Mortars are often ordered based on compressive strength; but even more important properties are bond strength and flexibility. Bond strength and flexibility work together, holding masonry units in place yet flexing in response to lateral loads or expanding and contracting in response to temperature swings.
Premix or self-mix? Mixing cement and lime on-site provides flexibility to a skilled mason but requires careful measuring. Pre-mixed masonry cement ensures consistency but rules out on-site fine-tuning of the recipe.
What is mortar made of?To achieve the balance of properties for a particular application, you mix different proportions of portland cement, hydrated lime, sand and water. Portland cement yields greater compressive strength but lower water retention during the cure, thus risking shrinkage cracks. Lime yields lower compressive strength but greater bond strength and flexibility. Sand, the aggregate, adds volume and minimizes shrinkage as the cement dries. Water makes the mix workable and activates hydration, the chemical reaction that hardens the cement.
Type M: high compression strengthType M has the highest proportion of portland cement, with 3 parts portland cement, 1 part lime and 12 parts sand. Type M has a high compressive strength (at least 2500 psi) and is recommended primarily for walls bearing heavy loads, but also, due to its durability, for masonry below grade or in contact with the earth: foundations, retaining walls, sidewalks and driveways.