Mortar consist of Portland cement, hydrated lime, sand and water. Different types of mortar consist of these four elements but in different proportions. More Portland cements provides better compressive strength but increases the risk of cracks. The lime give the mortar better bond strength and flexibility, while sand decreases shrinkage. Water makes the cement harden. Changing the proportions makes the mortar more conducive to your project.
Three parts Portland cement, one part lime and 12 parts sand give Type M mortar a high compressive strength of at least 1134 Kilogram per square inch. In other words, it can withstand heavy loads, making it appropriate as above-grade masonry such as exterior and interior load-bearing walls as well as non-load bearing walls. This mortar also possesses great durability so it works well for at or below grade jobs (portions of a structure at ground level or below it) such as foundations, retaining walls, manholes, sewers, paving sidewalks and driving ways.
You often use Type S mortar for at-grade or below-grade masonry such as foundation walls and retaining walls, sidewalks, patios and driveways. It contains two parts Portland cement, 1 part hydrated lime and nine parts sand. This combination of elements gives the cement maximum ductile strength, which makes it stand up against wind, soil, pressure and earthquakes. It also gives the mortar a high compressive strength of about 816 Kilogram per square inch and a high tensile bond strength so it can also be used for above-grade work such as exterior load-bearing walls.
Type N mortar only possess a medium compressive strength of around 340 Kilogram per square inch with its one part Portland cement, one part lime and six parts sand formula. Type N mortar can stand up to severe weather so it works extremely well for exterior above-grade masonry such as chimneys and exterior walls---both load-bearing and non-load bearing. However, you can also use it for interior walls such as foundations or for pavement.
Made from one part Portland cement, two parts lime and nine parts sand, Type O has a low compressive strength of about 159 Kilogram per square inch. With such a low compressive strength, you should only use this type of mortar on interior non-load bearing walls or sparingly on exterior non-load bearing walls. Don't use it for foundation walls, sewers, manholes, pavement or other at or below grade masonry.