Concrete; a primary building material is extremely energy intensive to make and transport, and produces a significant amount of the world’s greenhouse gas emissions. Concrete’s impact on the environment starts when limestone is blasted in quarries to make cement – the binder, or substance that sets and hardens it into a useful building material. Cement accounts for 7 to 15% of concrete’s total mass by weight and is made by superheating (in coal-fired kilns) a mixture of limestone and clay and then grinding the resulting substance into a powder. When this power mixes with water, it forms strong calcium-silicate-hydrate bonds that can bind other particulates, like sand or gravel, to make concrete. The cement-to-water ratio determines the strength of the concrete.
Once limestone has been blasted and mined it is then transported to a cement plant, where the fuels used by the plant and machinery produce CO2 emissions. Next the limestone, or calcium carbonate, releases CO2 when it is heated to make the cement. Forty percent of CO2 emissions from the cement plant come from the combustion process and Sixty Percent of CO2 emissions come from the calcination process. Since calcination is intrinsic to the process, they must focus on reducing energy use associated with the manufacture of concrete.
Concrete producers also say that as concrete ages, it carbonates and reabsorbs all the CO2 released during calcination – but this process takes hundreds of years. The general consensus is that cement manufacturing produces about 5% of global CO2 emissions generated by human activity, and 3% of global emissions of all greenhouse gases. By comparison the transport sector is responsible for about 15% of global greenhouse gas emissions, so concrete has a pretty hefty share of the pie considering it’s just one material.To overcome all these effects, green cement is produced.
This cement is based on an intermediate product, clinker, which is produced with minor additions of mineralizes to the kiln..These mineralizes are Calcium Sulphate (CaSO4) and Calcium Fluoride (CaF2).With the addition of these mineralizes, the energy consumptions are reduced by 5%. Also the CO2 emissions get reduced. It is found that there is a 5-10% increase in 28-day strength of the cement.
A material that resembles Portland cement both chemically and physically is fly ash, which is the by-product in the combustion of coal. Fly Ash has been used in concrete construction for years. However, variations in properties related to type of coal being burned and the nature of the combustion process at any power plant have considerable limited its use as a replacement for Portland Cement. Typically, fly ash is used to replace 25% of Portland Cement in a concrete mixture. They can be used in higher dosages, thus reducing the environmental impact of both concrete construction and coal-fired power plants.
Green cement can tackle global warming on two fronts. It would eliminate the need to heat limestone, which releases CO2. And harmful emissions can be siphoned away from power plants and locked into the cement.