3.4 Cement
Cement is covered next because together with steel it drives nearly half of all global CO2 emissions. Most of the demand is in rapidly developing countries such as India and China. As mentioned earlier cement suits a rapidly expanding population, as buildings can be put up cheaply and quickly and suit a versatile spread of building designs when cement is incorporated. It must be noted however that the quality and lifetime of buildings put up in this way are severely limited.
Cement is produced when limestone is heated to 1000oC which leaves lime. Lime mixed with water and sand harden when exposed to the atmosphere. This is cement, a versatile and very effective building material that can be used for a variety of purposes.
When cement is mixed with stones or blocks (called aggregate) and poured into a mould it's called concrete. Concrete has a lower embodied energy than cement as 75 – 80% is crushed aggregate (which does also have an embodied energy due to the energy used when mining it).
“With its versatility and low cost of materials, construction, and maintenance, concrete has emerged as the material of choice for new construction in the 20th and 21st centuries. With over 10 billion tons of concrete being produced annually, the concrete industry is the largest consumer of natural resources and one of the biggest contributors to greenhouse gas emissions worldwide. ”
(Source: Lauren Midori Kuntz The green of The cement Industry [see reference 4])
Cement and steel go well together because concrete is weak in tension but steel has compression and tension strength. They bond well together, and the concrete protects the steel from corrosion. Reinforcing bars for use with concrete are the biggest application of steel worldwide.
Concrete is big emitter of CO2, because the reaction of turning limestone to lime itself gives off CO2 (50% of emissions). The energy required for this reaction involves the burning of fossil fuels which gives off more emissions (40%). The final 10% is from the energy used in grinding and transportation.
When walking around any urban centre it is impossible to go very far without seeing a building that has used concrete in its construction. Indeed any civil engineer will find it very difficult to be involved in any infrastructure or construction projects that don't use concrete.
A problem with cement, apart from the CO2 emissions is its low recyclability. At the end of its life it cannot be melted down to form new concrete. However, it can be broken up and used as aggregate for more concrete, but this often not the case. Concrete is also a very hazardous material to work with, and can cause burns to human skin.