"Hello, The cement mortar sounds like it was too dry, stopping it binding the tile to the floor properly. If they aren’t sticking to the floor at this point, it’s probably best to restart at this point. Generally, I would advise that you should be using a floor tile adhesive instead. Ideally having a thick layer to ensure that the tiles are laid evenly on your floor, if it is noticeably uneven. However, if you need a lot of the adhesive, this can be expensive, so you may wish to consider alternatives. Specifically, using a mix of cement and sand on natural stone, although this would need to be a full-wet mix to be fully effective.
Answered on 28th Feb 2019 - Member since Mar 2019 - report
Additionally, it could be useful to give the concrete a coating of watered down PVA for some extra sealant, keeping any moisture away from the concrete flooring beneath. A cement/sand mixture could be a cheaper solution overall, but if you can afford it the adhesive will likely be better. (It may also be cheaper if the adhesive is not a fast-setting adhesive.) Good luck with your flooring!"