Final answer:
For masonry below grade and in contact with soil, Portland cement-based mortars are recommended due to their superior strength and durability, offering better resistance to moist and potentially acidic conditions.
Step-by-step explanation:
The type of mortar recommended for masonry work below grade and in contact with soil, which may require higher water resistance and strength, is generally a mix that includes Portland cement. Portland cement-based mortars are stronger and more durable than other types, making them suitable for use below ground level where moist conditions prevail. Unlike the mortar known from ancient Rome, modern mortar has evolved to include different compositions for specific applications.
Traditionally, masonry mortar was a mix of lime, sand, and water; however, modern practices have improved on this by adding cement to create cement-lime mortars, which offer better resistance to the elements. For masonry below grade, mortars often include a higher proportion of cement, which gives them the durability needed to withstand the moist and sometimes acidic conditions found in soil.