Question
I live in NW Iowa and have an awesome patio on the south side of my house that was built up with lime stone blocks. I'm noticing what I think is mold or something that is turning them dark on top. The area gets lots of shade from a large maple close by. Can I make them white again?

Answer
Hello,

The best cleaner to try would be a commercially prepared limestone cleaner. Use an alkaline or neutral ph cleaner,as limestone is highly alkaline and an acid base cleaner may erode the surface. Do not use muriatic acid.

Another cleaning method is a diluted bleach solution; 1 part bleach to 1 part water. Let the bleach sit on the stained area for 1 to 2 minutes,then agitate with a brush or cloth and then rinse well with clean water.

Test all cleaning chemicals or sealers on an inconspicuous spot for 24 hours before cleaning the limestone.

When the cleaning is finished,apply a limestone sealer. This will insure that the stains do not return and a sealer can provide this protection for many years. Use a high quality impregnating limestone sealer. Two coats may be required.

Limestone is a very soft and porous rock and so do not use high pressure when rinsing off cleaners. Use a gentle spray from a garden hose. Do not pressure wash. It is important that any efflorescence (salts and other residue) is washed from the surface before applying a sealer. Commercial limestone cleaners contain surfactants that will remove this residue.

Sounds like mold has taken hold in the pores of the limestone and are taking advantage of the low light and possibly damp conditions of the stone. A sealer will rob the mold of any pores to latch onto and also deny the mold a damp environment by keeping water out of the surface of the stone. Good luck. Let me know if I can be of further help.

Mark Harshman
http://www.mahdrafting.com
Email: [email protected]

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