There are numerous bonding mortars available on the market today. Although each has its own characteristics, their main purpose is to bond stone to an acceptable substrate. There have been so many advances in bonding mortars, it has become confusing trying to decide which mortar is right for a specific job.
Dry-set mortars ANSI A118.4 are commonly called thin-sets and are formulated of Portland cement, sand and water retentive chemicals. This mortar can be limited in that it only forms a mechanical bond to the thin stone and substrate. (Unlike a modified mortar, which forms both a mechanical and adhesive bond.) With the array of thin stone tile types now available — many with dense bodies and low absorption — and regulations limiting and impairing proper preparation of a substrate, it has gotten to a point where mechanical bonding becomes almost non-existent. (To picture a “mechanical bond,” think of Velcro®, with the “barb” side being the mortar. As it hydrates, it grows microscopic arms or “hooks” into the stone and substrate, turning three different layers into one homogenous layer. The adhesive bond is like the adhesive on the back of the Velcro.)