Adhesion of Air Lime–Based Mortars to Old Tiles: Moisture and Open Porosity Influence in Tile/Mortar Interfaces

The use of tiles in Portuguese façade coatings began in the nineteenth century with industrialized production of tiles. Nowadays, many nineteenth century façades are degraded and in urgent need of maintenance. New bonding mortars must be developed in order to substitute the existing ones. Compatibility, durability, and reversibility with the old materials must be respected. The tile/mortar adhesion strength is a very important characteristic in old tiles reattachment. It mostly depends on characteristics of mortars, porosity, and moisture content of tiles. Mortar in contact with a wet tile will have different binding characteristics than that in contact with a dry tile. This work studies the adhesion of lime-based mortars to old tiles and the influence of moisture and open porosity in the tile/mortar interface. New tiles with similar porous characteristics as nineteenth century tiles were used. A newly designed test was used to test the tile/mortar adhesion strength. The composition of mortars and moisture content of tiles was varied. Some trends of variation of mortar compositions and moisture conditions of tiles were observed in tile/mortar adhesion strength. The increase of moisture in tiles causes the increase of mortars' open porosity. Mortars applied in wet tiles have higher values of adhesive strength than mortars applied in dry tiles. The tile/mortar adhesive strength increases with increasing open porosity of the mortar in the interface.