Concrete is a human-made substance, with a key ingredient being Portland hydraulic cement. Cement itself is created through a chemical reaction induced between clay and limestone. The calcium oxide (lime) mixture is mixed, dried and then fired in large kilns until it breaks down into a fine dry powder. This extremely energy-intensive procedure for cement produces much CO 2 gas. One tonne of CO 2 is produced for every tonne of Portland cement that is made, so one cubic metre (m3) of concrete will emit approximately one tonne of CO 2 in the process and end up weighing around 2500kg.