The substrate-tile pair
The variable that decides everything is not the adhesive brand: it is the combination of what is underneath and what will be laid on top. Porcelain does not absorb like a clay tile, and a freshly poured screed does not behave like an existing floor with years on it.
That is why the adhesive catalogue is organised by use case and not by price range. Choosing on price without looking at the substrate is the fastest route to a floor that starts sounding hollow within a year.
Why porcelain is different
Porcelain has very low absorption. Conventional adhesives develop part of their grip because the tile absorbs some water and the mortar anchors mechanically in its pores. If the tile barely absorbs, that mechanism does not happen.
Hence the existence of fixers for porcelain and porcelain stoneware, formulated specifically for that condition. Using tile adhesive under porcelain is the most common cause of hollow units, and the problem does not appear on installation day: it appears when the busiest area starts to give.
When a semi-flexible is needed
When there is some movement in the substrate, when the surface is difficult or when the tile has low absorption. Polimor is a semi-flexible adhesive mortar of medium controlled grain, modified with polymers and special additives that deliver a strong chemical adhesion conventional adhesives do not offer.
That chemical adhesion is what allows work where mechanical anchorage alone falls short. On floors over a slab that will deflect somewhat, on coverings over mixed substrates or with large-format tiles, it is the difference between a floor that lasts and one that lifts.
Pools and immersed surfaces
Pool adhesive is semi-flexible, of medium controlled grain, with polymers and special additives that deliver great adhesion under water immersion. The key word is immersion: a surface that gets wet is not the same as one that lives underwater.
No conventional adhesive is formulated to work permanently submerged. Saving the difference on a pool almost always means draining it and relaying the covering a couple of seasons later.
Why tiles sound hollow
Because the adhesive did not cover the whole face of the tile. There are usually three causes: the adhesive was combed badly, the open time was exceeded and the mortar skinned over before the tile was bedded, or a product without enough workability for the format size was used.
The answer is not more adhesive. It is using the right adhesive, combing in the correct direction, respecting the open time and checking every few tiles by lifting one to verify transfer. That two-minute check saves replacing an entire floor.
