BONDING CERAMICS TO GYPSUM AND PLASTER WALLS
Gypsum and plaster have been used for thousands of years. Their advantages are obvious: they are abundant in nature, offer good thermal and acoustic insulation and are also a good barrier against fire. They are therefore one of the most widely used coatings worldwide.
The inventiveness of manufacturers currently offers many options for the formation of vertical walls with gypsum as the main component: plasterboard panels, bricks already externally coated with gypsum, sprayed gypsum, etc.
The laying of ceramics on plaster was solved in the middle of the last century with the appearance of cement-based adhesives.
In order to formulate a special cement-based adhesive for bonding on gypsum, its specific physico-chemical behaviour has to be borne in mind. During the drying process of the facing, the water reacts with several soluble salts in the gypsum and transports them to the outside. At this point, the water that comes into contact with the atmosphere evaporates and the salts re-crystallise, giving rise to the so-called whitish efflorescence or crypto-florescence.
It should also be noted that once the tiles have been laid, a reaction may occur between the cement and the gypsum if the latter is too wet. This reaction leads to the appearance of the chemical compound, ettringite. Ettringite is a crystalline salt that causes a large increase in volume at the ceramic-cement interface.
Finally, it is well known that unlike cements, which undergo a plastic shrinkage of around 10/00, gypsum undergoes an expansion of 1% in volume, so that there is always a differential between the expansive action and the other of contraction.
A special tile-to-gypsum cement-based adhesive will be well formulated if it takes all these facts into account.
It is also important to follow a protocol for gypsum facings that are going to be tiled.
At present, and following the only compulsory standard regulating the application of gypsum products: in the General Specifications for the Application of Gypsum and Plaster in Construction Works (“Pliego General de Condiciones de Recepción de yesos y escayolas en las obras de construcción – O.M. de Mayo 1.985”), it is very important, before placing tiles, to control aspects such as:
- the absence of dust and/or surface efflorescence (rub with esparto grass)
- the residual humidity does not exceed 0.5% (ideally using a carbide hygrometer)
- the surface hardness (a simple punch will make a deep scratch)
- check that future outbreaks of moisture will not affect the gypsum
- check that the mechanical stresses to which the facing will be subjected are lower than the tensile strength of the gypsum (expansion joints, etc.)
At Fixcer we suggest using the FIX PORCELANICO FLEX or FIX PORCELANICO GEL to glue tiles, stoneware or porcelain stoneware with reduced or no absorption.
