As William noted, there are three types of mortars available: portland cement-lime (PCL) mortar, masonry cement mortar and mortar cement mortar. The following is from BIA Tech Note 8Rev (2003):
“Mortar cements are hydraulic cements, consisting of a mixture of portland cement, plasticizing materials such as limestone or hydrated or hydraulic lime, and other materials intended to enhance one or more of the properties of mortar. In this respect, mortar cement is similar to masonry cement. However ASTM C 1329, Specification for Mortar Cement, includes requirements for maximum air content and minimum flexural bond strength that are not found in the masonry cement specification. Because of the strict controls on air content and the minimum strength requirement, mortar cement and portland cement mortars are treated similarly in the Masonry Standards Joint Committee (MSJC) Code and Specification.”
The key here is the difference between masonry cement and mortar cement. Masonry cement appeared in the 1920’s and 1930’s. The intervening decades have been filled with acrimony between the masonry cement makers and lime producers. The proprietary nature of masonry cement led to charges that it was full of mysterious stuff. In the mid-seventies, Clayford Grimm, among others, did some testing that revealed very high air content (over 18 percent) for some manufacturer’s products. Investigation also revealed masonry cements typically achieved lower bond strengths than PCL mortar. Masonry cement producers continued to claim equality with PCL mortars. Finally, after UBC rebuffed their efforts to have masonry mortars approved for use in seismic-reinforced masonry, the manufacturers decided they would have to do something to address the issues of air content and bond strength. Their response was to develop mortar cement (as opposed to masonry cement). The chief difference is that mortar cements are limited to 8-to-12 percent air content (vs. 3-to-5 percent for PCL) and flexural bond strength equivalent to PCL. These changes were codified in 1996 (I think) in the ASTM C 1329 standard referenced above.
Today, UBC and IBC recognize (and require) either PCL mortar or mortar cement mortar for seismic reinforced structures.
William also mentioned the recent testing of bond strength. I admit I’ve stopped paying close attention to this issue in the past few years, but as of 2000, the initial results done on face brick indicated that brick with a low IRA (initial rate of absorption, or “suction”) actually had better bond with mortar cement mortar than PCL mortar. On the other hand, PCL provided superior bond with high suction brick. This is probably because a workable PCL mortar contains more water than a workable mortar cement mortar (at least of those tested at that time). But, while it’s true that good bond is critical to the performance of a wall, its entirely possible to put too much faith into simply achieving the strongest bond possible.
For example, the solution to water penetration issues related to masonry veneer on steel stud framing construction was not to increase the bond strength of the mortar but to reduce the deflection of the steel studs. Likewise, it would be better to carefully select brick with good IRA values rather than obsessing about whether mortar cement (or even masonry cement) has better bond than PCL mortars. The key is not only to avoid specifying brick with an exceptionally high or low IRA, but also to avoid blends that combine very high suction brick with very low suction brick. Such blends ensure that the mason can’t take any measures (i.e. wetting brick, slightly increasing or decreasing water content of the mortar, etc.) to achieve good bond, since what helps bond for one brick hurts it for the next.
So, to address Brett’s concerns (finally!)
First, there is no inherent reason to fear mortar cement (or masonry cement either). If your structure is seismic-reinforced, mortar cement is mandatory. Otherwise, I can tell you that in the Metro Detroit area, masonry cement has held the vast majority of the market for decades (even before the advent of mortar cement) without any obvious problem emerging. That said, if your mason hasn’t had much experience with whatever type of mortar you finally accept, your project will be on the wrong side of his or her learning curve. Ask about the extent of their experience. At best, they should have experience using the same mortar used for your project, with masonry units having suction rates comparable to those on your project, and under similar weather conditions. You can boost your confidence by requiring more extensive mock-ups than you would otherwise, reviewing the results in person, and discussing special methods he plans to use (mortar water content adjustments, etc.).
If you are using brick, the other thing you can do (at least in the future) to decrease your risk (regardless of mortar type used) is to use make sure the IRA is not too high or low and to avoid blends where the individual brick are vastly different.