Astm c-91 is a specification that the Masonry Cement producer is held up to. (usually a cement manufacturer)
C-270 is based on labratory specifications for Mortar,
which is the finished product when you combine masonry cement with sand and water or portland cement and lime with sand and water.
C91 is a "Product Specification" that represents the minimum requirements for Masonry Cement. Please note that mortar made from masonry cement will have different properties than mortar made with portland cement and lime under the proportion specification of C270.
C270 is a "Performance Specification" for mortar, whether referenced as a proportion specification or as a property specification.
In my experience of testing mortar, both field and laboratory, mortar made with masonry cement will not likely meet the property specification of C270 (compressive strength under Type S or Type M). Part of this is due to the problems with field sampling and testing mortar (likelihood of sample disturbance can be great), and part of it is due to the "diluted" properties of masonry cement as compared to a 1 part portland cement + 1/4 to 1/2 part lime conventional mortar mix.
There are several precautions that must be taken to achieve good quality mortar. The most prominent of these is the technique used by the masonry contractor to measure his material quantities. Most often this is done by breaking a bag of masonry cement into a mixer and then adding shovels of sand (and water from an ever-present hose) until the mix "looks about right". As you can imagine, a shovel of sand in the morning when the sand is damp, will contain a different volume of sand than in the afternoon when the sand pile has likely dried out a bit from exposure (assuming no rain!). This leads to highly variable mortar properties. My recommendation is to fabricate a 1/2-cubic foot box (or you can use a 1-cubic foot box if you enjoy back strain!)and measure ALL dry materials with it, including cement. This allows for consistency in the volumetric proportions of the mortar which will then vary only by the moisture content of the sand (which won't vary by more than about 1 percent by weight). This leads to much better consistency in the properties of the mortar as well.
For specifications, decide whether you want proportion or property specs and proceed to enforce such. If you do not specify one or the other, the proportion specification under C270 is the default. Specify the evaluation of the mortar under ASTM C780.
