Pre-Mixed Thinset Basics

• Pre-mixed thinset is not true mortar in that it is a concrete-based mortar. Instead, pre-mixed thinsets are a blend of organic adhesive materials combined with sand. As a general rule they can be used in any area where you would traditionally use normal thinset, but they are recommended by professionals to be reserved for wall applications only because of the drying time.
  
  

Drying Time

• Drying time is the primary concern for pre-mixed thinsets. Whereas normal cement-based thinset will cure within 24 to 72 hours, pre-mixed thinsets take around 5 to 7 days to cure. In humid areas or construction projects where the air is cold, pre-mixed thinset can take up to 10 or 12 days to fully cure enough to allow foot traffic. For this reason, you should always plan ahead accordingly if you are thinking of using pre-mixed thinset.
  
  

Natural Stones

• One exception to the rule is natural stones. You should never use pre-mixed thinsets with natural stones, as the organic adhesive compounds within the solution will soak into the stone and change the color of the material. This effect, known as "bleeding through," will not only stain the color of natural stone, but it also projects some of the adhesive material into the stone itself, rather than keeping it in the thinset where it cures as part of the bonding strength.
  
  

Warnings

• Master craftsmen recommend against using pre-mixed thinset for shower pans mostly because of the drying time. However, this inherently long period of drying has its own side effects, such as creating a buffet for mold and mildew. If you do decide to use pre-mixed thinset for your shower floor, be sure to spend the extra money and purchase an anti-fungal mixture to battle the natural elements while the thinset dries.