Not sure if that would be a good ideal Rosi.
Are you talking about ceramic or porcelain tiles? I would just make sure clean the cement before hand and as long as there are no cracks, tile right over it.
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The answer I received was no, it is not. I had sealed my floors due to a radon issue and was told that the thinset would/may not adhere properly. I am in the middle of a laminate install and glad it turned out that way.