Concrete – and in particular cement, concrete’s key ingredient – is catastrophic for the environment. Versatile and long-lasting, concrete buildings and structures are in many ways ideal for climate-resilient construction. But concrete has a colossal carbon footprint — at least 8% of global emissions caused by humans comes from the cement industry alone, more than any country other than China and the US – and somewhere between four and eight percent of all global man-made carbon emissions.
These are largely due to the way that limestone, which is the main ingredient in cement, is processed. First, The rock is crushed and burned to extract calcium, which is the binding agent used in cement, releasing the carbon into the atmosphere in the process. Concrete is made by adding sand and gravel to cement, whisking the mixture with water and pouring it into moulds before it dries. Making the cement is the most carbon-intensive part: it involves using fossil fuels to heat a mixture of limestone and clay to more than 1,400 °C in a kiln. Also, when limestone (calcium carbonate) is heated with clays, roughly 600 kilograms of carbon dioxide is released for every tonne of cement produced.
The recipe for concrete has been largely unchanged since the 19th Century: you just need a mixture of large aggregate (stones), small aggregate (like sand), cement – which binds it together – and water. “The main issue with concrete is the production of cement, because if you want to get a cement, you need to have clinker,”… Clinker, typically a mixture of calcium carbonate, clay, and gypsum is mixed and heated in a kiln. “You need to heat clinker at a very high temperature, maybe at 1500 degrees, and by doing this, you are producing lots of CO2 emissions.” Inside the kiln, the clinker undergoes calcination: the calcium carbonate breaks down into calcium oxide, releasing even more CO2.
Cement Sustainability Initiative: (CSI) is a program of the World Business Council for Sustainable Development that has been considered a model for the sectoral approach to climate change mitigation The cement industry is a significant GHG emitter. Th e data suggest that CO2 emissions per produced ton of clinker decreased, 6 percent between 1990 and 2006. Thermal energy efficiency improved by 14 percent over the same period. But the emissions of CSI members increased by 35 percent because their output grew by 50 percent in the same period.