Checking the progress of repair of a sandstone wall, I noticed, where
delaminated sandstone had been removed but the setting bed was still
partially in place,that the top surface of the mortar (a lime mortar) had a
smooth black surface, almost shiny. Surprised, I dug around at an adjacent
location and found the same thing. I couldn't find it in the collar joints.
Has anyone come across the same, and do you understand why?
The sandstone has a considerable iron content, and the repair was a
response to the damage caused by a failed gutter above, left unattended for
a considerable time.
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