When it comes to installing floor tile, you need a variety of materials for installation. Thinset mortar is the foremost product, as it binds the tile to the substrate beneath. Grout is the finishing element used to fill the joints between the tiles. And while grout cannot be used for mortar because of the lower cement content, mortar can be used for grouting floor tiles, although there are limitations.
A variety of polymers, latex additives, pigments, sand, cement and lime, thinset mortar is the base element for tile installations. Its basis is the old-fashioned cement mixture used over the years in building block structures, with the cement and lime bonding together to create a sticky adhesive that hardens over time and binds whatever materials are stuck to it during installation.
Grout is similar to thinset mortar except that it has a higher quantity of sand, a lower amount of lime and it is mixed with more water to create a slightly soupier mixture. Grout is intended to fill the voids between the tiles rather than bond the tile surface to anything, so it is less sticky than traditional mortar. Grout also cures differently because of the additional water.
You can use mortar as a grout in limited instances. For example, while mortar can be used with joints up to 1/4-inch, beyond that, it is prone to shrinkage because of the lower amount of sand within the mixture. Grout has higher levels of sand specifically to counter shrinkage over time, with wider joints requiring more sand. Using mortar beyond 1/4-inch joints will lead to the mortar cracking over time.
Mortars lack color, which is another reason why grout is traditionally used over mortar even in instances where mortar can be used as a grouting element. Thinset is either gray or white. There are no pigments added to either of the mixtures.