Muriatic acid is also known as hydrochloric acid, it's what helps digest your food in your stomach.
Scare stories aside, it can be rinsed out of a block and it is used to clean scale out of a block, this is quite common. Of course if the block in question has a rusty pinhole, the acid will find it.
It is used to etch concrete. It reacts with the alkaline based cement. Will it work on block filler is a good question and depends on what the block filler is primarily made of.
The factors for metal removal are concentration, time and temperature. Increase any of them and metal removal increases.
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