Cement is a binder, a substance that sets and hardens as the cement dries and also reacts with carbon dioxide in the air dependently, and can bind other materials together.Cement is the best known binding material. Over the period of time, different varieties of Cement have been prepared to accommodate all needs. Portland Cement is the most commonly used Cement and all other variants are derived from Portland Cement.
Portland cement is produced by burning a homogenized mixture of clay and limestone, resulting in intermediate product called clinker. This inturn is mixed with gypsum in a specified proportion, and ground into fine powder to form the final product called Portland cement. This product is very useful in construction and engineering works of housing, buildings, bridges and cement concrete roads.
Following are the types of Portland Cement Ordinary Portland Cement {Type I},Modified Portland Cement, Rapid Hardening or High Early Strength Portland Cement (Type III), Quick Setting Cement, Low Heat Portland Cement (Type IV), Sulphate Resistant Portland Cement (Type V), Water Repellent Portland Cement, Water Proof Portland Cement, High Alumina Cement,Portland Slag Cement, Air Entraining Portland Cement (Type I-A, II-A, III-A), Pozzolana Portland Cement, Supersulphated Cement, Masonry Cement and Expansive Cement.
The major raw materials required for the production of cement are limestone, clay, sandstone, gypsum and pumice. These raw materials are proportionally mixed at different stages of the production process inorder to produce an intermediate product called clinker and final product of cement.
The technology of cement production involves several unit operations, and are complex to enumerate. However, the major unit operations are the following:-
– Preparation of raw mix
Quarrying, crushing and transportation
Proportioning of the ingredients
Grinding of raw mix
– Calcinations and storage
Homogenizing of raw mix
Burning of raw mix in vertical shaft kiln and storing of clinker
– Production of cement
Grinding of clinker and gypsum (96/4%)
Packing of cement
Cement plants are usually located closely either to hot spots in the market or to areas with sufficient quantities of raw materials. The aim is to keep transportation costs low. Basic constituents for cement (limestone and clay) are taken from quarries in these areas.
Basically, cement is produced in two steps: first, clinker is produced from raw materials. In the second step cement is produced from cement clinker. The first step can be a dry, wet, semi-dry or semi-wet process according to the state of the raw material.
The raw materials are delivered in bulk, crushed and homogenised into a mixture which is fed into a rotary kiln. This is an enormous rotating pipe of 60 to 90 m long and up to 6 m in diameter. This huge kiln is heated by a 2000°C flame inside of it. The kiln is slightly inclined to allow for the materials to slowly reach the other end, where it is quickly cooled to 100-200°C.
Four basic oxides in the correct proportions make cement clinker: calcium oxide (65%), silicon oxide (20%), alumina oxide (10%) and iron oxide (5%). These elements mixed homogeneously (called “raw meal” or slurry) will combine when heated by the flame at a temperature of approximately 1450°C. New compounds are formed: silicates, aluminates and ferrites of calcium. Hydraulic hardening of cement is due to the hydration of these compounds.
The final product of this phase is called “clinker”. These solid grains are then stored in huge silos. End of phase one.
The second phase is handled in a cement grinding mill, which may be located in a different place to the clinker plant. Gypsum (calcium sulphates) and possibly additional cementitious (such as blastfurnace slag, coal fly ash, natural pozzolanas, etc.) or inert materials (limestone) are added to the clinker. All constituents are ground leading to a fine and homogenous powder. End of phase two. The cement is then stored in silos before being dispatched either in bulk or bagged.
Cement produced in Nigeria should comply with NIS 444.1:2003. Strength class should be 42,5N or higher because the concrete must develop strength as rapidly as possible. The quality of cement is regulated by the Standards Organization of Nigeria {SON}.
Nigeria cement production capacity is between an estimated 26 million tons and 28 million tons per annum and the industry has witnessed 95.6 per cent growth in the span of 8 years, beginning 2005 to 2012.
It is widely agreed that one of the most basic needs of man is shelter and in time past man has experienced with different material in order to meet with his housing needs. In modern day history, the use of cement along with sand and gravel as come to be accepted as one of the best construction raw material because they provide durability, firmness and can withstand harsh weather conditions though other materials like steel and glass are also used.
The ever increasing deficit in housing supply in Nigeria which is estimated at over sixteen million {16,000,000} units and the need for the construction of other important infrastructure if the country is to meet up with the vision 20:2020 project of becoming amongst the top 20 economies in the world by 2020 continues to drive the need to cement, an essential construction raw material.
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