Coloured crystals emerging from concrete

[Chris Hogg]

Ditto flint I think, although there may be some short-range order in the structure there.

[R D S]

Yes, you'd be forgiven.

If the 'glass' was merely embedded in the concrete I could go along with the idea that it may have been in the sand and I didn't see notice (it would be a stretch), but it's the stuff that's sitting on top that's doing my head in, it was smooth and painted. As said, the paint has peeled off, is it possible there's been some sort of reaction between paint/sand/cement/weather?

[Chris Hogg]

No!

[Nikki Smith]

The colour of the as-cast concrete is always determined by the finest constituent. This used to be the cement, but with increased use of GGBS as a cement replacement this now determines colour. GGBS is used quite a bit in fancy exposed concrete as it is lighter than OPC. (PFA darkens the concrete). This is the case for cured concrete, but also valid for wet.

Crushed glass is fine as a small aggregate, and if it comes in an all-in bag then you wont see it - the sand will determine the overall colour.

As for the wall, it looks like an over-sanded mix which was placed too wet or was rained on. The top surface might also have been over-trowelled. Whichever way, the top surface of the wall will have had a far too high water/cement ratio, so bugger all strength. Fine to receive paint, but useless for any longevity. Frost action + rain has now washed the loosely cemented sand, leaving the fine glass aggregate behind - too heavy to be washed away.

This crap-crete will continue to be washed away until either it gets down to something that has a higher cement ratio in it (usually 5mm down or so), or if the mix really did have insufficient cement until its all a pile of sand and recycled glass at the base of the wall...

[Tim Lamb]

In message [e-mail], Chris Hogg [e-mail] writes

Umm. Towards the end of the link posted by martin there is a hint about a possible reaction between glass and cement in the presence of moisture. Also that glass being lighter than stone would tend to migrate to the surface during tamping/agitation.

As laid and painted, any glass would be hidden and forgotten until exposed by the coating deterioration.

>

[alan_m]

It it just the surface? In the photo there appears to be lumps under the very surface layer of the sandy structure - when the sand is washed off possibly these lumps will be identified as more crushed glass.

What you may be seeing on top of the walls is erosion by rain etc. and the weak sand/cement has been washed away leaving the heavier glass particles resting on top. The paint has just flaked off because there was nothing sound beneath it to stick to - you just painted over sand.

Get a hose with a gun attachment and aim a fierce stream of water at a small portion at the top of the wall. See if the water causes a small crater and if glass can be seen in the walls of this crater.

You may be able to rescue the wall by painting it with a coat of SBR and letting that dry before repainting (or perhaps with a slurry of cement and SBR)

Wickes tend to hide the small (1 litre) tub of SBR in a different aisle/section to their normal building materials section where you will find the 5L and 25L SBR containers.

[GB]

So, the OP made a weak mix with loads of water, and with a small amount of glass in the mix, and most of that floated to the top?

[The Natural Philosopher]

What has happened is that the glass *was* in the concrete, but at some point the top layer of the concrete that seems to contain no cement at all, has got wet, frozen, turned into sand, lifted the paint and washed away.

Leaving the glass behind.

if you want a flat concrete surface to stay concrete you need a 2:1 or

3:1 mix.

[The Natural Philosopher]

+1

[Theo]

If it was a crystalline solid, for clear crystals you'd expect a particular shape. The crystal would grow by gradual precipitation of the fluid it was dissolved in, leaving the crystal behind. You would either get an opaque blob, amorphous due to tiny crystals (like kettle scale - aragonite, a form of calcium carbonate) or a shape determined by the crystal structure (eg salt crystals, NaCl having a cubic structure). You wouldn't get clear lumps without discernible flat faces set at angles to each other.

Theo

[Chris Hogg]

And they'd all be the same colour, probably white or transparent, not shades of brown and especially not green. Brown bottle glass is achieved by a combination of iron and sulphur in the glass melt (basically a soda-lime-silica combination with a few minor extras to control viscosity, durability etc, at circa 1650°C), and green from chromium in the same melt.

I wonder how many time RDH will have to be told that it's crushed cullet before he gets the message!

[R D S]

It isn't! Or there'd be none sat loose above the previously smooth surface that got painted.

[alan_m]

If the surface of the concrete was sound the paint wouldn't have flaked off in the way it has done. You have effectively painted on a compacted sand and there has been no bond strength at the paint to wall layer, nor in the of the concrete.

[Chris Hogg]

<sigh>

[Chris Hogg]

Pick out the green ones - they're emeralds, and the clear transparent ones are white sapphires. The brown ones are probably brown diamonds - very rare. Take them all to your local auctioneer, they're worth a fortune.

[Andy Burns]

Run a hose on it while attacking it with a stiff scrubbing brush, see what emerges ...

[Martin Brown]

My impression from looking at it is that the mix was too sand rich and hadn't got enough cement in to bind the sand. The paint resin wetting it out made a thin surface layer moderately strong and when it fell off it took a layer of the sand/cement mix off with it revealing your crystals.

Glass makes concrete potentially weaker. I have had the same problem with some work done a few years ago (but without the ornamental glass inclusions). The guys doing it used a slightly too sand rich mix on the roof and it is now blocking the gutters.

Only a physical one of the paint film being more attached to the weak concrete underneath it than the concrete was to its own bulk. When it fell off it took a chunk of the material off with it and/or the mix isn't quite stable to the effects of wetting, frost, wind and rain (though there has been so little frost this year I discount that)

[Martin Brown]

Simplest test that might satisfy the OP that they really are pieces of glass is to put a few pieces on a fire brick and use a blow torch to melt them.

My money is on a bright yellow sodium flame test and a blob of red hot molten glass forming PDQ. They really don't look like crystals at all.

[Chris Hogg]

Oh I doubt that would persuade him. He's got his mind set on it being something that has crystallised in the crapcrete that he mixed, despite being told repeatedly that they're not crystals as such and that they're multi-coloured bits of glass cullet sometimes used as aggregate in concrete. The construction industry has never seen anything resembling the appearance of 'crystals' like that anywhere, ever.

He cannot accept that one of the components in his mix had some cullet in it and he just didn't notice it.

[The Natural Philosopher]

People find it hard to accept they screwed up, used a product wrongly, used the wrong product or were deliberately and cynically lied to for profit.