From Joe Norton
Use muriatic acid to clean cement off stone walls. This is a very powerful chemical. Use extreme caution when using muriatic acid.
How to clean cement spills effectively is a problem that the DIY er often encounters. Cement products can be messy. Whether you're using concrete or mortar, building a stone wall, patio, or undertaking some other project, there is a good chance you'll make a mess.
But do not worry, there are ways to clean stray cement off your masonry structure. In this article, we'll zero in on one of them, using as an example how to clean dripped cement off a stone wall. You can apply these same techniques to most masonry projects involving cement products.
To clean cement off a stone wall, we'll be using a magical potion. It's called muriatic acid. It will clean cement off stone without damaging the stone (if used properly), even after it has dried.
A word of caution. Muriatic acid is a handy cleaning option. But is also dangerous. Read my article on how to use muriatic acid safely. Now that I've frightened the bejeezus out of you regarding this product, let's do some cleaning with muriatic acid.