Water Absorption
The production of tile today is carefully engineered to produce tile with controlled technical aspects. This includes tile with a high water absorption rate, which is a particular advantage when producing wall tile as the higher porosity enhances bonding to a vertical surface. Interior floor tile produced for medium traffic in dry areas is engineered to achieve porosity between 3-7%.
This water absorption level produces a floor tile that achieves the necessary technical strength, while being cost effective due to its ease of cutting on site and increased bondability due to the more open pore structure of the body. This range of tile requires less polymer additives in the mortar as well as reducing the need to back butter the tile.
Tile produced for excessive traffic, exterior use or use in a wet environment is constructed to achieve 3% water absorption or lower. Although technically the strongest type of body construction, tile in the impervious 0.5% and lower category such as porcelain tile, usually requires a full two-part polymer mortar and should always be back buttered to ensure bonding.
Always select a tile that has been constructed for the specific area of use determined by the expected level of wear and environmental conditions. There is not one single tile that is best for every installation. When selecting tile for exterior use, it is necessary to determine exactly what level of performance is required and to balance this need to the correct level of absorption.
Usually, a lower water absorption percentage means that the tile's performance with respect to mechanical strength and frost resistance will be better. Tile in the vitreous range with water absorption equal to or lower than 3% or an impervious tile with 0.5% or lower are considered to have superior technical and resistance qualities.
The thickness of a tile can also have relevance to these two attributes particularly when a tile is produced by the extrusion method. Some extruded tile may have an absorption level above 3% but easily pass the frost resistance and mechanical loading tests due to the structure and dimensional thickness.
Dust pressed tile however, must be in the range of 3% water absorption or less to be tested for frost resistance under EN 202 or ISO 10545/12. This standard subjects the material to 100 cycles of freeze-thaw. Taking the tile rapidly between -15 degrees C to +15 degrees C. Tile used on the exterior should be tested, rated and approved for use by the manufacturer as opposed to using water absorption solely as the indicator.
Technical advances in recent years have allowed manufacturers to control the clay particle shape and size, increase compression at the presses as well as firing temperature and time.
Using these techniques, manufacturers produce tiles that can be guaranteed to have specific and reliable technical aspects appropriate to the intended use.
Clay
The combination of clay, sand and minerals is spray dried or atomized in order to create molecules of clay that are precisely the same in size and shape. This uniformity enables the industry to produce tight compact clay bodies.
Pressing
Depending on the type of tile being formed, manufacturers will adjust the pressure per square inch (psi) at the presses. Applying more pressure during pressing allows the maximum amount of air to be forced out of the tile body. The spherical clay molecules can be compressed extremely tightly as in a porcelain body or less compacted as in a wall tile bisque.
Firing Time and Temperature
Specific clay formulas are prepared from a combination of minerals, clay and sand. Certain minerals with a high melting or sintering temperature that also have an advanced crystalline phase are employed in the formulas for commercial or heavy use tile. The high firing temperature literally melts these minerals and the molten glass-like material fills almost all of the remaining voids between each clay molecule. This produces a tile body that is dense, compact and has an extremely fine micro porosity. While producing the most highly technical body, it should be understood that bonding this body type is more challenging than for a tile with a higher porosity. Polymer additives and forcing the mortar into the back of this tile is almost always recommended. It is imperative to note the relationship between water absorption in the body of a tile and other practical considerations.
The higher the porosity, the more enhanced bondability becomes. Conversely, the lower the porosity, the more challenging it is to bond.
The lower the porosity, the harder the material is to cut and very dense tile requires cutting using a wet-saw diamond blade.
The lower the porosity, the higher the resistance to damage caused by freeze-thaw or thermal fluctuation.
The lower the porosity, the higher the compressive strength and therefore the ability to withstand live and dead loads.
Specifying a tile with extremely low absorption rates when technically or mechanically not required, can add unnecessary costs to the project both in material and installation pricing