Final answer:
Type N mortar, similar to ancient Roman concrete, can be used for load-bearing walls not exposed to moisture and freeze/thaw cycles. It is made of lime mortar and sand and is known for good strength and durability.
Step-by-step explanation:
For load-bearing walls that will not be subjected to moisture and freeze/thaw cycles, a type of mortar that could be used is one similar to the Roman concrete, also known as opus caementicium. This material is a mixture of lime mortar, sand, volcanic ash (pozzolana), and water.
It's known for its exceptional strength and durability, as evidenced by many ancient Roman structures still standing today. However, for contemporary construction, a basic mixture of lime mortar and sand would typically suffice for such interior load-bearing conditions where moisture and freezing are not concerns.
Hence, the usual modern mortar for such applications is Type N mortar, which is a general-purpose mortar with good strength and durability for above-grade, load-bearing walls.