Sorry for one of my first posts here being an urgent one, but it's the first warm day we've had in awhile and our tile contractor is back to repair some work on an outside patio. The patio has ceramic 12" square tiles and when they were installed the bullnose pieces along the edge were installed with a tilt up. The rest of the patio sloped away from the house, but those last 4 inches had enough of a slope upward that it caused water to pool out on the patio, which we first noticed when we saw water coming out through the siding below.

Now they are here to remove the tiles and thinset along the edge, redo the mud, and replace it at the correct angle. But when they started pulling up the tile, we found what you see in the attached image, that the thinset under the tile had turned blue. My concern is that the water which had permeated the grout kept the thinset wet for several months (we had to cover it with plastic to keep it from leaking through into the structure, but the plastic also trapped some moisture inside.

The installer has never seen thinset turn blue before and has no idea what it is. Can anyone here tell me if they've seen that before, if it's mildew of some sort, and if we need to keep removing more tiles? The tiles all feel very sturdy, they're not loose, and the tile pattern has been discontinued so we're kind of in a bind if we remove more tiles than they have available replacements.

Would appreciate any advice, thanks!

--Jason

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