Building Wastes for Reducing CO2 Emissions

In the previous post, I lamented that little was being done to reduce CO2 emissions. Some options that everyone ignores are reasonably simple to execute, although they require effort and investment, but in a recent paper from Nature (vol, 629, pp1055 – 1061) Dunant et al. propose what is, in my opinion, a more difficult option: recycling of cement and steel from building wastes. Currently, cement production causes about 7.5% of global CO2 emissions, for two reasons. First, they take lime  and heat it to drive off the CO2, and second, they use coke or powdered coal to heat it and suitable silicates to sufficient temperatures. Portland cement is essentially calcium silicate, preferably with high levels of alite (Ca3SiO5) but usually a small amount of calcium sulphate is added to control the set time. Also often present is tricalcium aluminate and tetracalcium alumino ferrite, which are a consequence of the presence of aluminium and iron oxides/silicates in the clays. These are also cements in their own right. In a building, concrete is made by mixing water, cement, sand and aggregate and pouring it into a space with steel reinforcing. When the building has passed its “use-by” date, it is demolished and all the wastes have to be disposed of.

It is here that the rather novel proposal arises: recycle the cement with the steel in an electric arc furnace. Usually, steel recycling uses a lime-dolomite flux, and this cement paste can act as a partial substitute for this flux. The authors claim the resultant slag “can meet existing specifications for Portland clinker and can be blended effectively with calcined clay and limestone.” Effectively, besides recycling steel they are also recycling the cement.

So, what could possibly go wrong? Basically, the curse of recycling: impurities. Rust from the steel may prefer to make an iron silicate such as in the pyroxene family than be reduced to iron, assuming there is carbon added as a reducing agent. Simple iron silicates are not cements.

Consider what the process would involve. First, the steel recycling. The authors say the scrap metal is melted and oxidised to remove dirt, carbon and phosphorus, then they say sulphur is removed (it is not clear how) and the steel is alloyed. In both steps a flux made from limestone and dolomite is added to protect the steel from air, provide the required basicity, protect the furnace and increase energy efficiency. It is not clear what “removal of carbon” means as if carbon is removed you no longer have steel but simple iron.

The recycling process involves reclinkering hydrated cement paste. The presence of sulphates reduces the production of alite, and the presence of chlorides will make the product of low value and exclude it from use in reinforced applications. The next problem is how to get the cement paste, as the cement is bound to rocks and sand. Currently, research into removing cement from aggregate is not to recover the cement but to recover the aggregate. A problem in recovering cement paste is the sand. The advance reported in the paper is that the higher temperature of the electric arc furnace are sufficient to remove the sulphates and chlorides in the vapour phase, and favours alite production. This produces a volume of molten steel, which can be run off and a lower density viscous slag that the authors claim that, if it is cooled rapidly, can be ground and used as cement.

That is all very well, but the higher temperatures will incorporate more silica and alumina if they are available, and herein lies, in my opinion, a significant problem. The long-term performance of concrete depends critically on the composition of the cement. Having a cement that binds aggregate is not that difficult to achieve, but does it provide the strength over a long time? The demand for cement greatly exceeds the plausible supply of recycled cement, so the standard manufactured product will be around for a very long time. Ask yourself that if you were about to build an elevated stand to park your car, knowing that a failure would wreck your car, would you use standard cement or recycled cement? I think I know the answer most would choose.