The specification for my job requires that the 28-day compressive strength of masonry be 2500 psi. If I make extra prisms and break them at 7 days, how strong do they need to be to assure that I meet this requirement?

You need to establish the relationship between the 28-day strength and the 7-day strength. This can be done by making and testing preconstruction prisms using the same mortar and units that will be used on the building. The procedure to follow is described in ASTM C 1314. ASTM E 447 can also be used for this purpose, provided that the test procedure is modified as noted in the 1995 MSJC Specification, Article 1.4 B.3.b. However, ASTM E 447 requires collection of additional detailed information that is not needed for compliance tests, and ASTM C 1314 is accordingly recommended over E 447 for field tests. Based on studies on masonry materials, the relationship should work out to show that the 7-day strength is roughly 70 percent to 85 percent of the 28-day strength.