What is cement made of?
For the bulk and continuous manufacturing and production of Portland Cement following mineral materials are used: calcareous materials (limestone or chalk), argillaceous material (shale or clay), blast furnace slag, silica sand, iron ore, and gypsum as raw material. Cement factories are established where these raw materials are available in plenty and facilities of long-distance transportation of raw and finished materials.
The manufacture of Portland cement is a complex process and done in the following steps: grinding the raw materials, mixing them in certain proportions depending upon their purity and composition, and burning them to sintering in a kiln at a temperature of about 1350 to 1500 ⁰C. During this process, these materials partially fuse to form nodular shaped clinker by broking of chemical bonds of the raw materials and recombined into new compounds.
The clinker is cooled and ground to a fine powder with the addition of about 3 to 5% of gypsum. By this process, the product formed is Portland cement.
Types of Manufacturing of cement
There are two processes known as “wet” and “dry” processes depending upon whether the mixing and grinding of raw materials are done in wet or dry conditions.
With a little change in the above process, we have the semi-dry process also where the raw materials are ground dry and then mixed with about 10-14 percent of water and further burnt to clickering temperature.
In the old age, because of the possibility of more accurate control in the mixing of raw materials, the wet process is popular. The techniques of the intimate mixing of raw materials in powder form were not available then.
Later, the dry process is wide uses, because of the modem development of the technique of dry mixing of powdered materials using compressed air.
The dry process requires much less fuel as the materials are already in a dry state, whereas in the wet process the slurry contains about 35 to 50 percent water. In the wet process, the drying process is fuel consumption. Here, below you can read both “wet” and “Dry” process in detail.
1. WET Process
In the wet process, first of all, the newly quarried limestone is crushed into smaller pieces. Then in the ball or tube mill, it is mixed with clay or shale with water to form a slurry. The slurry is a liquid of creamy consistency with a water content of about 35 to 50 percent, wherein particles, crushed to the fineness are held in suspension.
Then this slurry is pumped into slurry tanks or basins. In that tank, the limestone and clay particles are prevented to settle by blowing compressed air from the bottom of slurry are kept in ignited condition with help of rotating arms with chains.
This slurry is maintained at the required chemical composition in a tube mill by testing and correcting time to time. Also tested in the different storage tanks by blending the slurry. After that, in the final storage tanks by the agitation of slurry, it is kept inhomogeneous condition.
One of the important parts of the cement production plant is the rotary kiln. That homogeneous corrected slurry is applied by nozzle spray on the upper end of the rotary kiln. Rotary kiln is a 3m to 8m diameter steel cylinder, lined with fire resistance refractory materials, fitted on roller bearings and capable of rotating about its own axis at a designed speed.
The length of the rotary kiln may vary anything from 30 meters to 200 meters. The slurry on being sprayed against a hot surface of a flexible chain loses moisture and becomes flakes. These flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move from the upper end towards the lower end of the kiln subjecting itself to higher and higher temperatures.
The kiln is fired from the lower end. Normally for the manufacturing of cement, the fuel used are powdered coal, oil or natural gas. As the materials go ahead then, the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the temperature is in the order of 1500°C, about 20 to 30 percent of the materials get fused. Lime, silica, and alumina get recombined. This is where the oxides in raw materials will be combined to form compounds in the clinker.
The fused mass turns into a nodular form of size 3 mm to 20 mm known as clinker. Then, the clinker is cooled under controlled conditions in a rotary cooler. The clinker is stored in silos or bins. Generally, the clinker weighs about 1100 to 1300 gms per liter. The liter weight of the clinker indicates the quality of the clinker.
Then, cooled clinker ground in a ball mill. For the prevention of flash-setting of the cement 3 to 5 % of gypsum is added. Ball mill is a hardened steel ball that has different chambers that charged in sequence forward. The particles are crushed into the required fineness level. And particles are separated by air currents and taken to storage. Then, supply to the market as a final product and uses in various construction works such as Buildings, pavements, dams or other large work sites.
In the modern method of grinding, a well-graded cement particle is formed by the particle size distribution of cement particles.
As we all Civil Engineer know that for the formation of good concrete a good grading of aggregate is required, good grading cement particles are also required for good concrete.
I am also telling you that a well-graded cement also improves the strength of concrete.
The Fig.1. show the flow diagram of the dry process of the manufacture of cement.
1. Dry Process
In the dry and semi-dry process the raw materials are crushed in a dry state. Then, they are processed in grinding mill, dried and reduced to very fine powder-like. That dry power is further blended, corrected for the right composition and mixed with the flow of compressed air. Nearly about one hour of aeration, a uniform mixture is obtained, that aerated power tends to behave like a liquid.
Then, the blended meal is sieved and apply to process in a rotating disc called granulator. A quantity of water about 12 percent by weight is added to make the blended meal into pellets.
The blended meal is converted into pellets by the addition of water about 12 percent by weight. This is done for the easy exchange of heat by airflow during the further chemical reaction.
Smaller types of equipment are used in dry process comparative to wet process and dry process is also economical.
Nowadays many cement factories used modern production technology to increase capacity and quality of cement.
In modern technology, they use the equipment, such as a cross belt analyzer manufactured by Gamma-Metrics of USA to find the composition of limestone at the conveyor belts, high-pressure twin roller press, six-stage preheater, and vertical roller mill. The latest Technology with a modern control system includes stacker and reclaimer, on-line X-ray analyzer, Fuzzy Logic kiln, and other process control.
It is important to note that the strength properties of cement are considerably influenced by the cooling rate of the clinker. The rate of cooling influences the degree of crystallization, the size of the crystal and the number of amorphous materials present in the clinker.
The properties of this amorphous material for similar chemical composition will be different from the one which is crystallized. This all about the manufacturing of cement.
One of the interesting points about cement is that the strength properties of cement depend upon the cooling rate of the clinker, because the rate of cooling influences the degree of crystallization, the size of the crystal and the number of amorphous materials present in the clinker.
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