Type M mortar is best described as being used for which application?

• A. Above-ground decorative walls
• B. Underground masonry
• C. Interior non-load-bearing partitions
• D. Exterior sidewalks only

Answer: (B)

Type M mortar is the strongest of the standard mortar mixes and is designed for masonry that must resist high loads and significant moisture from earth. This makes it ideal for below‑grade work, where foundation and basement walls, or retaining walls, bear earth pressure and moisture over time. So underground masonry is the best use because it requires that higher strength and durability to prevent cracking and deterioration from soil, moisture, and frost. The other options involve applications that typically don’t need such high-strength mortar: decorative above-ground walls and interior non-load-bearing partitions use lighter, more workable mixes, and exterior sidewalks are generally built with mixes suited to flatwork and exposure rather than the highest-strength masonry mortar.