Which type of mortar would be used for work below grade?

Type S mortar is specifically designed for below-grade applications due to its higher compressive strength and good bonding properties, which make it suitable for environments exposed to moisture and pressure. This type of mortar is ideal for use in situations such as foundation walls, retaining walls, and any other structural elements that are underground or subject to heavy loads. The combination of strength and flexibility, along with its ability to resist moisture penetration, makes Type S an excellent choice for such conditions.

The other types of mortar, while valuable for other applications, do not possess the same characteristics that Type S offers. For instance, Type N is more suitable for above-grade applications due to its lower strength, Type M, while stronger, is often too rigid for some below-grade work, and Type O is low strength, making it inappropriate for structural elements below grade where higher load-bearing capacity is required.