Slag like other SCMs such as fly ash as been classified as a greener/sustainable materials because they are derived from wastes of industrial processes. However, since slag require more processing compared to fly ash can it really be classified as a green material.
Same also goes for metakaolin that is derived from the calcination of Kaolin
There is no such thing as a green material per se. The environmental benefits depend on the material's use.
Slag has a carbon footprint of 52-143 kgCO2/tonne vs 4-27 kgCO2/tonne for PFA. So, on that basis you could say that it is not as environmentally friendly as PFA. However, you can achieve higher replacement levels in slag blends than in PFA blends. So a 60% slag-cement blend will have a lower carbon footprint than a 20% PFA blend.
Then you need to consider whole life performance. In certain instances PFA blends may out-perform slag cements, but often slag cements show improved durability.
Finally, you need to think about the quantities of materials available. Supplies of PFA and GGBS are limited and there are insufficient quantities of either to replace cement at optimum replacement levels. So from this perspective, we should look to use whatever materials we can, so long as they help to reduce the overall environmental burden of concrete. More specifically, there are not ready supplies of PFA and GGBS across the globe, so you should also consider transport based emissions (although these are often trivial).
I've attached a book chapter I wrote a couple of years ago on various SCMs. It was published in the book "Sustainability of Construction Materials, 2nd Edition".
That's an excellent and comprehensive reply from Prof Black. I was wondering how ever regarding the nature of PFA and GGBFS being industrial wastes should not be considered to be material with zero carbon foot print per se if we ignore the processing, handling and transport.@leonblack
As I understand it, the emissions for pfa and ggbs are those associated with handling. Certainly there are no emissions allocated from the production of the primary products. It is the granulation and grinding processes which lead to ggbs having a higher carbon footprint than pfa.
It is high time to understand any material that is capable of reducing our dependence upon cement (which is basically responsible for huge carbon emissions) should be considered to be a green material in the context of saving our environment from collapsing.
Hence, in my view, slag is greener but to have knowledge regarding its embodied carbon/energy, is equally necessary.
Thank you.
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