Which type of mortar is considered low strength and suitable for general interior use in non-load-bearing masonry?

The type of mortar recognized as low strength and suitable for general interior use in non-load-bearing masonry is Type O. This mortar has a compressive strength of about 350 psi, making it less strong compared to the other types of mortar. Type O mortar's composition typically includes one part cement, two parts lime, and nine parts sand, which contributes to its lower strength.

This low-strength characteristic makes Type O ideal for applications where high structural support is not required, such as in interior walls, infill panels, and non-load-bearing partitions. It allows for some flexibility, which can help in alleviating strains in masonry that might result from thermal expansion or settlement.

In contrast, the other types of mortar, such as Type M, Type S, and Type N, are formulated for various degrees of strength, with Type M being the strongest and used in heavy load-bearing applications, Type S providing moderate strength for structural applications, and Type N offering good all-around performance for both interior and exterior environments, but still being stronger than Type O. This context underlines why Type O is the best choice for the specified application.