Thinset mortar is a Portland cement-based product to which manufacturers add chemicals such as latex and silicone to help protect against moisture and gain flexibility. As with any cement product, mortar must sit, or cure, for a time before use. How long to let thin set mortar cure depends on variables such as the humidity and temperature of the room.
Explaining Curing Time
When you do a tiling job, thinset mortar is what holds the tiles in place, preventing it from dislodging from whatever substrate is underneath. A mistake that many homeowners make on a do-it-yourself tiling project is moving on to the grouting stage of the project before the thinset mortar has time to cure.
Applying grout before the thinset cures essentially keeps air from reaching the mortar. Thinset mortar, like glue, needs time to dry and cure to ensure a tight and durable bond. So when you seal the air out before the thinset mortar cures, the polymer additives do not have the air they need to bond the tile to the substrate. Without this tight bond, the tiles will begin falling off the walls or will pull away from floors.
Thin Set Mortar Cure Time
Most manufacturers of thinset mortar suggest at least 24 hours of curing time before grouting. Higher humidity and heat make thinset retain more moisture and it takes longer for it to dry out. If the humidity in the room is high, like in a bathroom or if the temperature of the room is above 21.1 degrees C the day you use the thinset, it's best to wait two days. The extra time is necessary to dry out the excess moisture due to high heat and humidity.