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James Greyson

Nov 30, 2012
09:01

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Thanks Brian - you're a prolific CoLab proposer! Here's a Japanese pilot project doing this (plus rubbing to separate cement from aggregate) http://theconstructor.org/concrete/concrete-recycling/755/ Solar struggles to get past 150C for large volumes without expensive hi-tech (http://en.wikipedia.org/wiki/Solar_cooker ) so could consider other sources of process heat, for example the >300C waste heat from making charcoal/biochar from biomass? James

Brian Chow

Apr 27, 2013
03:40

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Hi James, I understand the article you linked mentions that the recycling process applies for crushed concrete (including aggregate rocks) and the heating process requires 300 degrees Celsius. However, there are ways around this. For example, the most economical way to re-use concrete is as aggregate itself, where in the splitting process only a large tumbler is needed. Gravity is very efficient at crushing concrete, whether it is from a tumbling process or random chunks being dropped from a high altitude. My own research has to do with lunar cements, which requires a different binding phase due to the extreme environments encountered there. However, from what I have seen, a weight fraction of less than 5% including aggregate is more than sufficient to achieve a structurally intact solid. The same must hold true for terrestrial concrete. That is, the proportion of cement must be similar to or even below 5% in weight fraction if the particle size gradation is controlled and aggregates are included. I then ask myself, can size gradation be economically controlled? I believe the answer is yes, because sieving is an inexpensive process applied to certain industrial processes. The 5% is key, and if the densities are similar between cement and aggregate (a reasonable assumption), then we can say that only 5% of volume of recycled concrete goes into a dehydration recycling process. The rest turns into raw aggregate. Generally it also that 5% which will be finely divided into a fine powder, which can be heated and water expelled from it quite fast. Suppose a very long cylindrical reflector is built, where the sunlight is focused on one line. These reflectors will be much larger than most personal solar cookers - because it is industrial-scale. Then, air carrying the particles can be blown across in a glass tube collinear with the focal line, and the gas speed is such that the small particles remain suspended in the air flow. Of course, there is the issue of particles accumulating on the glass tube, but I suppose this problem can be solved by an electrostatic "pulse" mechanism. I will add these ideas to an update of this description. Thanks, Brian

James Greyson

Apr 29, 2013
05:25

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Thanks Brian! I wonder how to sieve out the cement without getting loads of fine aggregate mixed in?

2013cementjudges 2013cementjudges

Jul 4, 2013
02:48

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Previous lab scale work has been done on cement recycling in the past, so it seems a promising approach. But the proposal does not give enough detail on how such a process would be scaled up going forward. There is no literature documenting that the process has been successful at scale and it's not clear what the feed in material would be. This is a challenging process technically and there was not enough documentation in the proposal to provide a convincing case for feasibility.