CEMENT

A cement is a binder, a substance that sets and hardens independently, and can bind other materials together. A construction material created by grinding calcined limestone and clay to a fine powder, which can be combined with water and poured to set as a solid mass or used as an ingredient in making mortar or concrete.

Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world, used as a basic ingredient of concrete, mortar, stucco, and most non-specialty grout. It usually originates from limestone. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards . 

Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaO·SiO2 and 2 CaO·SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass. 

White Portland cement or white ordinary Portland cement (WOPC) is similar to ordinary, grey Portland cement in all respects except for its high degree of whiteness. Obtaining this color requires some modification to the method of manufacture; because of this, it is somewhat more expensive than the grey product. 

In the manufacture of Portland cement, clinker is lumps or nodules, usually 3–25 mm in diameter, produced by sinteringlimestone and alumino-silicate (clay) during the cement kiln stage. 
Clinker consists of various calcium silicates, including tricalcium silicate (Ca3SiO5, also written CaO.Ca2SiO4) anddicalcium silicate (Ca2SiO4).

Tricalcium aluminate and calcium aluminoferrite are other common components. 

Comparison_of_Chemical_and_Physical_Characteristics

Portland cement clinker is made by heating in a rotary kiln at high temperature a homogeneous mixture of raw materials. The products of the chemical reaction aggregate together as molten minerals at the sintering temperature. 

The sintering temperature for modern cements is about 1450 °C. Aluminium oxide and iron oxide are present as a flux and contribute little to the cement strength. The major raw material for the clinker-making is usually limestone (CaCO3) mixed with a second material containing clay as source of alumino-silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of these limestones can be as low as 80%. Second raw materials (materials in the rawmix other than limestone) depend on the purity of the limestone. Some of the second raw materials used are: clay, shale, sand, iron ore, bauxite, fly ash and slag.

The process

Cement production involves the heating, calcining and sintering of blended and ground raw materials, typically limestone and clay or shale and other materials to form clinker. This clinker burning takes place at a material temperature of 1450 °C in kilns, which are inclined rotating cylinders lined with heat resistant bricks. Afterwards, the clinker is ground with a small amount of gypsum to give Portland

cement, which is the most common variety of cement manufactured in Europe. In addition, blended cements are produced by inter grinding cement clinker, small amounts of gypsum as well as materials like fly ash, granulated blast furnace slag, limestone, natural or artificial pozzolanas. Large cement plants produce of the order of 4,000 tonnes of cement per day. 

Depending on how the raw material is handled before being fed to the kiln, basically three different types of processes can be distinguished: the dry, semi-dry/semi-wet and wet process. The technology

applied depends on the origin of the raw materials. The origin/type of limestone/clay and the water content (ranging from 3% for hard limestone to above 20% for chalk), are particularly important.

In the dry process the feed material enters the kiln in a dry, powdered form. The kiln systems comprise a tower of heat exchange cyclones in which the dry feed is preheated (“preheater kiln”) by the rotary kiln’s hot exit gases prior to entering the actual kiln. The calcination process can almost be completed before the raw material enters the kiln if part of the fuel is added in a special combustion chamber

(“precalciner kiln”).

In the wet process, which is often used for raw materials with a high moisture content, the feed material is made by wet grinding and the resulting slurry, which contains typically 30-40% water, is fed directly into the upper end of the inclined kiln.