Limestone wall mortar turning black

Hi Everyone

I have a limestone retaining wall, two course high which consist of one meter reconstituded blocks. What I have recently noticed is that the mortar in between the blocks has gone black in places. I didnt seal the blocks or mortar as I didnt think it would go like this. Is there a special chemical that I can apply or do I need to remove the mortar and replace. Any assistance would be greatly appreciated. :)

Are the blocks in a moist shaded area? What soil is backfilled behind them? Does the black have any green in it when you look up close?

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Originally Posted by Burnsy

Are the blocks in a moist shaded area? What soil is backfilled behind them? Does the black have any green in it when you look up close?

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Hi Burnsy

Thanks for the reply, no it isnt in a shady area, gets full sun, the backfill behind the wall is just sand. I dont see any green there is only large black spots, looks like black mould in the mortar.

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Originally Posted by jessetia

Hi Burnsy

Thanks for the reply, no it isnt in a shady area, gets full sun, the backfill behind the wall is just sand. I dont see any green there is only large black spots, looks like black mould in the mortar.

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This is pretty common and you are likely to be right about the mould. Often the nutrients and moisture seep through feeding the mould. You can try using a chlorine or bleach solution to scrub it off but it will be ongoing if the back of the blocks is not sealed. Sealing the front will not always stop the problem.