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Cement Manufacturing Process

Cement is the substance which holds concrete together, which means that it is extremely widely used in our society.  It has been manufactured in New Zealand for more than 100 years, and during this century production has increased one hundred-fold. Portland cement (the only type of cement in common use today) is manufactured in a four step process. 

 

Step 1 - Quarrying

Limestone and a 'cement rock' such as clay or shale are quarried and brought to the cement works. These rocks contain lime (CaCO3), silica (SiO2), alumina (Al2O3) and ferrous oxide (Fe2O3) - the raw materials of cement manufacture. 

 

Step 2 - Raw material preparation

To form a consistent product, it is essential that the same mixture of minerals is used every time. For this reason the exact composition of the limestone and clay is determined at this point, and other ingredients added if necessary.  The rock is also ground into fine particles to increase the efficiency of the reaction. 

 

Step 3 - Clinkering

The raw materials are then dried, heated and fed into a rotating kiln.  Here the raw materials react at very high temperatures to form 3CaO SiO2 (tricalcium silicate), 2CaO SiO2 (dicalcium silicate), 3CaO Al2O3 (tricalcium aluminate) and 4CaO Al2O3 Fe2O3 (tetracalcium alumino-ferrate). 

 

Step 4 - Cement milling

The 'clinker' that has now been produced will behave just like cement, but it is in particles up to 3 cm in diameter.  These are ground down to a fine powder to turn the clinker into useful cement. Cement production has several quite serious environmental hazards associated with it: dust and CO2 emissions and contaminated run-off water.  Both cement works in New Zealand have measures in place to minimise these hazards.

 

Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world because it is 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 such as the European Standard EN197-1:

 

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.

(The last two requirements were already set out in the German Standard, issued in 1909).

 

ASTM C 150 defines portland cement as "hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition." Clinkers are nodules (diameters, 0.2-1.0 inch [5–25 mm]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of the limestone, shales, and other naturally occurring materials make portland cement one of the lowest-cost materials widely used over the last century throughout the world. Concrete is one of the most versatile construction materials available in the world.

 

Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 °C for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as Low Heat (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3 CaO·Al2O3) formed. The major raw material for the clinker-making is usuallylimestone (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. When a cement kiln is fired by coal, the ash of the coal acts as a secondary raw material.

 

Step 5 - Cement grinding

In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum oranhydrite) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the "specific surface area", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface area. Typical values are 320–380 m2·kg−1 for general purpose cements, and 450–650 m2·kg−1 for "rapid hardening" cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for 1–20 weeks production, depending upon local demand cycles. The cement is delivered to end-users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In industrial countries, 80% or more of cement is delivered in bulk.


 



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