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I need to rebuild a retaining wall, I broke down most of the wall. I
need to clean off the existing mortar from the brick.

I read many articles saying how dangerous muramic acid is. I saw
phosphoric acid isn't as dangerous. Has anyone tried phosphroic on
morter? Is there anything else to use?

Go and get a 3 or 4 inch cold chisel, a 2-lb hammer, and a masons hammer,
and a pair of leather gloves. Thick mortar is best removed mechanically. I
probably cleaned 30 or 40 thousand brick that way one summer as a kid. Once
you get the hang of it, it goes pretty fast. Often, you don't even need
tools- just strike the bricks together at the right angle, and the mortar
falls off. Muratic acid is for after you build the new wall, to wash down
the face and get off the smeared mortar. Be aware that you will need more
brick than came out of the old wall, since a decent percentage will break.

aem sends...