Upon tearing out approx. 1200 sq.ft. of pad & carpet on my basement slab, I find 12x12" slate tiles. They are in near perfect condition and water beads up so they appear to have been sealed. I wish to have terra cotta tiles installed but am told the thinset will not hold over a sealed slate. I am also told my only alternative is spending thousands of dollars to remove the slate. Is there not a product that can be used to make thinset adhere to a sealed slate? Any help is appreciated.