Aryabrat Ghosh observes that 2020 will be a year of green sustainability for cement industry where the ground work has already started.
Cement industry is one of the two primary industrial producers of carbon dioxide (CO2), creating up to 5 per cent of worldwide man-made emissions of this gas, of which 50 per cent is from the chemical process and 40 per cent from burning fuel. The CO2 produced for the manufacture of one tonne of structural concrete (using ~14 per cent cement) is estimated at 410 kg/cu m (~180 kg/tonne at a density of 2.3 g/cu cm) (reduced to 290 kg/cu m with 30 per cent fly ash replacement of cement). The CO2 emission from concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every tonne of cement. Cement manufacturer contributes greenhouse gases both directly through the production of CO2 when calcium carbonate is thermally decomposed, producing lime and CO2 and also through the use of energy, particularly from the combustion of fossil fuels.
One reason why the carbon emissions are so high is because cement has to be heated to very high temperatures in order to form clinker. A major culprit of this is alite (Ca3SiO5), a mineral in concrete that cures within hours of pouring and is therefore responsible for much of its initial strength. However, alite also has to be heated to 1,500¦C in the clinker-forming process.
Some research suggests that alite can be replaced by a different mineral, such as belite (Ca2SiO4). Belite is also a mineral already used in concrete. It has a roasting temperature of 1,200°C, which is significantly lower than that of alite. Furthermore, belite is actually stronger once concrete cures. However, belite takes on the order of days or months to set completely, which leaves concrete weak for an unacceptably long period of time.
Current research is focusing on finding possible impurity additives, like magnesium, that might speed up the curing process. It is also worthwhile to consider that belite takes more energy to grind, which may make its full life impact similar to or even higher than alite.
Another approach has been the partial replacement of conventional clinker with such alternatives as fly ash, bottom ash, and slag, all of which are by-products of other industries that would otherwise end up in landfills. Fly ash and bottom ash come from thermoelectric power plants, while slag is a waste from blast furnaces in the ironworks industry. These materials are slowly gaining popularity as additives, especially since they can potentially increase strength, decrease density, and prolong durability of concrete.
The main obstacle to wider implementation of fly ash and slag may be largely due to the risk of construction with new technology that has not been exposed to long field testing. Until a carbon tax is implemented, companies are unwilling to take the chance with new concrete mix recipes even if this reduces carbon emissions.
In India, typical usage of fly ash in PPC type of cement may range from 23 per cent to 26 per cent (average) whereas Bureau of Indian Standard (BIS) has allowed the limit till 35 per cent (in coming years the limit may exceed to 40 per cent), so there is a huge scope to increase fly ash. Chryso in India brings the solution (performance enhancers) to help reduce the issues given above like solutions to increase fly ash/slag content in PPC and PSC type of cement or to save energy by solutions like grinding aid for all type of cements (PPC/PSC/OPC) and also raw meal. With the help of its R&D centre in Turbhe, Navi Mumbai, the discussions has already started with many big cement groups to come up with solutions for the future where we are talking about complex type of cement product (addition of fly ash/slag/limestone in one type of cement) and increase of fly ash (to 40 per cent).
To summarise, one can say that 2020 will also be a year of green sustainability for cement industry where the ground work has already started.