Case Study #2
Those Troublesome Limestone Stains
Stain Due To Alkali Solutions. The phenomenon of alkali stain is characterized by a brown, rusty color on buff stone and gray-brown color on gray stone. Stain usually appears during and shortly after construction. Its cause is closely related to that of efflorescence, which sometimes develops on or near affected areas.
Alkali stain usually occurs in small areas on the building, although the condition causing it may exist broadly as a result of defective design, poor workmanship or unavoidable circumstances. Alkali stain is easy to avoid and in most cases disappears in a relatively short time after the causes are eliminated.
Soluble alkalis, generally potassium and sodium compounds, are natural elements of all cement. If ground or rainwater has opportunity to dissolve these alkalis and subsequently pass through limestone, stain may result. In some cases, water absorbed by the stone from the mortar or tuckpointing will produce stain.
This phenomenon requires comparatively large amounts of water to become a significant problem, and even then it is purely a matter of appearance. Conditions such as untopped, uncovered walls, unglazed window openings, wash from concrete pours and moisture absorption from mortar will produce stain.
Ordinarily, the best way to deal with alkali stain is to leave it alone after correcting the causative factors. If exposed to sun and rain, the natural process of weathering is most effective in its elimination.
Occasionally, prompt removal of alkali stains may be necessary. Often, mild stains can be relieved by washing with tri-sodium phosphate, followed by medium-pressure rinse (up to 800 psi). Make sure that all possible leaks are stopped. Darker stains may require other procedures.
Design Considerations To Avoid Limestone Stains
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Select materials for use adjacent to and in contact with limestone which will not bleed when oxidizing or provide pooling areas for rainwater.
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Provide adequate damp-proofing for back faces and under grade areas.
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Provide adequate sub grade drainage.
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Slope grade, walks and deck areas to assure runoff.