Recycling Of Portland Cement And Steel In Electric Arc Furnaces

The use of concrete and steel have both become the bedrock of modern-day construction, which of course also means that there is a lot of both which ends up as waste once said construction gets demolished again. While steel is readily recyclable, the Portland cement that forms the basis of concrete so far is not. Although the aggregate from crushed concrete can be reclaimed, the remainder tends to end up in a landfill, requiring fresh input of limestone to create more cement. Now a team of researchers from the University of Cambridge claim to have found a way to recycle hydrated Portland cement by using it as flux during steel production in electric arc furnaces (EAFs).

Not only does this save a lot of space in landfills, it also stands to reduce a lot of the carbon dioxide produced during cement and steel production, which is primarily from the use of limestone for cement and lime-dolomite for steel. The details can be found in the open access paper in Nature by [Cyrille F. Dunant] and colleagues. Essentially reclaimed cement paste is mixed with some fresh material to form the flux that shields the molten steel in an EAF from the atmosphere. The flux creates the slag layer that floats on top of the molten steel, with this slag after cooling down being ground up and turned into cement clinker, which is then mixed to create fresh cement.

The process has been patented by Cambridge, who call the product ‘Cambridge Electric Cement‘, with the claim that if using low-carbon power sources for the EAF like hydro and nuclear, it would constitute ‘no emissions’ and ‘no landfill’ cement. We have to see how this works out on an industrial scale, of course, but it would definitely be nice to keep concrete and cement in general out of landfills, while cutting back on limestone mining, as well as questionable practices like adding heavy metal-laden fly ash as filler to concrete.

Thanks to [cscott] for the tip.

24 thoughts on “Recycling Of Portland Cement And Steel In Electric Arc Furnaces

  1. I’m surprised to find that using crushed concrete as base stone in new construction isn’t more widespread. In my area, it is an affordable option to crushed rock … to the point where local contractors have recycled roadbed materials available to the public.

    Cheers!

    1. From the post: “Although the aggregate from crushed concrete can be reclaimed, the remainder tends to end up in a landfill, requiring fresh input of limestone to create more cement.”

      It’s the “clinker” which is the dehydrated element that reacts with water to form the cement, which is the key missing piece. The way we’ve so far tried to make concrete more “green” is by adding more and different fillers (“aggregate”) but until this research there wasn’t any way around the need for fresh clinker to hold everything together.

      1. Umm, all over the midwest at least, they grind up old concrete and use it as roadbed and basically as a replacement for gravel. It is highly desirable because the remaining cement dust packs and gets firmer from moisture.

        1. Oh and the rebar is not a problem because machines crush the concrete and the rebar is separated and recycled like any other steel. Crushed concrete here does not take up landfill space at all. It is used just like gravel and in fact the excess is sometimes used to refill old quarries. Yet another solution looking for a problem.

          1. >it is used just like gravel

            Which is basically landfill in various different senses.

            The problem with downcycling is that you eventually get more of the material than you really need, so you have to tip it somewhere out of sight. Maybe it’s filling up road beds or refilling quarries because there’s literally nothing else to do with the stuff. Eventually you run out of places to put it and have to open up a landfill to put it in.

  2. Lots of research into recycling concrete with “plans” to close all the limestone mines by 2050. The problem is that old rebar infested concrete also has a bunch of other contaminates so it is not as strong or durable as “fresh made”. Do you want your new 200 story building made out of something that “might be okay”?

    It can be done but getting pure calcium out of waste takes a lot of energy. Energy we need to make electricity for {cars. AI, air conditioning, robots}. The market economy will not choose putting massive amount of energy into making pure calcium so we’ll need heavy government regulation to tip the balance.

    Of course shipping our waste concrete/rebar to a country that can recycle it is not on because: A) it is really heavy and B) that has not worked out so well for plastic.

    Maybe by 2050 we’ll have unlimited amounts of cheap fusion electricity and we can recycle the concrete into iron and calcium at the demolition site … but until them don’t sell your limestone mining stock just yet.

    1. there’s no “might be okay” to it. this is a fundamental misrepresentation of engineering.

      the success of our marvelous concrete and steel world is that no one (well, except for the home depot weekend warrior like me) blindly uses “the best available grade” of concrete or steel. they test them! they determine the characteristics, and then the engineers use those characteristics when they design the structure. concrete made with different processes has different strength and flexibility. i don’t know the characteristics of concrete made with this proposed process (or what refinements or variations they may have in the future), but the characteristics will be determined and then in larger engineered structures it will only be used where it’s appropriate.

      it’s not the quality of ingredients, but rather whether the ingredients are matched to the design that determines whether it stands up.

      1. In the world, there is a spectrum of construction and engineering standards.

        Don’t pretend that there aren’t many parts of the world where pouring an additional couple of floors on top of an old building isn’t routine. Straw considered reinforcement by ‘inspectors’.

        Also parts of the world that can’t afford to knock down 80 year old flaktowers. Built too strong.
        Water jet cutting windows in considered ‘win’.

        In the middle, the good old USA.
        Design life.
        What is the design life of a tension slab?
        Tension cable diameter and expected corrosion rate?
        What year did tension slabs get really really common in American burbs south of frost?

        1. i was responding to a comment “Do you want your new 200 story building made out of …”. yes of course there are other standards in lower-stakes buildings. and while many buildings are not formally engineered, in practice they are subjected to the same test as the larger ones. people mimic the techniques of the building next door, because they know it is still standing. if a new source of concrete has a significant defect when used in this way, they will know it sooner or later. build until failure is just like build as engineered — it is a constant process of discovering the qualities of the available materials. there’s no building that isn’t in communication with that quality.

    2. It is just like plastic.

      At the end of the day, it’s just cheaper to burn the old plastic for power and make new from oil. Even if accounting is done in megatons of CO2.

      The same is likely true of old ‘crete.

    3. Concrete is recycled today right on site when the rip up old expressways. Crush the concrete, send the steel rebar back to the local steel plants for reprocessing and use the aggregate to repave the same expressway. It is happening right now in Illinois, Wisconsin at the moment and I am sure in lots of other states as well. I just used a bunch of this aggregate under my new garage slab. Thanks but I dont need to wait until 2050.

  3. Concrete is crushed in my area and used generally for driveways. It solidifies to some extent and also passes water easily to reduce runoff. It’s a low cost option that’s good for everybody. As far as I can tell, landfills here will not accept concrete and large batches are used to build up low areas where buildings are being planned.

  4. i get pretty frustrated with this kind of press release. i can’t really tell what’s novel here — it looks like an incremental improvement by combining steel and concrete processing together. there’s a lot of room for that kind of improvement and it’s good to see those steps taken.

    but the press release always obscures the real innovation by focusing on the completely novel never-before-thought-of (sarcasm) innovation of powering this energy-intensive industrial process using zero-carbon energy sources. no matter what you invent, you are forgotten, swept away in the back-patting prose of windmills and solar cells. bah humbug. i want to know what the one new thing is, and respect me enough to assume that i will understand that the gigawatt input can come from a variety of sources.

    (this is not hackaday’s fault, someone much closer to the origin of the story is pushing this zero-carbon energy angle)

    1. The paper (not a press release, mind you) is published as open access. Their key claim is that “It creates the first zero-emissions alternative to existing cement production, to our knowledge”, which may be slightly misleading.
      To me their work centers on going through the motions to provide quantified proof that the waste concrete matrix can replace part of the steel making flux (downcycling) and that this slag can subsequently be used as clinker in Portland cement making.

      You could toss in the aggregate as well and it would either melt and dilute the flux without becoming an active ingredient, or (like Quartz) it might as well stay a solid at these temperatures. So quantifying the properties and process-relevance of flux substitution with cement paste is one of the key contributions here.

      And if that doesn’t suffice, consider that many industries have rather high inertia when it comes to innovation, and convincing plant operators to significantly screw around with their recipes and processes is no easy feat. For the most part, it’s rigorous treatment of mostly boring stuff, and its documentation.

      I for one am very excited about it, since when you look closely, you’ll also notice that liquid slag reaches and exceeds 1500°C. This means it routinely reaches temperatures where asbestos is destroyed – something that is highly desirable, as it would otherwise be reintroduced into concrete over and over again.
      So if this process is compelling by virtue of being cost-competitive and highly scalable, it’ll suck up all kinds of rubble that would otherwise end up in a landfill. I like the thought of that.

      1. In the USA it is a federal crime to take gravel or sand from a navigable waterway without unobtainable permit.

        The bastards keep redefining ‘navigable’.

        Which is why I don’t have a creek on my Sierra Nevada property.
        It’s a drainage ditch.
        Goes dry regularly.
        I’ve got pictures!

  5. ‘Reuse’ and ‘recycling’ are not the same thing.
    Shame on you for continuing to use the same language that allows for these industries to deceive the public.

    The end product of recycling is the same material. The item is reinjected into the same cycle that created it.
    Aluminum must be the product when recycling aluminum.

    Reuse is… Just finding another way to use it.
    American (and elsewhere) “recycling” programs burn anything non-metallic to ‘produce power’, which IS reuse, but clearly not the [plastic > plastic] fantasy the public is sold on.

    This is not recycling the cement.
    This is reusing the cement.

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