I have a bottle of brick cleaner containing 18-20% hydrochloric acid. What can I add to it to make a gel or paste without unwanted reactions? The application is still just cleaning bricks as the product is intended, I just don't want it to run like a liquid.
Xanthan Gum can handle hydrochloric / muriatic acid (HCl) for a while, depending on how concentrated the acid is. Here is how I made an HCl gel to clean an old toilet that was heavily coated with lime scale...
I first dissolved a level teaspoon of xanthan gum into 1/4 cup of rapidly stirring water. After a few hours of letting it sit, with occasional stirring, I had a very thick, viscous goo. To make the HCl gel, I mixed one part of this goo with 1.5 parts of 31% hydrocholoric acid, by volume. This gave a gel of a suitable viscosity to get the lime dissolved off the toilet. I don't know how long this gel would last before the HCl broke down the gum, but my gel lasted several hours, which was enough to get the toilet clean.
The following URL gives more hints on how to make mineral acid gels: https://dowac.custhelp.com/app/answers/detail/a_id/4951
I tried the Xanthan Gum method using muratic acid to clean the calcium leaching on my pool, which was bad, having sat for 7-8 months. I used 1 lb. of Xanthan Gum (about \$11 on Amazon) and about 3 gallons of water to make a thick gel. I then added about 1 gallon of it to a 2nd bucket and added muratic acid, 2 then a 3rd gallon to make it stronger. It just didn't work very well. I don't know if it was from it being diluted, but I also wonder if muratic acid, which eats away at organic substances, interacts with Xanthan Gum, being an organic substance, and weakens the acid. It just didn't seem to help. I ended up using 100% muratic acid (33% HCL) and applied it with a home depot acid washing brush, wearing gloves, safety glasses, with a box fan and leaf blower at hand when fumes got bad. Fumes seem to sit in the pool and need to be blown out as you go. I found by dipping the brush and pushing it horizontally away from me, but not pulling back it would leave a nice coating. A little would drip down, especially from the beginning of the stroke, so I'd do a 2nd stroke just below the first to spread it out. You'll see the acid start bubbling with little dripping down. Then repeat starting at the 2nd stroke so it got good coverage. My pool took multiple coats of this. I noticed that after multiple strokes the part still not clean would bubble and the clean parts wouldn't. My surface is diamond brite double blue, with a doubling of the blue put in (quadruple blue for a more durable surface). It would sit for 30-60 minutes then wash it off. Apply sparingly and carefully around light or any other metal fixtures, they will quickly corrode and drip rust. Some may criticize this as being too aggressive, but they weren't here seeing what I was dealing with. It took me all day to do the pool, in south Florida's summer heat. I simply didn't have or want to spend more than a day doing this. And applying it weaker would just take more labor, probably more acid ( I used about 12 gallons), and eventually eat away the same amount. Test your area and find what dilution works for you. I ran a hose constantly to keep the rinsed muratic acid diluted in the deep end. I then used a \$50 utility pump with a hose and pumped it out into the grass. I now have to clean that area where water had pooled up, which I will do, then dilute further with water and pump out. Doing the entire pool I noticed it did eat away about 1 gallon of pool concrete and diamond brite blue specks. Acceptable for a 22k gallon pool. Pool is back to its beautiful blue color.