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