How can I get spilled concrete off a boulder?

Had a landscaping crew out a week ago to build a garden wall out of block covered with adobe. Just noticed that a boulder that was near their mixer got splashed with some concrete. I tried washing the boulder with a stiff brush, but no luck.

Any ideas how I can get this stuff off?
Thanks in advance!:thumb:
 
Depending on what the bolders are made of in your part of the world, muriatic (HCl) acid is the old standby for cleaning mortar, grout and concrete from surfaces.

If the bolder is hard like granite, no problem. If it's limestone you'll need to be on your toes. The acid might eat the bolder quicker than the spill.:eek:

You can buy muriatic at building supply stores.
 
Hey Jesse, muriatic acid is indeed the ticket, but google it and heed the precautions for handling. The best source is swimming pool supply stores.

I’ve used it for exposing aggregate on concrete slabs… which makes me think that our resident mud head will have some really good advice for you when he logs on.

If you have near the variety of rock in NM that is found in AZ, then you might well heed Ian’s admonition to stay on your toes if it is limestone. If it is, then I would chip off splatters and apply the acid with a spray bottle. Keep the hose handy.
 
muratic acid is usually mixed with water to dilute it before use, scrubbed with a stiff brush and washed off quickly for brickwork, try a butcher block wire brush and straight vinegar.
 
As has been suggested, I'd use a rock hammer and chisel to get the big stuff off, then muriatic acid and a stiff brush as others have suggested. That Sandia granite won't flinch when the acid hits it. When I was in the concrete testing business (in Albuquerque), we'd also use hydrochloric acid to clean up our testing equipment, but that's some nasty stuff, so I wouldn't recommend it.

Frank, my dad helped pioneer exposed aggregate cast-in-place concrete slabs when they built the Air Force Academy in Colorado Springs around 1957. That was the first in-place application of the technique. ;)
 
Muriatic is the ancient name for hydrochloric acid (think alchemy). These days it's how we refer to a fairly dilute (10-12%) concentration of HCl used as a cleaning aid. Concentrated industrial HCl is in the range of 38%.

Even home store muriatic at 10% or so needs to be further diluted. Follow safety instructions, pour acid into water and not vice versa, wear gloves and goggles.

Here's the EPA bulletin with full details Link

Have you thought about blasting the boulder out, Jesse ? :wave:

Table vinegar is approx 4% acetic acid - pickling vinegar is about 6%. I think it would likely evaporate long before it did anything to concrete. Could be wrong, tho.

I am not a chemist, but I did play one in high school. (and Wikipedia is my friend :D )
 
Muriatic is the ancient name for hydrochloric acid (think alchemy). These days it's how we refer to a fairly dilute (10-12%) concentration of HCl used as a cleaning aid. Concentrated industrial HCl is in the range of 38%...
Thanks for the info. Cool to know that, and shows I missed a lot of days in high school science class. I think the stuff we used to use in the testing lab was the strong stuff. It was wicked stuff, and of course we just wore rubber gloves and no respirator back then.
 
Thanks for the info. Cool to know that, and shows I missed a lot of days in high school science class. I think the stuff we used to use in the testing lab was the strong stuff. It was wicked stuff, and of course we just wore rubber gloves and no respirator back then.

I neglected to tell everybody that I blew the plywood cover off my chem lab sink. My lab partner did the fastest 30' dash I've ever seen! He was clear out in the hallway before the cover landed. Totally not my fault, of course :rofl:
 
Local rock is going to be sandstone (probably) though there is some limestone exposed in the Sandias so you might get that too (its the grey stuff). Brown, tan, or red is going to be sandstone. It may fizz a bit too from HCl, as the sandstone in our area has a lot of evaporitic minerals like make up limestone cementing the grains together.

If the boulder has a nice patina on it or some lichens, you may be out of luck, but acid would be the safest bet to preserve the look in that case. If chipping is ok, try the sledge and chisel (HF has pretty good rock chisels for a few bucks, just stay away from their hammers).

John
 
Thanks all. Went out to the boulder with some muriatic (1/3 acid, 2/3 water) a bucket of water, stiff brush, cold chisel and a hammer. If all that didn't work, I was going to raid the fridge for a coke.:rofl:

Anyhow, the muriatic took it off like a charm, with just a little scrubbing with a stiff brush. I think the rock was not sandstone, as it didn't seem to lose any surface, but I poured the leftover solution on some cobble and boy did some of those stones smoke:eek: :eek:

Thanks again for your advice - this forum is truly a remarkable place.
 
Jesse,
I’m glad those removal methods worked out for you. Thinking about this a little more, I see how those splatters could have landed on a boulder that was weathered or spalling, and where chipping or etching would have caused an unsightly discoloration.

Perhaps in those situations the rock could be chipped off or acid etched (maybe with a wax-resist application to define borders), to form a petroglyph.
 
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