Mortar Mix: Mortar Types
Mortar mixes are classified based on compressive strength and their bonding properties and flexibility. These are two very important characteristics of mortars, because they will hold and provide the strength on masonry units. Mortars are often ordered based on compressive strength; but even more important properties are bond strength and flexibility. Each mortar mix has a specific Portland cement, hydrated lime and sand proportions referred to volumes. However, the mortar mix is usually purchased by weight but they measured differently. Choosing a mortar mix is not an exercise based only on the resistance and strength of each mortar, it must depend on its use, adhesion and sealing requirements.
Mortar Mix Type N
A mortar mix type N is usually recommended on exterior and above-grade walls that are exposed to severe weather and high heat. Type N mortar mix has a medium compressive strength and it is composed of 1 part Portland cement, 1 part lime and 6 parts sand. A type N mortar is described as a general purpose mortar mix, used in above grade, exterior and interior load-bearing installations. It is also the preferred mortar mix for soft stone masonry. This is the mortar most often used by home owners. Type N mortar typically achieves 28 day strength in the range between 1500 and 2400 psi.
Mortar Mix Type O
Mortar mix type O is referred as that mix with a low compressive strength, about 350 psi, used on interior or non-load –bearing walls. Its exterior use should be limited due to its low structural capacity. In the appendix for ASTM C 270, Type O mortar is listed as an alternative to Type N for areas exposed to freezing. Type O Mortar mix is ideal when repointing due to its consistency and can be applied easily. . It is used in above grade, non-load bearing situations in both interior and exterior environments. Type O mortar provides a psi in the range between 750 and 1200, sometimes up to 2000.
Mortar Mix Type S
Offering a high compressive strength, over 1800 psi, and with high tensile bond strength, mortar mix type S is the ideal product to be used in masonry at or below grade. It performs extremely well to fight soil pressure, wind or under seismic conditions. This mortar mix can be used for below grade areas, for example masonry foundations, manholes, retaining walls, sewers, and brick patios and brick pavements. Type S mortars are required to have a minimum of 1800 psi and their mixes usually give you strengths of from 2300 to 3000 psi.
Mortar Mix Type M
Type M mortar mix has the highest amount of Portland cement and it is recommended primarily for walls bearing heavy loads. Type M mortar mix is used primarily for heavy loads, masonry below grade, foundations, retaining walls and driveways. Mortar mix type M will provide with at least 2,500 pound per square inch compressive strength. A type M mortar with its high strength yet poor adhesion and sealing can be a bad choice for one area of the job and just what is needed in another. Type M is preferred with stone because the strength of the mortar simulated that of the stone being used.
Mortar Mix: ASTM Requirements
Masonry mortars are specified by ASTM C 270, Specification for Mortar for Unit Masonry. Mortars are evaluated by ASTM C 780, Preconstruction and Construction Evaluation for Mortars for Unit Masonry. Following ASTM standards the minimum required compressive strength of the mortar types are:
- Type M 2,500 psi
- Type S 1,800 psi
- Type N 750 psi
- Type O 350 psi