How to install tile over carpet glue?

My client wants ceramic or porcelain tile on her back porch. The previous owner had glue-down green carpet/fake grass there. My question is: Can I put thinset down on top of the remaining skim layer of carpet glue?
An Internet search gives me advice all over the map: remove it with solvents/don't use solvents because they soak into the concrete and keep thinset from sticking; remove it with abrasives/don't because it just melts and wastes time; remove and replace an obscenely thick layer (1/4 - 1/2 inch) of concrete.
* The surface is smooth and even, and it is sloped properly. * I power-washed it awhile back, and the glue washed off just slightly. * I plan on scraping with a 4" razor in any case.
The fact that the glue dissolved slightly makes me hopeful that water-based bedding material will "bond" with it. On the other hand, it makes me worry, also, that it won't.
I'll take any advice I can get.
Steve Bell
New Life Home Improvement
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