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The main material that is used to keep the concrete together is cement. Cement starts hardening and strengthening when it is combined with water after some process. (Nzic, p. 1) There are many different types of cement; the most common type of cement which is used in construction is Portland cement. Portland cement can be categorised in eight different types according to its use, but generally it can be divided in two main groups of natural and artificial (Portland) cement. Indeed, natural cement is the precursor of Portland cement. Portland cement was first produced in 1824 by Aspdin (Cement Company, p. 1). Since it was harder than natural cement, it took the place of natural cement. The eight types of Portland cement are shown in the table below:
common cement for most reasons
Air entering modification of type I
Moderate sulphate resistance
Used to prevent moderate sulphate attack. Generates less heat than type I
Moderate sulphate resistance-air entering
Air entering modification of type II
High early strength
Used when high early strength is needed.
High early strength-air entering
Air entering modification of type III
Low heat of hydration
Used when hydration heat must be minimized in large volumes.
High sulphate resistance
Used to prevent severe sulphate action where soils and ground water have high sulphate content.
Table Types of Portland cement (Abdel M.O. Mohamed & Hogan E. Antia, 1998, p. 531)
One type of Portland cement is missing in the above table which is white Portland cement; the major difference is the colour (Cement Company, p. 1). The colour difference is due to the raw material that is used in the production and in commonly used for decoration and where cement can be seen.
Alternatively, natural cement can be divided in two main groups as:
Rapid natural cement (RNC): it consists of low clay content marl and is produced in temperature between 1000-1200 Ëš C for 12-20 hr. Also, it set so fast in less than 30 minutes. (M.J. VarasT, 2004).
Slow natural cement(SNC): it consits of high clay content marl, it is produced in 8-12 hr at 800-1000 ËšC. But it sets from 30 minutes to 12 hr) (M.J. VarasT, 2004)
2.1 Oxide composition
There are four major compounds used to manufacture Portland cement; lime (CaO), silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3) in which made up to 95% of the Portland cement. However, these four main compounds can make 4 different chemical forms which are shown in table below (newzeland institute of chemistry):
Table cement compounds (newzeland institute of chemistry)
But the amount of these materials varies due to the type of Portland cement. (Popovics, 1992, p. 14). But as said, these material made up to 95% of the portland cement and the other 5% is made from impuritis such as magnesia, sodium, potassium oxide, and titania, phosphorus and manganese oxide. (Popovics, 1992, p. 14)
2.2 major phase of composition
Composition of Portland cements consist of major phases that contains Tricalcium silicate (3CaO.SiO2), dicalcum silicate (2CaO.SiO2), and tricalcium aluminate (3CaO.Al2O3 (V.S.Ramachandram, 1995, p. 2) .The typical composition of different types of cement can be found in Table below:
Type of data
Compound composition (%)
Number of Samples
Table Composition of different types of cement (Popovics, 1992, p. 28)
The way cement is produced can vary; therefore, cement produced by solidification/stabilization technology can be called Portland cement. This procedure can be completed in organic or inorganic method; except inorganic way is used for producing Portland cement (Abdel M.O. Mohamed & Hogan E. Antia, 1998, p. 529) .The production of portalnd cement can be divided in to four mani steps that is shown rafly in the figure below:
Figure Cement production steps (Deborah N. Huntzinger, 2009, p. 669)
In the first step the raw materials for production of cement, which can be found in natutral rock mixture or limestone is used that make 80% of the cement, also the other 20% is made from clay or shale. The presence of lime and silica results in strength of cement and availability of iron reduces the reaction temperature and also gives grey colour to the product (newzeland institute of chemistry) (Nzic).
3.2 preparations of raw materials
After quarrying, second step is to arrange the raw material mixture or kiln provide for the pyro-processing action. (Epa, pp. 11.6-4). This process split in to five dissimilar methods: 1. Wet process, 2.dry process, and 3.semidry process, dry process with preheater and dry process with preheater/precalciner (Epa, pp. 11.6-4).
The two most common processes are explained in details below:
3.2.1 Dry process
In this process clay and lime stone are crushed separately in to small size sample ant then the sample is controlled in the laboratory for mineral analysis and to ensure the presence of raw materials. After all the mixture of rocks are fed into the mill to ground the rock until 85% of rocks are less than 90Âµm in diameter (Nzic, p. 4).
3.2.2. Wet process
In wet process, clay is mixed to paste in tank and pulverised in the presence of water and is known as wash mill. In the next step the crushed limestone is added to this mixture. Then both materials with big size are grounded together. In the end, the slurry is checked in laboratory by technicians for its composition (newzeland institute of chemistry) (Nzic, p. 5).
Clinkering is the most significant process in the production of cement according to pyro-processing which happens in clinkering. In this step, all the materials is dried, heated, and cooled. Since the product from dry process does not have excess amount of moisture, it can be dried in pre-heater tower. The moisture in the cement will be heated from 70 to 800 ËšC for around 30 seconds and the moisture will evaporate up to 20% of decarbonation and initial reaction appear. Following this, the powder is ready to be fed into the kiln (Nzic, p. 5). All the above explanation is governed when the material is prepared with dry process method.
However, if the preparation was done in wet process the scenario is different. This can took place in two ways. Either the slurry from wet process can be fed directly in the kiln and converts into dry balls by the heat and rotation of the kiln. Or else, the slurry from wet process can be preheated for drying. After drying the slurry are fed to the kiln. In general the kiln that is used for the wet process is longer than the kiln that is used for the dry process (Nzic, p. 5).
Moreover, the mixture of raw materials are sent to the kiln, kiln is machine that has rotating cylinder with furnaces, and material moved along the rotary cylinder and heated slowly with the furnaces. After the heating in high temperature, the material changed to either cementitious or hydraulic. Kilns can have ratio of length to diameter according to the type of process; we can have these ratios from 15:1 to 40:1. As explained before, in wet process the kiln can have length more than 210 meters long, but kiln from dry process is shorter (Epa, pp. 11.6-4).
After heating the kiln, a big cooler that uses air are supplied to decrease the temperature of the mixture from approximately 1000ËšC to 150ËšC (Nzic, p. 6). Therefore, the new mixture is made and is called clinker by means of all the Portland cement characteristics.
3.3.1 Reactions in kiln
When kiln is forming chemical reactions are taking place. These chemical reactions vary according to the temperature of the process. According to the temperature these reactions can be divided in to four major groups.
These reactions are listed below and are categorized in simple zones: (newzeland institute of chemistry)
Zone A: from 0 to 35 minutes, and temperature from 800 to 1100 OC
Decarbonation and melting of fluxing compounds AL2O3 and Fe2O3 took place at temperature over 900OC.
CaCO3 + heat CaO + CO2 (2)
Zone B: from 35 to 40 minutes, and temperature from 1100 to 1300 OC
Configuration of secondary silicate stage:
2CaO + SiO2 2CaO â-SiO2 + heat (2)
Zone C: from 40 to 50 minutes, and temperature of 1300 to 1450-1300 OC
2CaO â-SiO2 + CaO 3CaO â-SiO2 + heat + time (2)
3CaO â-AL2 O3 +CaO + Fe2O3 4CaO â-AL2 O3 â- Fe2O3 (2)
Zone B: from 50to 60 minutes, and temperature of 1300to 1000 OC
3.4 cement milling
After all, milling is the last stage of the production of the Portland cement. In this part clinker is sent to the rotating large tube mill and it would be mixed with gypsum, also up to 5% of natural anhydrites are added to the mixture during the grinding to control the setting time of finished cement (Nzic, p. 6) .This works as set retarder and ground for 30 minutes in the tube mill. The amount of grinding has direct relationship with volume of the cement, thus more volume results in coarser the grind will be. Clinker enters the mill from one side and Portland cement goes out from another side simultaneously. As the process takes place the temperature of mill and materials increases, so water is supplied to cool down both the materials and the mill. (newzeland institute of chemistry)
4 Technical challenges
Production of cement has wide range of use in construction society. But production of this compound involves emission of unwanted products that should be controlled very carefully. Clinkerization is very complex part in the chemical processes, XRD and IR techniques are used to understand the thermal analysis of clinkerization.
The primary emissions in manufacture of Portland cement are particulate matter (PM), nitrogen oxide (NOx), sulfur dioxide (SO2), carbon monoxide (CO) and carbon dioxide (CO2) (4). Rates of reactions are influenced by different factors, such as temperature. Above 1000C, water starts to evaporate, while at > 5000C, combined water from calcium hydroxide is ejected. After that, C2S, CA and C2F start to form by the consumption of calcium carbonate and magnesium carbonate at > 8000C. C12A7 is formed between 800 and 9000C, and at 900-10000C, C3A and C4AF start to appear and calcium carbonate decomposes completely. Above this temperature until 14500C, all the materials are formed (V.S.Ramachandran, 2002, p. 78) .The major resource of emission of PM in cement manufacturing plant is pyroprocessing system which contains kiln, clinker and cooler exhaust stacks (enviromental protection agency(epa)).
Combustion of fuels contains emission of nitrogen oxides, as temperature increases the emission increases as well. A raw material contains sulphur compounds and sulphur dioxide is produced from them (enviromental protection agency(epa)).
5 Product evaluation
After the production of cement the produced cement must be tested before using it in the construction. There are different types of tests that can be used for cement. These tests can be divided into six main types. In addition, these tests must be carried out to ASTM standards. Table below explains the test and the method of the tests:
Definition of Test
Method of Test
Fineness test finds the particle size of powders since the size of powders affects the accessibility of the area for water in the hydration procedure
1. By Turbidimeter
Tests cement to find out its volume expansion after hardness. The sample must be put under the pressure of 2.03 MPa for 3 hours and then measured the length due to the time
1. Autoclave Expansion
Used to find how cement paste sets must be down. Initial set and final set must be calculated
2. Gillmore Needles
Tests compressive strength: is carried out on 2 inch sample and compressive load is put on it until failure. It must last between 20 and 80 seconds
1.ASTM C 109
2. ASTM C 349
Tensile strength: direct tension test
Flexural strength: is carried out on the 1.57(inch)* 1.57(inch)* 6.30(inch) sample and put in the centre load by beam
1.ASTM C 348
Normally used for mixture calculation
1.ASTM C 188
Heat of Hydration
Measures the heat generated when water and cement react and is influenced by cement-water ratio, fineness, and curing temperature
1.ASTM C 186
Achieving composition of C3S+C2S >66.7%; C/S>2.0; MgO<5.0% for clinker
BS EN 196
The times after completion of mixing that cement paste shows resistances to the penetration of a needle by apparatus (3 p. 81) (figure 1 in appendix).
The most significant test of cement to determine the compressive strength that it produces in mortar or concrete. It depends on material used, the mix proportions, mixing procedure and mixing efficiency
Mortar strength test
A measure of the ease that a mortar or concrete can be place and compacted
ASTM C 143
Cement is considered to be unsound if the hydration of paste is eventually with excessive expansion, causing cracking and strength reduction
Le Chatelier apparatus
Heat of hydration
Important test when cement is being supplied for use in large concrete structures
ASTM C186 - 05
Table Different tests and methods (training.ce.washington, p. 1), (standard test method for heat of hydration of cement), (G.C.Bye, p. 87).
Furthermore, the quality of cement depends directly on factors that influence its rate of hydration and hydraulicity. So, as said before the cement must be tested before using it.
The standard test procedure of the UK is British standard BS 12:1996. Table below shows requirements of BS 12 which specifies chemical, physical and performance requirements for Portland cement of British standard 12 (G.C.Bye, p. 80).
Requirement: % m/m cement *
Deviation limit â-
Loss of ignition
Sulfate as SO3
Table Cement performance to BS Requirement of BS 12:1996 (G.C.Bye, p. 80)
Deviation limit â-
Table Compressive strength Nmm-2 (G.C.Bye, p. 80)
* Cement defined as clinker+ gypsum+ grinding aid (If any)
â- Maximum deviation permitted for individual result
R: high early strength sub class
Overall, cement has become one of the most considerable industries due to high demand of Portland cement in construction society. The process of producing cement has high amount of carbon dioxide produced and is realised in air, therefore, it can affect the global warming which is one of the most important issues in our daily life.
This process has become one of the very well investment fields for companies but it has high pay back in long term.
All in all, due to emission of gasses in this process, it should be done as environmentally-friendly as possible to reduce the effect of these produced gasses on the environment. Moreover, using other fuels rather than fossil fuels as energy sources for the process can make it more environmentally-friendly.