Sand, a binder, and water
Mortar is a mixture of sand with a binder, which is cement, lime, or both, plus enough water to make it workable. Sand is the bulk of it, which is why a mix is described as parts of binder to parts of sand.
The sand matters more than people expect. Bricklaying uses soft sand, whose fine rounded grains make a mix that spreads and holds under a trowel. Sharp sand has coarse angular grains and belongs in screeds and renders. Using one where the other is wanted produces a mix that either will not stand up on the trowel or tears as it is spread.
Its first job is spreading load, not sticking
Almost everybody pictures mortar as glue between bricks. It does bond, and the bond is doing real work against wind and sideways force. But it is the second function, and reading it as the first leads to the wrong instincts about strength.
Bricks are not flat. They are not even the same size as one another, especially handmade ones. Stack them dry and they touch at their high points, so the whole weight of the wall above passes through a few small contacts, and masonry that is enormously strong in even compression will chip and crack under a point load.
It is supposed to be the weakest thing in the wall
Every wall moves. Thermal movement, moisture movement, small settlement, a lorry going past. That movement has to be taken up somewhere, and the whole design of masonry assumes it will be taken up in the joints.
So mortar is deliberately the sacrificial component. When the joint is softer than the units, it cracks first, and a cracked joint is a maintenance item: rake it out, repoint it, done. When the mortar is harder than the brick, the movement goes through the brick instead, and there is no equivalent repair for a brick whose face has come off.
This is why "stronger mortar" is usually the wrong instinct and why cement repointing has quietly wrecked a lot of old buildings. The failure takes several winters to appear, by which time nobody connects it with the work.
Lime, cement, and why the old buildings are different
Before Portland cement, mortar was lime and sand. Lime is softer, more flexible and considerably more permeable, which suits a building with shallow foundations and solid walls: it moves without complaint, and moisture that gets into the wall can evaporate back out through the joints.
Cement is stronger, sets faster and is far less permeable. That is the right answer for modern cavity construction and the wrong one for a solid wall built in 1880, because water that used to leave through the joints now leaves through the brick faces instead. A gauged mix, cement and lime together, sits between the two and is the traditional compromise.
Mortar, concrete, render and grout are not interchangeable
All four are sand and a binder, and the differences are what decides which one a job wants:
- Mortar. Fine aggregate only, laid in a joint around 10mm. Beds masonry.
- Concrete. Coarse aggregate as well, placed in mass. Carries load in bulk, and the mixing guide covers why the water content decides its strength.
- Render. Mortar applied as a coating rather than a joint, usually with sharp sand, and the render and plaster tool handles coats and coverage.
- Grout. Fluid, poured or pumped to fill voids, cavities or the cores of blockwork, where a stiff mix would bridge instead of flow.
For the quantity and the mix itself, the mortar calculator is the page to use, and it is built around the trap that catches most people, which is that a mix ratio is by volume while cement is sold by weight. The brick and block calculator does the units the mortar is going between.
Common questions
What is mortar made of?
Sand, a binder, and water. The binder is cement, lime, or both together, and the sand is most of the volume, which is why a mortar mix is written as parts of binder to parts of sand. Soft sand, sometimes called building sand, is used for bricklaying because its fine rounded grains make it workable under a trowel. Sharp sand has coarser angular grains and goes into screeds and renders, and swapping one for the other changes how the mix handles more than most people expect.
Does mortar stick bricks together?
Partly, and that is not its main job. Mortar bonds to masonry and the bond genuinely matters for resisting wind and lateral force, but the first function is to make a continuous even bed so that load passes uniformly from one unit to the next. Bricks are neither flat nor identical, so a dry-stacked wall touches only at high points and concentrates its load there. Think of mortar as a bedding layer that happens to also adhere, rather than as an adhesive that happens to fill a gap.
Should mortar be stronger than the bricks?
No. Mortar is meant to be the weakest part of the wall, because every wall moves and that movement has to be accommodated somewhere, and it is the single most consequential thing to understand about it. If the joint is the softer element it takes the movement and can be raked out and repointed. If the mortar is harder than the brick, the brick takes it instead, and a cracked or spalled brick face cannot be repaired, only replaced.
What is the difference between mortar and concrete?
Coarse aggregate. Concrete contains stone as well as sand, which is what gives it compressive strength in bulk, and it is used in masses thick enough for that stone to work. Mortar has fine aggregate only, because it has to be laid in a joint around ten millimetres thick and a piece of stone in it would simply hold the bricks apart. They are not interchangeable in either direction: concrete will not lay a joint and mortar in a footing is expensive and weak.
Why do old buildings need lime mortar?
Because they were built to move and to breathe, and cement stops both. A lime mortar is softer and more flexible, so it accommodates the movement of a building with shallow foundations instead of transmitting it into the masonry. It is also more permeable, letting moisture that gets into a solid wall evaporate back out through the joints. Point an old solid wall in hard cement and the water leaves through the brick faces instead, which is what takes them off over a few winters.
What do the American mortar types mean?
The United States and Canada specify mortar by letter rather than by volume ratio: M, S, N, O and K, running from strongest to softest. The letters are every other letter of the phrase MaSoN wOrK, which is genuinely how they are taught and remembered. N is the general purpose one for most above-grade work, S is used where more bond strength is wanted, M for heavy load and below grade, and O and K are soft mixes largely reserved for historic masonry.