Wall tiling in adverse drying conditions

Ready-mixed adhesives are dispersions of polymers and inert fillers in water and do not gain strength until the majority of the water has dried out.

A solid bed of adhesive, as required for walls in wet areas, dries much more slowly since there are no air spaces and 'breathing' is inhibited.

If the area is cold or poorly ventilated, then the overall drying rate will also be slow.

Some of the water from the adhesive will initially be absorbed into the substrate and tile body.

If the tile is impervious, for example porcelain, or if the substrate is impervious, such as a layer of existing tiles, this cannot happen and it is necessary for all of the water to leave the system via the grout joints.

Water will leave the whole system by evaporating through the grout joints.

For a given tile 6 mm wide joints will have twice the surface area of 3 mm wide joints.

For a given joint width 150 mm square tiles will have twice the grout area of 300 mm square tiles.

If the joints are grouted before the drying is complete, water will be trapped. As it then dries through the grout the water will carry dissolved salts and cause a white deposit on the surface of the grout, known as efflorescence. This can sometimes happen anyway but the chances are increased if more water is trapped. It is also possible for water to carry other colours, such as a brown stain from wooden substrates, to the surface of the grout.

Paste adhesives offer superior ease of use but only gain strength by evaporation/water loss. In situations where the tiles are very large or the conditions are not conducive to evaporation, a cement-based adhesive is recommended.