement chemistry and the choice of kiln type. The layout of the different kiln types is shown on the figure on enclosure 2. It has been superceded by the more efficient preheater kiln systems 5.
The number of cyclones is 4 to 6. The last half of the calcination takes place in the kiln. This means that the necessary amount of heat exchange in the kiln is larger than in the kiln types with a separate calciner.
The kiln has to be larger for a given production. Due to the calcination in the kiln, the charge is fluidized by CO2 giving the material a chance to flow freely. A small calciner is built into the riser duct and the air for combustion is drawn through the kiln. The preheater is a single string with the calciner built into the riser duct. Combustion air is drawn from the grate cooler through a separate tertiary air duct.
A damper in the tertiary air duct allows. The calciner is placed in parallel to the kiln riser duct. The combustion air for the calciner is atmospheric air heated in the grate cooler and transported through a tertiary air duct. The gas from the calciner and the gas from the kiln are not mixed and pass through two separate strings of preheater.
The gas stream from the calciner is mixed with the gas from the kiln riser duct and pass through one string of preheater cyclones. There is only one main ID fan.
The system is a development of the SLC system. It has the two strings as the SLC system but with a calciner also in the kiln string. The firing in the two calciners will be: The ash from the coal used for combustion will be absorbed in the clinker. This is shown on the calculation sheet above table 8. The amount of ash absorbed is then: Some of the chemicals in the materials going into the burning zone evaporate.
Table of contents
The components can come from the raw materials, fuel and waste products. The four most important are: There are others but they are normally of minor importance. These minor materials, heavy metals or certain organic compounds can have important implications on the environment for a given plant. The plant should be aware of the different minor constituents to prevent any problems.
The four volatile elements mentioned above evaporate in the burning zone and condense again in the colder parts of the kiln system. The colder parts are the. Volatiles will also condense on the raw meal particles, as they are colder than the gas carrying the volatiles.
Some of the volatiles may escape from the kiln system partly being caught in the filter or escaping into the atmosphere. The volatiles that do not leave the system with the exhaust gas either remain in circulation in the kiln system or leave with the clinker.
The volatiles can accumulate in the kiln and preheater causing problems with in build-up in the cyclones and riser duct. It is important to be able to foresee any problems that may occur before the start up of a new plant or a conversion of an existing plant to prevent the possibilities of plugging.
A certain portion of the volatiles in the material flowing into the burning zone of the kiln will evaporate at the high temperature. The portion that evaporates is defined as the evaporation factor called epsilon. A portion of the volatile. This is referred to as a valve V. An internal circulation of volatiles takes place and the circulation factor is called K.
The part of the volatile leaving the kiln with the clinker is the residual part called R. A simple layout of a kiln system is shown below:.
For one unit of feed: R is the amount going into the clinker. It is possible to calculate the circulation K and the residual in clinker R when the evaporation factor and the valve V are known.
The enclosed table and figure give evaporation factors and valves for typical cases.
Evaporation factor Kiln valve,wet kiln,nodule operating Kiln valve,wet kiln, dust operating Long dry kiln Kiln valve,1-stage kiln Kiln valve,2-stage kiln Kiln valve,4-stage kiln Kiln valve,precalciner kiln Cyclone preheater valve 1stage Cyclone preheater valve 2stage Cyclone preheater valve 1stage Dedusting cyclone valve Raw mill valve Cooling tower valve Electrostatic precipitator valve.
The evaporation of alkalies is larger when chloride is high. This is at times used to increase the evaporation in the burning zone by adding CaCl2. Sulfur is difficult to evaluate.
Some sulfur in the raw mix is present free in various organic compounds or in pyrites. CaCO3 assisted by moisture catches some of it in the rawmill. SO2 in the preheater also reacts with calcium carbonate with a maximum around C. Sulfur in combination with alkalies behaves differently than SO2 from fuel.
Excess sulfur, sulfur not bound as alkali sulfates, can be calculated as:. To ensure trouble free operation of a preheater kiln the following limits apply: The bypass will take part of the kiln gas before the preheater and transport it to a separate cooling and dedusting system. The bypass gas has to be cooled immediately to oC to avoid clogging. The cooling takes place in a swirling chamber with atmospheric air.
Chemical control during operation of the kiln system is divided into the following: Large variations will result in irregular kiln operation resulting in problems with ring formation and coating in the preheater, as well as, requiring Chemistry Bible Rev. Performing a free lime analysis on an hourly basis monitors the product quality of the clinker.
The analysis can be made either on an average hourly sample or on an hourly spot sample. Environmental authorities stipulate emission control in many countries.The ratio of the change in sample weight between 30 and C divided by the weight at C, represents the hygroscopic moisture. The Chemistry Bible Rev. CaO and MgO are formed. If the raw mix is hard to burn, then the raw mix may have to be ground finer or the composition might have to be altered. Ria Micua.
The difference in the composition of the raw mix and the clinker is threefold. Radiation loss from all outer surfaces Heat loss with the gasses from the kiln Excess hot air from the clinker cooler Heat loss with hot clinker Heat effects the chemical reactions, the formation of solutions and changes in the state of aggregation such as melting or vaporization.
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