Concrete is very versatile, inexpensive, literally hard, and can be cast into almost any shape. It consists, in principle, only of sand, gravel, water, and the binder cement. The latter is made by the calcination of lime, clay, and some other components, and forms stable calcium silicate hydrates during hardening, which are responsible for the properties of concrete. However, the problem lies precisely in the calcination of lime, because for every molecule of calcium oxide produced, the so-called ‘burnt lime’ or ‘quicklime’, one molecule of the greenhouse gas carbon dioxide is released. For an annual world production of around 4.5 billion tons of cement, this is translated into 2.7 billion tons of carbon dioxide.
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Source: ScienceDaily