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Gravel recovery

To recover gravel/aggregate from waste concrete
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OK, so, if you didn't know, concrete is one of the most recycled materials (asphalt and steel are also at the top of the list - yup, it ain't aluminum or bottles). To put it very simply, it's pulverized into rocks, which can be used similar to gravel (it's pretty intuitive, it's already rock).

But it isn't quite as good as other kinds of gravel that are made from crushed natural rock.

So, I figure, to recover the gravel alone, you could pass concrete chunks between two plates vibrating/crushing, but that never come closer than the average size of the aggregate. I'm thinking that the cement and sand would pulverize off leaving intact the stronger gravel.

This might also sort of round out the gravel, which may give it interesting properties/make it more like river stone.

And of course there's questions of if it's monetarily worth it. Perhaps with special kinds of gravel, in areas where gravel is expensive. Like pea gravel from bagged concrete I believe is pretty pricey

now, I MAY have posted this idea before, I can't remember

EdwinBakery, May 09 2012

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       [+] worth looking into
FlyingToaster, May 09 2012
  

       Concrete can be destroyed by heat. It is a mixture of gravel and sand and cement; the binder is the cement, which is a "hydrate" molecule. Heat breaks the hydrate and forces the water out, basically leaving calcium oxide behind (which if you add water you can make new cement).   

       I suggest a solar-oven method of heating concrete enough to separate the original gravel that went into making it from the binder cement and sand. Use a sieve to keep them separated. Below the sieve, put the sand and the cement dust into bags and sell as pre-mixed cement. Use the gravel separately, as already mentioned in the main text.
Vernon, May 09 2012
  

       Maybe you could just put the chunks of cement into a gigantic pebble tumbler - I have a feeling that that would erode away the cement but leave the gravel undaunted.
MaxwellBuchanan, May 09 2012
  

       Crushing creates angular gravel rather than rounded which is actually more desirable as a road base. You will never get all the cement off the gravel just through crushing and other mechanical methods such as tumbling or a ball mill would be too slow - hence expensive - for such a bulk commodity.
AusCan531, May 09 2012
  

       Why not just dump the concrete on a beach somewhere and wait?
MaxwellBuchanan, May 09 2012
  

       If gravel is that expensive where you live then it might be best to move back onto land?
UnaBubba, May 10 2012
  

       Vernon, concrete isn't the same as the cement you're talking about. What you're talking about is lime. Concrete actually uses portland cement, which is different, though it has some similar ingredients. Portland cement deliberately include sand and clay, and an aluminosilicate forms. I do not believe the reaction is reversible just with heating like with lime.   

       Furthermore the use heat energy would be too expensive and defeat the purpose.
EdwinBakery, May 10 2012
  

       Aus, remember, I didn't say crushing, i said vibrating/crushing, but the plates NEVER COME CLOSER TOGETHER THAN THE SIZE OF THE ORIGINAL GRAVEL. The idea being that you only crush off the cement. I suspect that this process would also only chip away the edges of the gravel, rounding out the pieces.
EdwinBakery, May 10 2012
  

       [Edwin] I was wondering about that too. Since portland cement is a mixture of substances, what you get back after heating may have the same proportions of elements as the original, they won't be in the same molecular configuration (duh). Curse you, entropy! But you may still get something that sets. Someone with a forge or a furnace should give it a go.
spidermother, May 10 2012
  

       The gravel. regardless of how closely you go to crushing it without actually crushing it, will still be plastered with concrete (it sticks really well to the microcrystalline surface of fractured stone.) so it won't look like gravel when you're finished.   

       Crushed gravel is irregular in shape, so you'll never get close to any part of each particle of gravel than its longest dimension.   

       The other downside to this is that gravel is the cheapest part of the whole process. It's simply not worth recycling when it's cheaper to simply quarry and crush more.
UnaBubba, May 11 2012
  

       I specified using solar heat. The operating cost will be low regardless of the capital cost of the heating plant. But, likely, the quantity of material that you want to process may be too much more than can be processed by any reasonable-sized solar plant. The idea will still work. Heat DOES destroy concrete. IF there is enough heat.
Vernon, May 11 2012
  
      
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