What type of mortar is medium-low strength, used in non-load-bearing interior walls?

The correct choice for medium-low strength mortar used in non-load-bearing interior walls is O type mortar. This type of mortar is specifically formulated to provide adequate bonding and flexibility in situations where load-bearing capacity is not a critical factor. It is softer and has lower compressive strength than other mortar types, making it suitable for applications where movement or settling might occur.

Type O mortar has a maximum compressive strength of about 350 psi, which is well-suited for interior applications that don't require the high strength of other mortars. This characteristic allows it to accommodate potential building movements while providing sufficient adhesion for materials such as bricks and blocks within non-load-bearing structures.

In contrast, other types of mortar, such as N, S, and M, have higher compressive strengths and are thus designed for more demanding structural applications, supporting load-bearing walls and ensuring durability against harsh environmental conditions. Therefore, O type mortar is the ideal choice for the specified use in non-load-bearing interior walls.