Is there a certain type of mortar mix I need to use?

Q: I’d like to build a stone landscape wall. Is there a certain type of mortar mix I need to use?

A: There are several types of mortar mixes available for different applications, classified based on compressive strength, bonding properties and flexibility. Each mortar mix uses a specific proportion of Portland cement, hydrated lime and sand during mixing. Mortar mix type N is the best one for general application and the choice most often used by homeowners. Composed of one part Portland cement, one part lime and six parts sand, type N has a medium compressive strength recommended for exterior and above-grade walls including soft stone masonry exposed to severe weather and high heat. Type N mortar typically achieves 28-day strength in the range of 750 psi.

Mortar mix Type O has a low compressive strength, about 350 psi, used on interior or non-load-bearing walls. Type O is used for repointing brick because its consistency makes it easy to apply, but its low structural capacity makes it best suited to interior environments and not areas subjected to high winds.

Mortar mix Type S has a high compressive strength (over 1800 psi) and high tensile bond strength, making it ideal for use in masonry at or below grade because it performs well against soil pressure, wind and under seismic conditions.

Type M mortar mix has the highest amount of Portland cement and is used for heavy loads, masonry below grade, foundations, retaining walls and driveways. Mortar mix type M will provide with at least 2,500 pounds per square inch compressive strength. However, Type M is also known to have poor adhesion and difficulty sealing.

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