Two ceramic tile characteristics condition tile installation by direct adhesion:
- Water absorption capacity [E in % according to EN 14411], since the anchoring of cement hydrates in cement or cement and lime mortars, as well as that of cementitious adhesives with low polymer resin content, depends on the water absorption capacity and surface micro-roughness. In very porous tiles, adhesion may also be jeopardised by incomplete hydration of the cement, when the ceramic body absorbs the water needed for the maturing of the mortar or for the cementitious adhesive without water retainers (the so-called ‘adhesive cements’ or cementitious adhesives only appropriate for interiors).
- Tile surface area, deduced from the size, for the shear stresses that could develop once the bonding material has hardened, caused by thermal and/or moisture swings in the tiles or dimensional instability of the substrate. In cement mortars and adhesives with low polymer resin content, such shear stresses could lead to rupture of the adhesive bond. The larger the surface area, the greater will those stresses be at the tile perimeter. In addition, in thin-bed fixing with resin-based dispersion adhesives [D1 and D2 of standard EN 12004], large-sized tiles hamper evaporation of the water and/or solvent, and impede full maturing of the adhesive in the time foreseen by the manufacturer.
The choice of tile installation technique depends on the above two characteristics: thick-bed with cement and/or lime mortar or thin-bed with adhesive, the latter case being conditioned by the choice of type of adhesive. If further information is needed, the Reader can consult the document Ceramic tile and tile installation
Other tile characteristics should also be taken into account in relation to tile intended use, in order to ensure tile durability and avoid impairment of appearance through correct selection of the materials and the tile installation technique:
- The coefficient of moisture expansion, particularly in very porous and glazed tiles [BIII GL or AIII GL according to EN 14411], in relation to the appearance of delayed crazing if the glaze is subjected to tension owing to compression of the substrate, caused by an overlaid cement conglomerate shrinkage and increase in tile volume by water and humidity. In this situation, the use of deformable cementitious adhesives may serve as a good preventive measure.
- The formation of efflorescences caused by migration of soluble salts to the surface of the unglazed porous tile, or the break-up of the body/glaze interface if those salts crystallise in that interface in glazed porous tiles, in coverings subject to the action of water and humidity from the substrates. In this case, waterproofing of the fixing surface and tile installation with an adhesive are preventive solutions.
The Reader can access the help questionnaire
to obtain a precise recommendation regarding the materials and the tile installation technique.