Do you know what is cement, its types and mixtures and hydration of cement?If you are an engineer you must read this article.
We know that cement is used as a binder between the coarse aggregate. It is believed that early Egyptians mostly used cementing materials, obtained by burning gypsum.
The cement and hydration of cement are two familiar terms for a civil engineer.Portland cement was first patented in 1824.
Let’s read the definition of hydraulic cement first,
“A hydraulic cement produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, and a small amount of one or more forms of calcium sulfate as an interground addition”
Lets read several properties of hydraulic cement and non-hydraulic cement
Hydraulic cement (e.g., Portland cement)
- Hardens by reaction with water
- Hydrated products are resistant to water
- Do not require pozzolanic materials to develop the resistance to water
Non-hydraulic cement (e.g., Gypsum and lime cements)
- Hardens by reaction with water
- Hydrated products are NOT resistant to water
The cement ingredients are mainly calcareous and argillaceous materials.
Calcareous material – Containing CaCO3 (primary source – limestone); impurities such as iron and alumina are sometimes present
Argillaceous material – Containing clayey matter, source of SiO2, Al2O3
Gypsum – Added in the final stages of manufacture as a set regulator
Sometime, ground limestone is also added to cement
Manufacture of cement
Calcareous materials such as limestone and chalk and argillaceous materials such as shale or clay are used as the raw materials for the manufacture of cement.
The location of the cement factories are selected based on the availability of the these raw materials.
The manufacturing process of cement consist of grinding of these raw materials and mixing them in certain fixed proportions based on the purity and composition of the raw materials.
After mixing these raw materials are heated in a kiln at a temperature of about 1300 to 1600 degree celcius. After this process, the clinker is cooled and ground to fined powder with the addition of the 5% gypsum.This product is known as Portland cement
Based on the availability of moisture content at the grinding stage the grinding process is termed as dry or wet process.Also there is one more process semi-dry process.
The dry process of cement
The dry process is more economical compared to the dry process, because the total consumption of the coal for the burning process is only 100kg, whereas the coal consumption in the wet process is 350kg.
In the dry process of cement, the raw materials are crushed in the absence of water and put them into grinding mill with correct proportions, the proportion can be determined based on the purity and composition of raw materials.
The mix is converted into fine powder by the grinding mill and dry powder is obtained. This dry powder is also known as raw meal and it is mixed in the presence of compressed air.
After one hour of aeration the mixture is obtained. The blended meal is further sieved with the help of rotating disc called granulator.
The wet process of cement
In the wet process of cement, the clay or shale is mixed with the limestone from the quarries in a tube mill. Then they are grounded into fine slurry with the addition of water. Slurry is a creamy substance with water content of 35 to 50 percent.
Hydration of Cement
Hydration is the chemical reaction between cement and water and aggregate unless it is mixed with water content to form adhesive property. The quantity, quality, stability and rate of formation of hydration products are important. Cement acquires adhesive property only when it is mixed with water.
The chemical reaction in the hydration process is exothermic. The reaction liberates heat. Solid state type of mechanism and through solution type of mechanism may occur during the hydration process.
The heat during the hydration can be easily measured with putting freshly mixed concrete in a vacuum flask.
It is found that the temperature inside the mass freshly concrete is 50 degree celcius more than the temperature at the time of placing concrete.
The heat of evolution is due to the reaction between aluminates and sulfates.
- Limestone and clay are quarried, crushed, stockpiled and ground separately. In the wet process, slurries are made and blended. However, this is uneconomical. In the dry process, the grinding is performed with dry materials but some water maybe added to facilitate handling.
- The ground and blended material is fed into a rotating inclined kiln. As the material slowly moves down the kiln, evaporation, calcination, clinkering and cooling take place. (Clinkering is a heat treatment where partial melting occurs.)
- The clinker (dark porous nodules of 6-50 mm diameter) is further cooled with air or water. It is ground to a powder in a ball mill, along with a small amount of gypsum, to obtain portland cement. (Gypsum is added to avoid the flash set of the ground clinker.)