Joined: July 2004
||Posted: 17 Sep. 2019,12:55
You should run a search on this all-too-common topic using the button above - it seems to crop up at least once a month.
No easy answer other than very, VERY careful and multiple repetitions of cleaning with a weak citric or phosphoric acid-based cleaner. Even then, it doesn't always work - either the cement is too stubborn to shift or the acid-wash is too strong and it 'bleaches' the limestone - so don't go building-up your hopes!
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