How Cement is Made - Cement Manufacturing Process

Cement is a key ingredient in concrete, which is used to build everything from houses and bridges to sidewalks and sewer systems. Concrete is made by mixing cement with aggregate (sand, gravel, and water), which forms a paste that hardens over time. Cement is necessary to give the concrete its binding properties, which allow it to harden into a strong, durable material. The cement manufacturing process is a complex one involving several steps. Each step in the process plays an important role in the overall quality of the finished product. In this article, we will describe how cement is made with six cement manufacturing process stages.

Stage of Cement Manufacture

There are six main stages of the cement manufacturing process.

Stage 1: Raw Material Extraction/Quarry

The raw cement ingredients needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, mill scale, and bauxite. The ore rocks are quarried and crushed into smaller pieces of about 6 inches. Secondary crushers or hammer mills then reduce them to an even smaller size of 3 inches. After that, the ingredients are prepared for pyroprocessing. 

Stage 2: Grinding, Proportioning, and Blending

The crushed raw ingredients are made ready for the cement-making process in the kiln by combining them with additives and grinding them to ensure a fine homogenous mixture. The composition of cement is proportioned here depending on the desired properties of the cement. Generally, limestone is 80%, and the remaining 20% is clay. In the cement plant, the raw mix is dried (moisture content reduced to less than 1%); heavy wheel-type rollers and rotating tables blend the raw mix and then the roller crushes it to a fine powder to be stored in silos and fed to the kiln.

Stage 3: Pre-Heating Raw Material

A pre-heating chamber consists of a series of cyclones that utilizes the hot gases produced from the kiln in order to reduce energy consumption and make the cement-making process more environment-friendly. The raw materials are passed through here and turned into oxides to be burned in the kiln.

Stage 4: Kiln Phase

The kiln phase is the principal stage of the cement production process. Here, clinker is produced from the raw mix through a series of chemical reactions between calcium and silicon dioxide compounds. Though the process is complex, the events of the clinker production can be written in the following sequence:

1. Evaporation of free water.
2. Evolution of combined water in the argillaceous components.
3. Calcination of the calcium carbonate (CaCO₃) to calcium oxide (CaO).
4. The reaction of CaO with silica to form dicalcium silicate.
5. The reaction of CaO with the aluminum and iron-bearing constituents forms the liquid phase.
6. Formation of the clinker nodules.
7. Evaporation of volatile constituents (e. g., sodium, potassium, chlorides, and sulfates).
8. The reaction of excess CaO with dicalcium silicate forms tricalcium silicate.

The above events can be condensed into four major stages based on the change in temperature inside the kiln:

1. 100°C (212°F): Evaporation of free water
2. 100°C (212°F)-430°C (800°F): Dehydration and formation of oxides of silicon, aluminum, and iron
3. 900°C (1650°F)-982°C (1800°F): CO2 is evolved and CaO is produced through calcination
4. 1510°C (2750°F): Cement clinker is formed

The kiln is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes. By the time the raw mix reaches the lower part of the kiln, clinker forms and comes out of the kiln in marble-sized nodules.

Stage 5: Cooling and final grinding

After exiting the kiln, the clinker is rapidly cooled down from 2000°C to 100°C-200°C by passing air over it. At this stage, different additives are combined with the clinker to be ground in order to produce the final product, cement. Gypsum added to and ground with clinker regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill walls. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid, and dodecyl-benzene sulphonate.

The heat produced by the clinker is circulated back to the kiln to save energy. The last stage of making cement is the final grinding process. In the cement plant, there are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred to these rotating drums and ground into such a fine powder that each pound of it contains 150 billion grains. This powder is the final product, cement.

Stage 6: Packing and Shipping

Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains, or ships, and only a small amount is packed for customers who need small quantities.

Chemical Reactions during Cement Manufacturing Process

The reactions that take place (after evaporation of free water) between the reactants in the kilning phase of cement making process are as follows:

1. Clay Decomposition:
   Si₂Al₂O₅(OH)₂ → 2 SiO₂ + Al₂O₃ + 2 H₂O (vapor)
   KAlSi3O8 (orthoclase) + 0.5 SO₂ + 0.25 O₂ → 3 SiO₂ + 0.5 Al₂O₃ + 0.5 K₂SO₄
2. Dolomite Decomposition:
   CaMg(CO₃)₂ → CaCO₃ + MgO + CO₂
   KMg₃AlSi₃O₁₀(OH)₂ + 0.5 SO₂ + 0.25 O₂ → 0.5 K₂SO₄ + 3 MgO + 0.5 Al₂O₃ + 3 SiO₂ + H₂O (vapor)
3. Low-Temperature Calcite Decomposition:
   2 CaCO₃ + SiO₂ → Ca₂SiO₄ + 2 CO₂
   2 MgO + SiO₂ → Mg₂SiO₄
   Ca₅(PO₄)3OH + 0.25 SiO₂ → 1.5 Ca₃(PO₄)₂ + 0.25 Ca₂SiO₄ + 0.5 H₂O (vapour)
4. Alumina and Oxide Reaction:
   12 CaCO₃ + 7 Al₂O₃ → Ca₁₂Al₁₄O₃₃ + 12 CO₂
   4 CaCO₃ + Al₂O₃ + Fe₂O₃ → Ca₄Al₂Fe₂O₁₀ + 4 CO₂
   4 CaCO₃ + Al₂O₃ + Mn₂O₃ → Ca₄Al₂Mn₂O₁₀ + 4 CO₂
5. The reaction of Remaining Calcite:
   CaCO₃ → CaO + CO₂
6. Sintering:
   Ca₂SiO₄ + CaO → Ca₃SiO₅