What Is Mortar Made Of? Cement, Lime, Sand & Water Explained

Mortar is made of three things: a cementitious binder, fine aggregate (masonry sand), and water. Most modern masonry mortar adds a fourth, hydrated lime, and many bagged products also carry small quantities of admixtures.

Scope: this covers mortar for laying brick, block and stone under ASTM C270, the standard that governs unit masonry mortar in the US. Render, plaster, tile-setting mortar and repair mortars are different products with different rules.

The one sentence worth keeping is this: cement is an ingredient of mortar, not a synonym for it. Sand is the largest part of the mix by a wide margin, and the binder is a small fraction that holds the sand together.

The four ingredients and what each does

Assumptions: proportions are for cement-lime mortars mixed to the ASTM C270 proportion specification. Bagged masonry cement and mortar cement products combine the binder ingredients before the bag is filled, so their on-site ratio is cement product to sand only.

Sand is most of it

Sand is 70 to 75 percent of the volume of a standard mortar. Its job is not filler in the dismissive sense. It gives the mortar the body to hold a joint open, limits shrinkage as the paste cures, and controls how the mortar behaves under a trowel.

Masonry sand is specified under ASTM C144, which sets a grading envelope. It is finer and more evenly graded than concrete sand, because a mortar joint 10 mm wide cannot accept the coarse particles that concrete sand carries. Sand that is too coarse makes a harsh mortar that will not spread. Sand that is too fine needs more water and more cement to reach the same workability, and shrinks more as it cures.

Sand also carries a hidden variable: moisture. Damp sand bulks up, often by 20 to 30 percent, so a bucket of damp sand contains noticeably less actual sand than a bucket of dry sand. That is worth knowing when you batch by volume.

Portland cement is the primary binder

Portland cement, specified under ASTM C150, is the powder that turns a pile of sand into a wall. Mixed with water it hydrates, a chemical reaction that grows crystals through the paste and locks the sand grains and the masonry units together. The reaction starts within a couple of hours, gives usable strength in a day or two, and continues for weeks.

Cement gives mortar its compressive strength and much of its bond strength. Raising the cement content raises the strength, which sounds like an unqualified good and is not. A mortar considerably harder than the units it beds is a liability, because the joint stops being the part that gives way first.

Lime does the work cement cannot

Hydrated lime, specified under ASTM C207, is the ingredient that makes cement-based mortar pleasant to lay and durable in service. It contributes four things:

• Plasticity. Lime makes the mortar smooth and cohesive on the trowel, so it spreads without tearing and clings when the trowel is turned over.
• Water retention. Brick and block are absorbent. Lime holds water in the mortar against that suction so the cement has enough left to hydrate properly and form a real bond.
• Flexibility. A lime-bearing joint accommodates small movements from settlement, thermal cycling and wind without cracking straight through.
• Autogenous healing. Free lime can recrystallise in fine cracks over time and partly seal them, a slow self-repair that a straight cement mortar does not have.

Lime also introduces a naming trap. Hydrated lime comes in Type N and Type S, and mortar comes in Type M, S, N and O. They are separate classifications that happen to share letters. Hydrated lime Type S is “special hydrated lime” with a guaranteed plasticity and soundness, and it is what most masonry work uses, but Type S mortar does not require Type S lime.

Water is a reactant, not just a wetting agent

Water is what starts hydration, and it also sets the workability. Mortar is unusual among cement-based materials in that it is not specified by a water-to-cement ratio. ASTM C270 specifies mortar by proportion or by property, and the water is whatever it takes to reach a workable consistency for the mason and the units on that day.

That is why there is no single correct water figure, and why the mortar water ratio guide gives a starting range you then finish by feel. Water needs to be clean and free of meaningful amounts of acids, alkalis or organic material. Potable water is fine.

The binder is not always cement

The question “what is the binding material in mortar” has more than one right answer, and the correct one depends on the mortar in front of you.

Assumptions: this covers mortars for bedding masonry units. Gypsum, clay and bituminous binders appear in historic and specialist masonry but are outside ASTM C270.

Two of those rows deserve a note.

Masonry cement is a factory blend of portland cement with a plasticising material, usually finely ground limestone, plus an air-entraining agent. It replaces the cement and the lime in one bag, which is why a masonry cement mortar is proportioned as cement product to sand rather than as three ingredients. It is convenient and consistent, and the air entrainment that makes it workable also tends to lower bond strength compared with a cement-lime mortar.

Mortar cement, covered by ASTM C1329, is a similar product with a limit on air content and a minimum flexural bond strength requirement. Where a project specifies bond strength, mortar cement is usually the reason.

Admixtures

Bagged mortars and many site mixes contain small additions that change one property without changing the basic recipe.

• Air-entraining agents create microscopic bubbles that improve workability and freeze-thaw resistance. Air also reduces bond strength, which is why ASTM C270 caps air content by mortar type.
• Pigments, specified under ASTM C979, colour the mortar. Iron oxides do nearly all of the work, dosed as a percentage of the cement weight and capped at 10 percent.
• Set retarders extend the working window, and they are what allow ready-mixed wet mortar to be delivered in tubs and used over a day or more.
• Water repellents are used where the units carry an integral water repellent, so the joint and the unit behave the same way.

Site-added antifreeze compounds containing chlorides deserve a warning rather than a description. They corrode embedded metal, ties and reinforcement included, and are prohibited or restricted in most masonry specifications.

Why mortar is not simply cement

Cement on its own is unusable in a joint. It shrinks badly, it costs several times what sand costs, and it produces a brittle material that would crack the units it is meant to hold. Sand is what makes the mixture stable, and lime is what makes it workable.

The proportions are set out in the ASTM C270 proportion specification, which fixes the cementitious materials and then ties the sand to them:

Aggregate = not less than 2¼ and not more than 3 times
            the sum of the separate volumes of the cementitious materials

That single rule is why every familiar mortar ratio looks the way it does. Type N at 1 part cement, 1 part lime and 6 parts sand puts the sand at exactly 3 times the 2 parts of binder. Type S at 2:1:9 does the same. So does Type O at 1:2:9. The proportions are not folklore; they are the top of a range in a standard.

Assumptions: proportions by volume of dry, loose material, matching the ratios the Mortar Calculator uses. ASTM C270 also allows a property specification route, where a mortar is qualified by laboratory testing rather than by proportions.

The strength differences between the types come almost entirely from the ratio of cement to lime within the binder, not from how much sand is in the mix. That comparison is worked through in the Type N versus Type S guide.

The same ingredients, different traditions

The recipe shifts by region, and none of the versions is wrong.

US practice leans on bagged masonry cement or on a cement-lime mortar proportioned to ASTM C270, and quotes mortar by type letter: M, S, N or O.

UK and European practice more often specifies a cement, lime and sand mortar by designation, or a cement and building sand mix such as 1:4 or 1:6, with mortars classified by compressive strength class under BS EN 998-2. Hydrated lime is common but a proprietary plasticiser frequently takes its place on site.

Conservation practice across both goes back further, to lime alone. Non-hydraulic lime sets slowly by absorbing carbon dioxide from the air rather than by hydration, and natural hydraulic lime sits between that and cement. Both are softer and more permeable than cement mortar, which is exactly why they suit old, soft masonry.

What stays constant is the structure of the recipe: a binder, a fine aggregate at roughly three times the binder volume, and enough water to make it workable.

Getting the ingredients in the right quantities

Knowing the recipe is the easy half. Working out how much of each material a wall needs is the other half, and it depends on the unit size, the joint thickness and the wall area rather than on the recipe itself.

The Mortar Calculator takes those dimensions and returns the cement, lime and sand quantities for your chosen mortar type, alongside the equivalent count of pre-blended bags. If you are mixing from raw materials for the first time, the step-by-step mixing guide covers the order the ingredients go in and why the dry blend comes before the water.

The distinction that trips people up

Mortar, cement and concrete get used interchangeably in conversation and they are three different things. Cement is a powder and an ingredient. Mortar is a mixture of cement, sand and water designed to bond masonry units and stay slightly weaker than they are. Concrete is a mixture of cement, sand, coarse aggregate and water designed to carry load.

They are not substitutes for each other in either direction, and the mortar versus cement versus concrete comparison sets out what happens when they are swapped.