Yes, the mortar will ruin the particle board. I absolutely will not install ceramic over a floor with particle board in the subfloor. Some people will cap it with plywood first, then install the backerboard, but particle board is way too unstable for my taste. If you want to make sure the floor will be there as long as the house, remove the particle board. Replace it with plywood and proceed as usual. It's a little more work, no more money (since u'r considering installing plywood anyway), and the peace of mind is well worth the extra effort.

i was told that if i prim the sub floor and then use a non water base mastic that

would be ok.

what do you think?

thanks to everyone