The four strengths
ASTM C270-19a Table 1 is the property specification. It gives each mortar type a minimum average 28-day compressive strength:
- Type M: 2500 psi
- Type S: 1800 psi (12.4 MPa)
- Type N: 750 psi
- Type O: 350 psi
From highest to lowest the order is M, S, N, O. The common slip is putting S ahead of M or N ahead of S; the numbers settle it, since 1800 psi is below 2500 psi and 750 psi is below 1800 psi. O, N, S, M is the same list run backwards, lowest to highest.
Laboratory mortar only
Table 1 applies to laboratory-prepared mortar only. C270 §3.2 forbids judging field-sampled mortar against it. Cubes made from mortar taken off the mason's board cannot be declared failing because they test below the Type S 1800 psi figure, and they cannot be passed by comparing them with the 750 psi Type N figure either; that uses the wrong type as well as the wrong test. C270 has no rule that field mortar needs half the table value. Treating field cubes as a Table 1 test is the error the exam is looking for.
Water retention
Minimum water retention is 75% for all four types, M, S, N and O alike. There is no 90% figure for Type M, no 50% figure for Type O, and the requirement is not limited to M and S.
Air content
Maximum air content depends on the mortar's cementitious system as well as its type:
- Cement-lime mortar: 12% for M and S, 14% for N and O.
- Mortar cement and masonry cement mortar: 18% for M and S, 20% for N and O.
So Type N cement-lime mortar is limited to 14%. The trap is reaching for 18% or 20%, which belong to mortar cement and masonry cement, or 12%, which is the cement-lime limit for the two stronger types only.