Last year I made a wall by pouring/shuttering concrete, later painted it, over winter the paint flaked off and what appeared like ice could be seen on the bare concrete, I assumed moisture had come out of it but the ice didn't melt and I realised they were solid and crystal like. Now I have a range of colourful ones too, has anyone seen this before? What's happening?
Well that's obviously 'concrete' made with scrap broken glass and sand and very little cement, instead of proper glint ballast and a decent amount of cement
Looks to me like you have added assorted broken bottle glass to the mix!
+1 It does look like recycled glass. Possibly the mix was much too wet when poured and/or too little cement in the mix. Was the mix vibrated after pouring into the shuttering to remove air?
Crushed cullet. Where did you get the concrete? Was it a pre-mix or did you source the ingredients separately? If the latter, how come you didn't notice that the aggregate was mostly glass cullet?
If the op poured it as he claimed he'd know perfectly well what it was
There is glass, but I can't tell if it's embedded in the concrete or resting on top?
If it was some sort of dry pre-mix from a bag, it wouldn't have been very obvious what the aggregate was.
It looks if the light/soft bits (sand) has washed away just leaving the more heavy bits (glass) sitting on top.
If it's loose and movable, it is resting on top -rather obviously.
If you fully zoom in you will see some sitting on top of the surface, some partially embedded and some just showing and fully embedded. You will also notice the sandy structure of the concrete.
If the OP mixed his own using ballast then anything could be mixed in it. As the ? Paving expert? says frequently the sweepings off the yard floor. Ballast is OK for non- critical situations eg. Fence posts etc. but careful mixing of gravel and fines is essential for any exposed concrete subject to wear and tear.
Richard
The concrete was made by myself with bags of cement and sand (although I didn't use chippings so it's probably not technically concrete). I'm fairly certain there was no broken glass in the sand, surely i'd have noticed especially when troweling it smooth and painting it.
I first noticed the clear crystals as the paint i'd applied flaked off over winter and I noticed the coloured ones more recently.
To be fair I think the mix may have been too wet, there was a fair bit of water sitting atop but I am fairly certain there was no glass in any of the ingredients.
It had set when I painted it, I was concerned it was too sandy (because it was too wet?) but there was no glass or such visible, what you see in the picture seems to have formed within and in some part emerged over the winter.
I poured it roughly this time last year.
It's glass - some brown, some green, some clear - typical container glass colours, typical crushed glass appearance. It hasn't 'crystallised' since the concrete was poured - they don't look like crystals as such, just amorphous chippings.
How it got into your mix, only you can answer that.
If it was cement and sand, where did the aggregate come from?
If it was rounded stones, are you near the coast by any chance? It's possible they hoovered up the sea for gravel extraction and hit upon some 'sea glass'. You might not notice those for being glass, especially if covered in dust.
Although that sounds unlikely - sea glass is a rare and sought after thing:
Theo
This article suggests that up to 20% of the cement in concrete can be replaced with fine glass particles without any issues.
Those pieces in his concrete are not that fine though.
20um particles vs 5mm is a hell of a difference in size.I thought it was a wind up when he first posted but apparently not...
Fine as in like sand I doubt it would really cause a problem but chunks of macroscopic glass in concrete will create weak lines along the shiny surface of the glass. Smaller particles will bind less well than sand but good enough to still result in a decent strong concrete.
It is a matter of surface area to volume ratio and smoothness. Not many things in nature are as smooth on the surface as a glass!
Obsidian is off hand the only natural glass example I can think of.
With glass in concrete, there can also be a problem with 'concrete cancer', aka alkali-silicate reaction, and the finer the glass, the more surface area to react and the greater the problem, I would think.
Geologically speaking, 'glass' just means amorphous, ie lacking in visible crystals. It's common any time volcanic rock has cooled rapidly. If it has come into contact with air or water you may get structures like pumice (which is technically a form of foamed glass), but if it's below the surface you get solid volcanic glass.
Opal is another amorphous mineral (caused by deposition of dissolved silica in surrounding rock from hot volcanic water).
Theo
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