What type of mortar mix is used for underground masonry?

The correct choice for underground masonry is Type M mortar mix. This type of mortar has a high compressive strength, typically around 2,500 psi or greater, which makes it particularly suitable for load-bearing applications, including below-grade (underground) masonry work. Type M mortar is specifically formulated to withstand harsh conditions, including moisture from the surrounding earth and potential hydrostatic pressure.

In addition to its strength, Type M mortar is highly durable and can resist the effects of freeze-thaw cycles, making it ideal for use in situations where it may be exposed to water or varying temperatures. This makes it essential for maintaining the structural integrity of underground masonry walls, such as foundations and retaining walls, and helps prevent cracking or deterioration over time.

Other types of mortar mixes, such as Type S, Type N, and Type O, have different properties and strength ratings that may not be suited for underground applications. For instance, Type S is lower in compressive strength compared to Type M and is typically used for masonry that requires good tensile strength above ground. Type N is a medium-strength mortar mix typically used in above-ground applications and is not ideal for high moisture environments. Type O has low compressive strength and is primarily used for interior, non-load-bearing