Manufacturing process < All about Cement

All about Cement

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Manufacturing process

From the limestone quarry to the delivery of the end product, follow every step in the cement manufacturing process.

Step 1: extraction of raw materials
The raw materials needed to produce cement (calcium carbonate, silica, alumina and iron ore) are generally extracted from limestone rock, chalk, clayey schist or clay. Suitable reserves can be found in most countries. These raw materials are extracted from the quarry by blasting. They are then crushed and transported to the plant where they are stored and homogenized.

Step 2: raw grinding and burning

Very fine grinding produces a fine powder, known as raw meal, which is preheated and then sent to the kiln. The material is heated to 1,500°C before being suddenly and dramatically cooled by bursts of air.

This produces clinker, the basic material required for the production of all cements.

CO2 and cement

Why does the manufacture of cement produce CO2?
Cement manufacturing is the source of 5% of global CO2 emissions. The cement industry is a natural producer of CO2:

60% of emissions are due to the transformation of raw materials at high temperatures (the "decarbonation" of limestone),
40% result from the combustion required to heat the cement kilns to 1500°C.

Kiln reliability

Ensuring a good yield
The reliability coefficient of a kiln is calculated by comparing the number of lost time hours to the number of operating hours.
Improving this indicator will be key to achieving higher yield and productivity targets set in the Excellence 2008 program. Lafarge Cement France saw its efforts pay off when the reliability coefficient of its kilns passed from 92.8% in 2002 to 96.5% in 2006.

Step 3: cement grinding and shipping

A small amount of gypsum (3-5%) is added to the clinker to regulate how the cement will set. The mixture is then very finely ground to obtain "pure cement". During this phase, different mineral materials, called "cement additives", may be added alongside the gypsum. Used in varying proportions, these additives, which are of natural or industrial origin, give the cement specific properties such as reduced permeability, greater resistance to sulfates and aggressive environments, improved workability, or higher-quality finishes.

Finally, the cement is stored in silos before being shipped in bulk or in bags to the sites where it will be used.

Recycling

Recycled materials in cement

In 2006, the cements manufactured by Lafarge had an average additive content of 23%. These additives were of natural origin, such as limestone and volcanic rock, or industrial origin, for example blast furnace slag (a steel industry by-product) or fly ash (from coal-fired power plants).

The use of additives:

enhances certain cement properties,
recycles materials that would otherwise have been sent to landfill,
reduces the consumption of natural raw materials,
reduces CO2 emissions.

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