Final answer:
Type S mortar is commonly used for block walls in basement foundations below grade, due to its high tensile bonding strength and resistance to soil pressure and moisture.
Step-by-step explanation:
The type of mortar used for block walls in a basement foundation below grade is often a specific blend of mortar that is durable and can withstand the pressure of the earth against it
It must also be resistant to moisture penetration. A typical choice for this application is Type S mortar, which is a mixture of Portland cement, hydrated lime, and sand. Type S mortar provides high tensile bonding strength and is capable of resisting soil pressure and wind load after curing. When erecting block walls for foundation purposes, professionals choose a mortar with greater compressive strength to ensure the long-term stability of the structure.