Which cement is better suited for tile work
Tiles: flexible adhesive and their properties
With large porcelain stoneware wall tiles, flexible adhesives ensure high stability. Photo: PCI
The demands on laying mortar for ceramic tiles have increased continuously over the last few decades. Ever larger formats, the increasing popularity of porcelain stoneware tiles, the desire for ceramic floors also in outdoor areas: this can no longer be achieved with conventional cement adhesives. This is why so-called flexible adhesives were developed.
In the case of tile adhesives, a distinction is made between cement-based adhesives and non-cement-based adhesives such as dispersion adhesives and reaction resin adhesives. In this article we limit ourselves to the cement-based products. They are also referred to as glue, although from a purely material point of view it is actually mortar.
A special form of cement tile mortar is the flexible adhesive. It is usually more expensive than standard cement mortar and, in contrast to this, contains plastic additives, which ensure that it has a much greater adhesive strength and also hardens more flexibly. Hence the name: Flexible adhesive stands for flexible cement tile adhesive. What this means is explained below.
Trend towards porcelain stoneware
A major reason why flexible adhesive has increasingly replaced the simple cement adhesive is the fact that more and more porcelain stoneware tiles are being laid in Germany. This ceramic material is denser than tiles made of conventional stoneware and therefore absorbs less water. The difference to the previously widely used earthenware tiles, which were far more porous and therefore more drinkable than modern materials, is even greater.
The dense material structure and low absorbency of porcelain stoneware have the advantage that the tiles are, among other things, more frost-resistant in outdoor areas. In addition, the coverings are harder and more resilient, which is an important property, especially in the floor area. On the other hand, the dense, smooth tile backs can no longer be safely glued with pure cement-based adhesive. Conventional laying mortar simply hardly finds any pores into which it can penetrate, so that the interlocking creates a firm bond between tile and mortar.
Plastic for more adhesive strength
As a solution to the processing problem with low-pore tile coverings, the first flexible adhesives came onto the market in the 1980s. As mentioned above, these are cementitious tile adhesives with plastic additives. The plastics produce a significantly stronger adhesive force without the mortar having to penetrate the covering material.
Fine stoneware tiles can also be securely bonded to the substrate with flexible adhesive. They are also better suited for critical substrates such as old ceramic coverings or other smooth surfaces. After all, the durability of the covering not only depends on the adhesion between the tile and the mortar, but also between the mortar and the substrate. The increased adhesive strength of the Flex products also proves itself when it comes to tiling walls with large-format panels without them slipping off.
The plastic-modified adhesives are also ideal for highly stressed floor areas. Photo: PCI
In addition to the stronger adhesive force and the associated better adhesion, even on critical substrates, flexible adhesive has another very advantageous property: The plastic-modified installation mortar is significantly more elastic and thus more malleable than purely cement-based adhesive. After drying, the material is less rigid. It can therefore better absorb vibrations, tensions and other movements of the subfloor, which occur especially with "working" subfloors such as dry screed elements or fiber cement and plasterboard panels. This reduces the risk of individual tiles flaking or cracking.
According to the "Guideline for flexible mortar" issued by Deutsche Bauchemie, cement-based tile adhesives must have a deformability of at least 2.5 mm so that they can be designated as flexible adhesives. This elasticity also has the advantage that freshly laid wet screed can be laid much faster with tiles than is possible with pure cement-based adhesive. The manufacturer PCI advertises that with its “Flex Adhesive S2” cement screeds can be covered with tiles and slabs after just three days without any problems. Although the screed continues to shrink for a while as it dries, it is compensated for by the deformability of the adhesive mortar.
Rules of technology
Incidentally, many modern flexible adhesives are more efficient than the recognized rules of technology in the tiling trade currently provide (as of September 2017). They recommend the craftsman to wait at least 28 days before laying tiles on a cement-based wet screed. As I said: with the S2 product from PCI, tiling should be technically possible after just three days! In order to prevent liability problems, however, the manufacturer expressly recommends that the fabricators make a corresponding written agreement with the client if the screed is actually covered earlier than after a waiting period of 28 days.
You can find more on the subject of construction chemistry in the overview
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