Mortar

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. Premixed 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.
The Types


Type M: high compression strength
Type 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.

Type S: compression and tensile strength
Type S is sometimes specified for masonry at or
below grade, but offers another quality. S has
high compressive strength (1800 psi) but adds
high tensile bond strength. S contains 2 parts
portland cement, 1 part hydrated lime and 9
parts sand, and yields maximum flexural strength
to fight wind, soil pressure or earthquakes.

Type N: for exterior, above-grade walls
Type N is a medium compressive-strength (750
psi) mortar made of 1 part portland cement, 1
part lime and 6 parts sand. Type N is
recommended for most exterior, above-grade
walls exposed to severe weather, including
chimneys.

Type O: for interior or non-load-bearing use
Type O has a low compressive strength (about
350 psi), containing 1 part portland cement, 2
parts lime and 9 parts sand. O is recommended
for interior and limited exterior use in
non-load-bearing walls.