Practical problem with acid and granite

photos, stained area and unstained area if you can. Use flickr and post URL here if you cannot do it here.

there are a lot of trace elements in the granite, expecially if it is porous. Some can have a valence state changed. You mah have bleached an item to an acid state.

Of method is to take a piece of the same granite that you do not need on the wall, (if you can get it). Mark a strip and treat with acid and make the stain. Use this as a test piece.

Then try a drop of drain cleaner to neutralize the acid. Let sit and then wash...see if any difference. Try other neutralisers. Try a few drops of oxalic acid. and H2O2 Hydrogen peroxide

Household ammonia might also work as the NH3OH might be more mobile in the granite than NaOH. Ammonia is also less risky

Hello Cromwell. I'm not a chemist but I did a quick google on 'restoring granite + acid ' and found such links as https://www.baneclene.com/professionals/Content.aspx?xps=NDYy.

My limited knowledge is that chemical reactions are not necessarily reversible. Also it seems that granite can be confused with other materials. Testing limited areas is always the best method. I wonder from the description if the cement on the granite was accidental, or there for some reason? It may give a clue perhaps. Best of luck - I'm quite envious of you having such a great project to do! Kris

Someday, someone will enjoy and celebrate your mistake as a craftsman's artifact. Leave it alone and move on.

This a photo reaction (sunlight warmth) due to the chlorine in the acid, reacting with minerals in the granite and UV light from the sun. (yellow colour) Have you tried mild detergent these are often ammonia based. Polish with steel wool.

Is there scope to use mechanical removal, such as a high-pressure water jet, rather than a chemical method of removing the cement, perhaps?

'Granite' like most rock types can mean a whole variety of materials . If it really is Granite , porosity and permeability are relarively low compared to other rock types (note that porosity ≠ permeability ). Several mm penetration doesn't sound consistent with Granite . I still think it's worth being certain what it is , although that still leaves you with the stain - which I will stick my neck out and say you are stuck with. As a last resort , can you add tone to the yellow stained bits to camouflage a bit - Do you have a chunk in a skip you can experiment with.

http://www.asgranite.com/counter-faqs.html

Hello there, I think you are going to be stuck with the stain... chemical reaction cannot be reverse all the time. You can try water jetting 5000 PSI with a TSP solution at 1% concentration, sometimes it works.

I would use a wet sandblast (the small one) and locally try to remove the stain or I should say attenuate the color of the stain. Then apply some colored stain matching the original color. This is a job for an artist... take your time and you should get good results.