For thousands of years, tiles were a few inches across because those sizes were easiest to make and fire in a traditional kiln. As recently at the 1980's, 8"x8" floor tiles and 4"x4" wall tiles were typical. The introduction of the roller-hearth kiln changed all of that. There was no longer a size restriction on how large a tile could be and manufacturers raced to make tiles larger to meet changing consumer preferences. Today, tiles of 12"x24", 8"x36", and even 24"x48" are common. Smaller tiles are easier to install with a thin layer of mortar. Larger tiles (tiles larger than 15" long on any side) and heavy tiles (tiles that weigh more than 5 pounds per square foot) caused installation products manufacturers to develop new products and methods that could be troweled thicker.
Why do Large and Heavy Tile mortars need to be different? Small tiles are typically set with thinset mortars (click on the photo to see it larger). The Tile Council of North America defines thinset as "...a blend of cement, very finely graded sand, and a water retention compound that allows the cement to properly hydrate. Tile set by the thinset method is adhered to the substrate with a thin layer of "thinset" cement. This type of cement is designed to adhere well in a thin layer - typically not greater than 3/16th thick." Tile substrates are allowed 1/8" of variation from one area to another (and typically have more than that). Large tiles are allowed almost 1/8" of bow from the edges to the middle. Adding those together, you realize that there can be almost 1/4" of gap which can't be filled with 3/16" of mortar.
What's different about Large and Heavy Tile Mortars? These mortars are designed to do two things well, be applied thicker and not to shrink much while they cure. The first part of this equation requires the mortar to be Thixotropic. While they are being troweled or the tile is being adjusted, the mortar acts like a liquid. Once the movement has stopped, they act like a solid holding the tile in place either vertically (see photo) or horizontally. You can think of this like ketchup, shake the bottle and it moves, stop shaking and it doesn't. By tinkering with the chemistry of the mortar, we can adjust its thixotropic behavior. Once thixotropy is holding the tile in place, it needs to stay there while it cures to avoid lippage. This is done by adding ingredients to reduce shrinkage during curing.
How do I know if a mortar is suitable? The product Technical Data Sheet, references ANSI and ISO designations for that mortar. If the designation contains a "T", the product is Thixotropic and satisfies part 1 of the above. For example, our Ultraflex LFT achieves and A118.15T rating. In 2019, ANSI is also publishing standards that include an "H" rating for mortars that satisfy part 2 of the above. They have been tested to produce minimal lippage when used with large and heavy tiles. In the past these have been called 'medium bed mortars' but that terminology has been replaced with 'Large and Heavy Tile Mortars' going forward. If you are interested in more information about MAPEI's Thixotropic mortars, please see our brochure or contact us to discuss the proper mortar selection for your situation.