Cement & lime
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Cement & lime

Cement is a fine powder which makes mortar when mixed with water and, when combined with sand and aggregates, forms concrete. To make cement, manufacturers start by identifying and quarrying minerals such as limestone, clay and sand which contain calcium, silicon, aluminium and iron. Limestone is the principal ingredient but some limestones, such as those containing high levels of magnesium, are not suitable. After quarryiong, large rocks are broken down into fist sized chunks which are then blended to achieve the desired chemical composition of the end product.
 
The chunks are ground down further before being fed into a preheater tower which makes use of the hot gases exiting the kiln. The granules then tumble down through the large rotary kiln which slowly turns and reaches a temperature of 1480C at its hottest. Through the process of calcination, what emerges are red hot particles known as clinker. The clinker contains hydraulic calcium silicates. It is cooled and then, with a little added gypsum to delay setting, it is ground down into cement powder in a ball mill.
 
The rotary kiln was introduced in the 1890's. It consists of a large steel cylinder tilted at a slight angle from the horizontal and lined with refractory bricks. These bricks have to withstand the stress induced by the rotation, the abrasion from the mineral granules as well as the high temperatures. These kilns are designed for continuous use and can run for a year or more before the refractories need to be replaced.
 
A side product of cement making is a large volume of carbon dioxide. It is estimated that 5% of the world's man made carbon emissions come from cement manufacture. Various steps are being taken to reduce the carbon footprint of cement such as including powdered limestone in the mix without additional treatment, putting some of the carbon dioxide back into the cement, recycling concrete as aggregate and even investigating a biological process whereby bacteria create cement at room temperature.
 

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