There are generally two classifications of tile set mortar; modified and unmodified. The difference between them is unmodified or dry-set mortar is comprised of cement, sand, and a water retention agent, while modified mortar contains a polymer which can improve the bond strength and durability.
Not sure which kind is right for you? Here are the main pros and cons of each.
Pro- Due to the added polymer, modified mortar improves binding, strength, and durability.
Con- Modified mortar relies on air to cure properly, so it is not ideal for membrane flooring systems such as ones found in heated floors.
Pro- Using unmodified mortar in membrane flooring systems will prevent cracking from shifting of the subfloor.
Con- Generally, unmodified mortar is not as strong as modified varieties, and thicker layers are required in order for the mortar to bind properly.
The Schluter Ditra Membrane is a polyethylene membrane with a unique multi-layered grid structure which can be used on both concrete and wood subfloors. The advantages of using the Schluter Ditra Membrane are that it is designed specifically for ceramic tile and stone, plus both DITRA and DITRA-XL serve as an uncoupling layer, waterproofing membrane, and vapor management layer, providing a barrier to prevent against heat loss.
While the Schluter Ditra Membrane is an excellent choice for many flooring needs, there are some things to keep in mind. It is not recommended for use with thin-set mortar that is not also Schluter brand, and it is not ideal for coupling with modified thin-set mortar.