This looks like a classic case of efflorescence and a classic example of why I really don't like sealing stone on an exterior applications. Of course there are exceptions but sealing exterior stone, especially when the stone is in contact with soil is very risky. You asked what you did wrong? In my opinion, you did nothing wrong and it would have done this anyway since the moisture is coming from the underneath. The sealer will only protect water from the surface down. In some cases sealing the stone can restrict moisture migration from underneath. This is true even if it is breathable. If the stone was not sealed it may have not happened as quickly but would have happened eventually. As far as what to do? I would explain to the customer what I just outlined above. If need be I can write a letter for you to present to your customer. As Chief Technical director of Stone and Tile Pros, one of you benefits is to use me when you need to in cases like this.. Of course all our partners get a discount off my regular rates. Let me know if you need my help.
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Frederick M. Hueston PhD
www.stoneforensics.comwww.stoneandtilepros.com
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