Cement Production Technology
Introduction
What is Cement?
Cement, is a grey mineral powder, is fine and milled. It contains limestone, clay, and marl as its important proportions [1]. The principal application of cement is in the construction industry, where many modern civil engineering projects are established in both residential and commercial structuring [2].
Cement VS. Concrete
Although these words can be interchangeably used. The fact that concrete is the final product which consists of cement as its ingredient. Cement is used to bind, or hold, the concrete, mixing together in order to improve concrete strength [3].
History of Cement
Back in the 19th century when cement was invented by Joseph Aspdin, a British stonemason who mixed ground limestone with clay and heat them in his kitchen stove, resulting in a fine powder from the concoction by pulverization [4]. He patented Portland cement in 1824, which was produced from synthetically mixed limestone and clay as mentioned above [5].
However, Aspdin’s invention can not be classified as a true Portland cement, due to the conversion that was lightly burned. The real one was created by Isaac Charles Johnson, who lived in southeastern England about 1850 [5].
Portland Cement
Calcium oxide (CaO, Lime), silicon dioxide (SiO2, Silica), and aluminium oxide (Al2O3, Alumina) are essential components for Portland cement.
We can acquire the lime from a lime-containing raw material called calcareous, and other oxides from a clayey or argillaceous material. Silica sand, iron oxide (Fe2O3), and aluminium hydroxide (Al(OH)3) are supplemental raw materials that may be applied in order to archive the requirements of the composition.
Gypsum is another mandatory raw material, which is required to burn in the cement clinker during the grinding process in order to regulate the cement’s setting time. In the place of calcium carbonate, there will be a combined process with sulfuric acid using anhydrite or calcium sulfate, and also can be made as Portland cement. Sulfuric acid is derived from sulfur dioxide which was originally produced in the flue gases in the burning process.
There is a raw material which must be low in the proportion of Portland cement due to its limitation. That is magnesium oxide (MgO, magnesia). Additionally, there are other impurities in Portland cement, which must be restricted e.g. fluorine compounds, phosphates, metal oxides and sulfides, and excessive alkalies [5].
Types of Portland Cement
There are several types of Portland cement, which is evidence of the flexibility of this material and can be manufactured to meet a variety of both physical and chemical requirements.
There are eight specific types of Portland cement, which are provided by the American Society for Testing and Materials (ASTM) Specification C-150 (ASTM C150 Standard) [4].
- Type I — For use when the special properties specified for any other type are not required.
- Type IA — Air-entraining cement for the same uses as Type I, where air-entrainment is desired.
- Type II — For general use, more especially when moderate sulfate resistance is desired.
- Type IIA — Air-entraining cement for the same uses as Type II, where air-entrainment is desired.
- Type II(MH) — For general use, more especially when moderate heat of hydration and moderate sulfate resistance are desired.
- Type II(MH)A — Air-entraining cement for the same uses as Type II(MH), where air-entrainment is desired.
- Type III — For use when high early strength is desired.
- Type IIIA — Air-entraining cement for the same use as Type III, where air-entrainment is desired.
- Type IV — For use when a low heat of hydration is desired.
- Type V — For use when high sulfate resistance is desired.
There is another type, the white Portland cement. This is used when requirements of architecture prefer white or colored concrete or mortar. It is equal to typical grey-colored Portland cement in terms of proportion.
Additional data about Types of Portland Cement:
Portland Cement Specifications [Types of Portland Cement] — ANSI Blog
Cement Manufacturing Process
Extraction and Raw Material Processing
Quarrying is the key to extracting raw materials which are applied in the manufacturing of cement in the case of hard rocks such as limestones, slates, and some shales, with the assistance of blasting when mandatory. The underground process is used to mine some deposits. Excavators are used to digging directly with softer rocks e.g. chalk and clay.
Trucks, railroad freight cars, conveyor belts, or ropeways, are used to transport the excavated materials to the crushing plant. The pipeline can be employed to move the raw materials in the state of wet or slurry as well.
Froth floatation can be used to remove excess silica or alumina and so upgrade the limestone in some region where limestones of insufficiently high lime content. On the other hand, this process is costly and is applied only when inevitable [6].
Manufacturing Process
There are four processes in the manufacturing of Portland cement [6]:
1) Crushing and grinding
All except hard materials are often crushed in two stages, following by soft materials, and then usually ground in a tube mills with a charge of steel grinding balls, rotating, or cylindrical ball. Depending on the process, this grinding is done wet or dry, unless the dry grinding may need to be dried in rotary or cylindrical dryers.
In wash mills, vigorous stirring with water brake down soft materials, and then produce a fine slurry, which is penetrated screens in order to remove improper particles.
2) Blending
Selective quarrying and control of the raw material provided to the crushing and grinding plant is influenced to a first approximation of the chemical proportion demanded for a specific cement. Drawing material from batch containg mixed raw materials of gradually various proportion is influenced finer control.
Silos are used to storing theses mixes in the dry process while the wet process prefer slurry tanks. Agitation and powerful circulation of compressed is applied to ensure the completely fined mixing of the dry materials in the silos. Mechanical means or compressed air or both are used to stirring the slurry tanks in the wet process that contains water between 35 to 45 percent. Sometimes filtering is required, reducing the water content to 20 to 30 percent, and the filter cake is then provided to the kiln. Fuel consumption for burning is reduced due to this process.
3) Burning
Bottle klins are the earliest klins, where cement was burned in batches, followed by chamber kilns and then by continuous shaft kilns. In some countries, the improved form of shaft klin is still operated, but the rotary klin is dominated the burning means.
These 200-metres long with 6-metres diameter klins are used for wet process while there are shorter version for dry process, which consists of a steel and cylindrical shell lined with intractable materials. They circulate tardily on an inclined axis with a few degrees to the horizontal. Introducing the raw material at the upper end, it moves gradually down the klin to the lower end or the firing end.
Firing process is used with the fuel, which may be fine coal, oil, or natural gas by inject into a pipe. The temperature is employed in the range of 1,350 to 1,550 °C depending on the raw material at the firing end. To enhance heat transfer to the raw materials that feed in to the klin and decrease the heat lost in the waste gases, some form of heat exchanger is regurlarly applied at the end of the klin.
The klin produces the burned products as small promontories of clinker. These clinkers pass into coolers, where the heat transfer is occured between hot product and cool air, resulting in cooling systems for product. These may be pulverized to cement or collected in stockhouses for later use.
The raw materials in the form of promontories containing water between 10 to 15 percent are transported onto a traveling chain grate before passing to the shorter rotary kiln in the semidry process. These processed raw materials absorb the hot gases coming from the klin, resulting in the preheated promontories.
Major interference from cement klins can be the source of dust emission. Cyclone arrestors, bag-filter systems, or electrostatic dust precipitators between the kiln exit and the chimney stack are usually installed in the plant that located in inhabited areas. The cement production caused more than 50 percent of this emission, which genuinely linked to the manufacturing of clinker. Additionally, by-product of the chemical reaction of the current process is another cause. Alternative materials are applied to blend clinker as the potential optional method in order to decrease the need of clinker itseft and hence help mitigate the environmental impacts from the cement manufacturing process.
Complex control instrumentations are installed in many modern cement plants to handle the burning process. Some plants are equipped with computer, which is able to sampling raw materials and managing the proportion of raw mixes automatically.
The biggest rotary kilns have outputs surpassing 5,000 tons per day.
4) Grinding
The amount of gypsum and the clinker is required to achieve the fine powder in horizontal mills, which similar to those used for grinding the raw materials. The material may pass into through the mill (open-circuit grinding) directly, or more rough material may be sorted out from the fine product and returned to the mill for additional grinding (closed-circuit grinding).
The feed material may be added a small amout of a grinding aid. The addition of an air-entraining agent is employed in order to make air-entraining cements.
Pneumatical pump is used to transfer manufactured cement to storage silos from which it is pulled for packing in paper bags or for dispatch in bulk containers.
Conclusions
Cement is suitably useful material for construction and widely acceptable for many applications. However, it is evaluated that around 4–8 percent of the world’s carbon dioxide (CO2) emissions come from the production of cement, resulting it a main contributor to global warming. Some of the mitigations to these greenhouse gas emissions are similar to other sectors:
- Improving the energy efficiency of cement plants
- Substituting fossil fuels with renewable energy
- Capturing and sequestrating (plus utilization) the CO2 that is emitted.
Moreover, given that a major section of the emissions are an intrinsic part of the production of clinker, novel cements and alternate formulations that decrease the need for clinker are an important area of focus [6].
For more information about Global Warming and Carbon Capture and Utilizations, you can also read this article:
Global Warming and Carbon Capture and Utilizations | by Suppawat Boonrach | TechNounia | Medium
References
[1] Cement, its nature and origin (heidelbergcement.com), retrieved 2 September 2022 from https://www.heidelbergcement.com/en/cement
[2] Cement production | Climate Technology Centre & Network | Tue, 11/08/2016 (ctc-n.org), retrieved 2 September 2022 from https://www.ctc-n.org/technologies/cement-production
[3] What is Cement? Types of Cement — Concrete Network, retrieved 2 September 2022 from https://www.concretenetwork.com/cement.html
[4] History of Cement | Construction Materials — CEMEX USA | CEMEX, retrieved 2 September 2022 from https://www.cemexusa.com/products-and-services/cement/history-facts
[5] cement — History of cement | Britannica, retrieved 2 September 2022 from https://www.britannica.com/technology/cement-building-material/History-of-cement
[6] cement — Extraction and processing | Britannica retrieved 5 September 2022 from https://www.britannica.com/technology/cement-building-material/Extraction-and-processing
Further Readings
1. Textbook: Cement Production Technology: Principles and Practice by Anjan Kumar Chatterjee (English)
2. Article: Cement Production by Climate Technology Centre & Network (English)
Cement production | Climate Technology Centre & Network | Tue, 11/08/2016 (ctc-n.org)
3. Article: Cement Industry by PTT Lubricants (Thai)
4. Article: กรรมวิธีผลิตปูนซีเมนต์ by CPAC Academy (Thai)
