Concrete is a ubiquitous construction material, from ancient times through today. The primary expenditure of energy in concrete production is in the manufacture of cement (the gray powdery substance that binds the aggregates together after addition of water). And, as mentioned in an earlier post, with respect to CO2 emissions, its impacts extend beyond the emissions from the heat necessary to produce it since turning limestone, CaCO3, into lime, CaO, releases CO2. It's estimated that, worldwide, cement production is responsible for about 5% of CO2 emissions, and that on the order of a ton of CO2 is released in the production of a ton of cement.
It's difficult to conceive of a replacement for concrete, so efforts to mitigate the impact of concrete construction on CO2 emissions revolve around replacement of the most CO2 intensive components of the cement. As mentioned in the article linked above, one possibility is "Eco-Cement" which uses magnesium carbonate instead of calcium carbonate (limestone) to produce cement. The topic of this post is the partial replacement of cement with ground granulated blast furnace slag ("GGBFS").
GGBFS is a by-product of producing iron from ore or from producing certain types of steel. The slag typically consists of silicates, aluminosilicates, and calcium-alumina-silicates. The molten slag can be cooled slowly or quickly, but must be quenched (cooled very quickly) to prevent the formation of crystals. The slag must be "vitreous" (glassy) in order to have cementitious properties. The vitreous, granulated slag is then ground to an extremely fine powder for use as a substitute for portland cement in proportions as high as 70% in concrete.
The concrete produced using GGBFS has many desirable characteristics, including increased strength, increased density, decreased permeability and (arguably) increased resistance to sulfate attack and the dreaded alkali silica reaction. Its only significant drawbacks are increased setting time and lower early strength. These can be mitigated where necessary by the addition of appropriate admixtures. Further, the slow setting and strength gain is at least partially due to the relative large (compared to portland cement) grain size using current economically feasible technology. As grinding technology improves, this limitation will diminish. And slow setting is sometimes an advantage, for example, when long transit routes are necessary or during hot weather concreting. That slow setting is sometimes a benefit is demonstrated by the fact that there are currently marketed set retarding admixtures.
This use of what has, in the past, been a waste product of iron and steel production for an environmentally and structurally beneficial application seems to be a win for all concerned. But can it save the world? I wouldn't go that far. When utilized in concrete with equal portions of portland cement and GGBFS, the overall CO2 emission can be roughly halved. This is accomplished by both a reduced need for heating energy and by a reduction in the amount of limestone to be calcined for a given amount of cement.
So what's the potential? Using very rough figures, about 320 million metric tons of slag of suitable chemistry are produced worldwide each year. Some of this is used for other purposes, but we'll use that amount to set a cap on the replacement possible. Approximately 2.5 billion metric tons of cement are produced annually, and roughly 0.8 tons of CO2 emission are avoided for each ton of GGBFS utilized in place of portland cement. Obviously, plenty of capacity is available to use of all the slag produced if this could actually be accomplished. This would result in reduction in CO2 emissions of about 250 million metric tons or, roughly, 1% of world CO2 emissions. It won't save the world but with the G8 agreeing today to reduce emissions 80% by 2050, we have to start somewhere.