How long does concrete floor need to dry before tiling?

Concrete "Cures" instead of drying! In most cases a specification will call for 28 days as a Cure Period. During this cure time the intent is to maintain water inside the mix to allow for complete hydration of the key ingredient: Portland Cement! Complete hydration will result in the compressive strength achieving the mix design specifications. A mix for a home floor slab will be around 3-4,000 psi, where an industrial floor may be 8-10,000 psi.

Curing membranes, evaporation retarders and wet curing blankets are all used to keep the surface of a concrete from allowing evaporation of the mix water out of the matrix. Premature evaporation of mix water causes shrinkage cracks and curling, that can ruin a slab.

Consider use of a penetrating sealer as soon as possible after the finishing operation is completed. "Thumb Print Hard" is a rule of thumb for time. Temperature, humidity and wind speed [all affecting evaporation] may come into play, but generally speaking, as soon as possible. Use a sealer that doesn't form a film on the surface but rather wicks inside to form a solid inside the gel-pore/capillary system. This prevents evaporation, allows for complete hydration and curing without applying some compound that will have to be removed at a later date. The sealer should also warrant waterproofing of the slab and no adhesion problems of your tile set bond material. I use such a sealer that allows for painting or tile setting 48 hours after its application!

This procedure will allow for a tile set or coating to be applied in about three days after your concrete was poured. This also prevents internal chemistry of the concrete from migrating up and out to the surfaces, where it can cause "Alkali Burn" of a coating or a dis-bond of a thin set material. Check with the specifier on your project to get a buy in on this product and procedure !! You may have to educate him ?? Otherwise you can waste 28 days waiting for the "Normal Time Period" and still have vapor emission problems with your slab because the gel-pores are still open and the slab was not waterproofed!