Mastic is an organic adhesive that comes from the resin of the mastic tree. The resin is very sticky and used in construction, commercial applications, and tile installations as a bonding agent or adhesive. It is available as a thin-liquid, glue, or sticky paste and comes in a pail, tube, or tube similar to caulk.
Using mastic has its advantages and disadvantages over thinset. Unlike thinset, mastic comes premixed and ready to use straight out of the pail, making it a time-saver when laying tile. It tends to be stickier and set more quickly than thinset, making it ideal for vertical surfaces where you would want the tile to stay in place immediately, such as a kitchen backsplash.
While mastic is easy to use and has a high bond strength, it also has several drawbacks. Because mastic can react with water and re-emulsify, it is not appropriate for areas that have more than limited contact with water. If the mastic begins to re-emulsify, the tile can start to lose adhesion and eventually fall from the wall. This can become problematic in shower and tub surrounds even if there is not constant contact with water. Tile not sealed properly, or a crack in the grout, can allow moisture to penetrate to the mastic. Additionally, because mastic has organic components, it can be more prone to harboring mold if there is moisture present.
Another drawback to using mastic is that it will not provide much structural support. The surface must be completely level in order to prevent "tile lippage" from occurring. Mastic cannot be built up to level off minor imperfections in a floor like thinset can, and in general, most floor applications should be done with thinset and not an adhesive.