We pulled up a carpet in preparation to install new tile flooring and found the concrete surface underneath to be sandy and cracked. The room seems to be a former porch which was enclosed and about 3-4 inches of substandard concrete poured over the original slab. We chipped out a hole and can see a good hard original slab under the new layer. There is a hollow sound when the floor is tapped so it obviously hasn't bonded to the original floor. It is quite clear that tile won't stick to this sandy surface. I thought perhaps Hardie backer board might be the solution but see on their website that it has to be laid with thinset too, which wouldn't adhere to a sandy surface. I am wondering if there are any sealing products which anyone could recommend or any other possible solution other than breaking up the concrete and repouring.