What are the types of mortar and their uses in masonry?

Mortar is the binder that assembles bricks and blocks. According to CSA A179, mortars are classified by type:

Mortar types:

• Type N (general purpose): The most commonly used. Compressive strength: 5.2 MPa (750 PSI). Proportions: 1 volume Portland cement, 1 volume lime, 6 volumes sand. Used for non-load-bearing exterior walls, partitions, residential chimneys. The most versatile
• Type S (high strength): Strength: 12.4 MPa (1,800 PSI). Proportions: 1 cement, ½ lime, 4.5 sand. Used for load-bearing walls, foundations, retaining walls, and manholes
• Type M (very high strength): Strength: 17.2 MPa (2,500 PSI). Proportions: 1 cement, ¼ lime, 3 sand. Used for heavy infrastructure (thick retaining walls, paving)
• Type O (low strength): Strength: 2.4 MPa (350 PSI). Used for repairs and repointing of old masonry (mortar must be weaker than the brick)

Important rule: Mortar should never be stronger than the brick — otherwise the brick will crack under thermal stress. For old bricks (soft brick), use Type O mortar.