Concrete, It's Not so Easy Being Green

Gray Merriam | Feb 17, 2021

Concrete like your foundation or garage floor added carbon dioxide to our atmosphere and is heavily involved in climate change. We use about 4 billion tonnes of cement each year to build with. That concrete is responsible for about 8% of the global emissions of carbon dioxide --- about 800 million tonnes --- the cause of climate change.

How does cement production produce carbon dioxide? The first step in producing cement is "burning" limestone. Pulverized limestone is calcium carbonate powder. It is heated to about 1450 degrees Centigrade (2700 degrees Fahrenheit). Chemically, the calcium carbonate of the limestone is converted to lime (calcium oxide) releasing carbon dioxide to the air above the kiln. About half of the CO2 from cement making is released here. Pioneer builders did not know.

The other half of the CO2 released during cement manufacture is from burning some sort of fuel (not always clean fuel) to get the limestone up to 1450 degrees C.

Each kilogram (~2.5 pounds) of Portland cement involves releasing 0.93 Kg (~2.3 pounds) of CO2. About 98% of all concrete used globally is Portland cement.

The burned limestone, called clinker, is ground up and mixed with gypsum (calcium sulfate). It combines chemically and releases more than 800 kilograms of CO2 for each tonne of cement produced. When we mix this with water, it produces the "glue" (calcium silicate hydrate) that holds concrete particles together. Cooling and grinding the clinker releases about 10% of the total CO2 emitted by cement manufacture

We need cement to shelter people so, we need better ways to produce and use cement while reducing the CO2 emissions from its use. Some companies such as CarbonCure Technologies of Halifax have done the research and marketing needed to find ways to put CO2 produced by other industries back into cement manufacture to reduce CO2 emissions from concrete. This is the sort of technology needed to adapt us to the new world that we have created.