# Concrete to Dust - energy involved

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Hi all, I have been involved in this ridiculous debate which has raged on for quite some time, and so I would like to pick your collective brains.

As the title suggests, the debate is about how much energy would be required to turn a 1 Ton block of set construction concrete back in to particles of a 100 microns across.

I don't know if this is something which can be calculated in KWH (killowatt hours), but this is a unit of energy that I understand (probably because I see it every three months when I get my electric bill).

I realise this probably sounds a bit silly, but I could not think of any where else to go in order to get an answer.

Thanking you all in advance, Claire

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first, you would calculate the surface area of all the particles then take away the surface area of the block as that isn't involved in the binding. then i guess you would have to experiment to see how much energy it takes to pull appart a block of concrete with known cross sectional area. then it would be a matter of ratios.

this would be a very very very rough estimate likely filled with error but its around the best bet.

the surface are can be worked out with relative ease but i don't know if anyone has recorded the energy it takes to pull a bit of concrete appart and i might even be totally wronf in my method.

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how are you thinking of taking it apart?

Blasting it? crushing it? a large belt sander?

which...

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YT2095 Quote

"how are you thinking of taking it apart?

Blasting it? crushing it? a large belt sander?

which..."

Hi, I see your point, I did'nt realy think about that. Could it be explained by both blasting it (explosives), and expalined as crushing it (say dropping it from a great height)?

I'm not a science student so you'll have to forgive my lack of parameters (if that's the right word). I suppose I'm just trying to find out if it takes alot of energy, or not that much energy, hence my bizzare KWH request as a unit I understand.

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this is why i describedthe method of physicaly separating it grain by grain. it will give the minimum energy required.

a kWh can be a large amount depending on the scale of things. if you are working on a standard lab scale then it is a large amount (3600000 joules) but if you are on an engineering scale then its pretty insignificant.

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my Guesstimate would be in the region of a couple of megawatts, using the ideal Belt sander method.

a few Kilowatt motor being fed a 1 foot sqr surface of cement over a few hours (quite a few).

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That estimate seems to be a few tens of KWHr and nothing to do with a power measurement in Megawatts.

If anyone wants to find the answer it might have been measured. You need to know the energy of fracture (AKA work of fracture) for the concrete and also the area of a ton of 100 micron dust. Work of fracture is a fairly important property in engineering and concrete is pretty common so I imagine the value has been measrued. If not, you can measure it yourself.

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I'll admit, I'm struggling with some of the terminology used here.

Could the energy required be decribed in the amount of TNT needed to reduce the concrete to 100 micron particles, and could that then be converted in to KWH (or Mega Joules)?

I'm assuming that when you return cast concrete back in to particles, you are un-bonding those particles? Is that correct?

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TNT is a bad choice for efficiency, as after the concrete is cracked the rest of the pressure blows other stuff away needlessly. Belt sander is a nice idea, but I believe that the belt of the said sander is unlikely to last thus only reporting part of the energy involved (forgive my economic approach).

Are you allowed to temper with the block before hardening? Mixing with water or some other non-bonding chemical then freezing it should do. And it would only take a few watts.

Other than that, I think grinding the blocks with themselves is better, like churning the bits. Also, I'm not aware of the complete properties of the block, can it be vaporised? Sonic shatter? OK I'll stop now. How about quartz vibrating? OK, NOW!

Oh, how about cryo-freezing then shattering it?

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And what if the concrete block is dissolved by immersion in a suitable chemical ?

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then you dont get your 100 micron size, HFl occurred to me also, but I didnt post it for that reason.

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Right... concrete is a conglomerate of different materials; if to be dissolved to that fine particulate, the chemical would have to be capable of equally attacking all of the concrete components, sand grains, variety of stones, pebbles...

A plain ball/hammer/crusher mill could take care of the large particles after the chemical attack. Should not be much energy after that stage of the pulverization.

Guessing, a couple of horsepower mill (1500 Watts for 15 minutes =~400WH)should be capable of doing it.

Specifications in commercial stone mills should give better figures .

->Edited - added : Checked briefly, one of dozens of sites:

http://www.break-day.com/2-10.htm?gclid=CO6CzaHS0owCFQQRYwodQn1Ytw

Miguel

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OK, thanks everyone for your posts.

I think I can take from this that it requires some substantial measure of energy to reduce concrete to siad size particles, regardless of which method you chose to do it.

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