Selecting mortar the standard way...

The first question I get when I'm giving a talk on mortars is 'why do you have so many?' This one is easy to answer, it's because different mortars do different things. The followup question is usually 'how do I know which one to choose?' For that answer, we need to start diving into product standards. In the US, there are two standards groups to be aware of, ANSI and ISO.  Both publish standards for mortars, and both are referenced in the TCNA Handbook.  

Each mortar has a list of the standards to which it complies listed on the packaging and Technical Data Sheet.  These standards will help you choose the correct mortar. In ANSI, the type of mortars are divided into unmodified mortars (A118.1), polymer modified (A118.4), and high performance polymer modified (A118.15).  In addition, a mortar can also be rated for use over plywood (A118.11).  In 2012, the ANSI committee added letters to the ratings to help further differeniate mortars.  Extended set time (E) rated mortars allow for more working time.  Thixotropic mortars (T) don't slump after they are troweled and keep tile in place better for vertical installations.  Fast setting mortars (F) set up more quickly and allow for grouting earlier.  There are discussions underway to add an additional letter to address mortars for Large and Heavy Tiles, originally referred to as 'medium bed' mortars.  Other ANSI standards cover Reaction Resin (epoxy) mortars and Mastics.

In the ISO system, there is one standard for mortars.  There are additional letters that address the type of mortar (C for cement, R for reaction resin, D for organic).  A plywood compatible mortar gets a P and higher performance is designated with a 2.  In ISO, there is also a measurement for deformability that can give an indication of how much movement the mortar can withstand.  ISO mortars will either have an S1 (standard) or S2 (higher performance) designation depending on how they perform in lab testing.  

Although standards can seem overwhelming, once you understand the basics of ANSI and ISO classifications, they can help you select the correct mortar for your application.  For more information, please read this excellent article by Leigh Hightower:

Realta MAPEI article on ANSI and ISO standards