The Material Between the Bricks—and Why the Right Match Matters

Mortar is a workable mixture of water, fine aggregate—usually sand—and a binder such as lime, Portland cement, masonry cement, or mortar cement. Placed between brick, stone, terra-cotta, or concrete block, it hardens to bed, bond, and separate the units. It is not simply masonry glue: mortar also fills irregularities, distributes loads, accommodates limited movement, and influences how a wall manages moisture. The right mortar is therefore compatible with the whole wall—not necessarily the strongest one available.

Mortar, defined in one sentence

In practical terms, mortar is the workable bedding and joint material that becomes the hardened material visible between masonry units.

Fresh mortar is plastic enough to spread across a brick, conform to rough stone, or pack into a joint. As it hardens, it creates a continuous bed between individual units. That bed maintains controlled spacing while helping the units act together as a wall, pier, chimney, arch, or other assembly.

The binder may be lime, Portland cement, masonry cement, mortar cement, or a combination selected for the required performance. Sand commonly forms most of the solid volume, while water makes the mixture workable and enables the binder to harden.

This article concerns masonry mortar. The same word can also denote an indirect-fire weapon, but that meaning is unrelated to building materials.

What mortar does inside a masonry wall

Calling mortar an adhesive captures only part of its job. It is both a bedding material beneath masonry units and a joint material between them.

Picture an ordinary brick wall. Neither the bricks nor their bed surfaces are perfectly identical. Fresh mortar fills small hollows and variations, allowing one brick to sit evenly above the next.

The joints also affect the movement of liquid water and water vapor. Mortar is not a stand-alone waterproofing system, yet its composition, condition, profile, and workmanship all influence how a wall takes in, stores, and releases moisture.

In many traditional walls, the joint is intended to remain the more readily repairable part of the assembly. Weathered mortar can be removed and replaced through repointing; damaged brick faces and stone edges are harder to restore. This sacrificial-joint principle is especially important where the masonry units are old, soft, or porous. The National Park Service explains that compatible historic mortar should accommodate movement and permit moisture migration through joints rather than forcing deterioration into the surrounding units in its guidance on repointing historic masonry.

That does not mean mortar should simply be made as weak as possible. It must still satisfy the wall’s loading, exposure, and design requirements. Compatibility is a balance, not a reason to disregard structural performance.

What mortar is made from

The familiar description—sand, binder, and water—is simple, but each component changes how mortar behaves in the hod, under the trowel, and after hardening.

Component	Basic role	Selection caution
Sand	Provides the fine aggregate and body of the mortar; affects color, texture, workability, cohesiveness, shrinkage, and performance	Grain size, grading, shape, and color matter; unsuitable sand can change appearance and behavior
Lime	Acts as a binder component and generally improves workability, water retention, relative softness, and vapor permeability	Lime products and mortar systems are not interchangeable; suitability depends on the wall
Portland cement	Contributes binding action, earlier strength, and faster setting	Excessive proportions may produce mortar that is too hard or dense for soft or historic masonry
Water	Makes the mixture workable and participates in hardening	The amount belongs to the selected formulation; casual addition of excess water can alter performance
Proprietary binder	May combine cementitious materials and other ingredients in masonry cement or mortar cement	Follow the product data and project specification rather than assuming all packaged binders behave alike

Sand is commonly the largest mortar component by volume. It is not an inert afterthought. Its particles form the mortar’s skeleton, help control shrinkage, and strongly influence visual character. In restoration work, matching the original sand may be more effective than trying to correct an unsuitable aggregate with pigment alone.

Lime generally makes fresh mortar more workable and helps it retain water against absorbent masonry. Hardened lime-rich mortar is usually softer and more vapor-permeable than Portland-cement-rich mortar, although surface hardness alone does not reveal permeability.

Portland cement generally provides earlier strength and a faster set. Those qualities can be useful in an appropriate modern formulation, but more cement is not automatically an improvement. A dense, rigid mortar may be incompatible with masonry designed around a softer lime-and-sand joint.

Traditional mortar was commonly made primarily from lime and sand. Modern formulations may instead use Portland cement with hydrated lime, or packaged masonry cement or mortar cement mixed with sand and water. These are formulation categories, not synonyms for the strength or performance labels used in specifications.

There is no universal water quantity or binder-to-sand recipe suitable for every wall.

Mortar vs. cement vs. concrete

Mortar, cement, and concrete are related, but they are not interchangeable.

Term	What it means	Aggregate	Typical role
Cement	A binding ingredient used within a mixture	None when considered by itself	Helps bind mortar or concrete after mixing with water
Mortar	A finished masonry-joint material made from binder, fine aggregate, and water	Fine aggregate, usually sand	Beds and joins brick, block, or stone in relatively thin joints
Concrete	A composite made with cementitious binder, water, and aggregate	Fine aggregate plus coarse aggregate such as gravel or crushed stone	Forms larger elements such as slabs, footings, and foundations

A bag labeled “cement” is not a direct substitute for mortar. Cement supplies binding action, but mortar also requires properly selected fine aggregate, water, and—depending on the formulation—lime or other ingredients that establish its fresh and hardened properties.

Concrete normally contains coarse aggregate and is designed for placement in much larger masses. Mortar generally omits coarse aggregate so it can form workable, comparatively thin joints. Using concrete where mortar belongs, or ordinary masonry mortar where structural concrete belongs, ignores the different jobs for which the materials are formulated.

Mortar types are not a strongest-to-weakest shopping list

The labels M, S, N, and O identify mortar strength or performance classifications. Type K is another designation readers may encounter, especially in discussions of low-strength, nonstructural repair mortar. It should not be assumed that all five labels have identical standing, availability, or acceptance under every current standard and project specification. Manufacturer guidance describes Types O and K as not generally commercially produced and associates them with limited nonstructural replacement work.

These labels are not brands or complete ingredient recipes. Two mortars assigned the same classification may meet the required performance through different formulations.

The classification system is also distinct from product categories such as:

• Cement-lime mortar, made with Portland cement, hydrated lime, aggregate, and water
• Masonry cement mortar, made with packaged masonry cement, aggregate, and water
• Mortar cement mortar, made with packaged mortar cement, aggregate, and water

An engineering overview of mortar types and formulation categories likewise distinguishes the type labels from cement-lime, masonry cement, and mortar cement systems. A type label alone does not describe every relevant property, including workability, water retention, bond, permeability, and compatibility.

Higher strength is not automatically better. A wall needs sufficient strength for its loads and exposure, but excessive strength or rigidity can shift stress into the masonry units. Conversely, mortar selected only for softness may not meet the requirements of a heavily loaded or severely exposed modern assembly.

Selection should account for:

• The strength and absorption of the brick, block, or stone
• Structural loading and wall design
• Above-grade or below-grade exposure
• Wind-driven rain, freeze-thaw conditions, and persistent damp
• Existing mortar and previous repairs
• Joint dimensions and profile
• Project specifications, applicable standards, and local codes

For new structural work, begin with the governing drawings and specification rather than a general rule of thumb. The appropriate mortar depends on the masonry condition, loading, weather exposure, unit strength, wall design, and applicable requirements; an appropriately qualified mason, architect, or engineer should resolve uncertainty about structural loading, movement, or exposure. Historic walls, persistent dampness, and visible movement may also require a preservation professional or building specialist before mortar is selected.

Why old masonry needs compatible mortar

Historic masonry often works differently from a modern brick veneer or concrete-block assembly. Traditional mortar commonly consisted primarily of lime and sand. Portland cement began entering mortar practice during the late 19th century and became common during the 20th century, so a building’s age alone does not establish what its joints contain.

For historic repair, replacement mortar should be evaluated against both the original joint material and the adjacent brick or stone. National Park Service guidance calls for repair mortar that is softer and more vapor-permeable than the masonry units while being no harder or less permeable than the surviving historic mortar. Its Preservation Brief on historic mortar joints also explains the development of lime and Portland-cement mortars and the need to match strength, permeability, sand, color, texture, and tooling.

An overly hard repair can remain intact while transferring stress to softer brick or stone. A dense, relatively impermeable joint may also redirect moisture and salts into adjoining units. Depending on the wall and exposure, associated deterioration can include cracking, spalling, delamination, or loss of the masonry surface. These symptoms do not prove that incompatible mortar is the sole cause; leaks, movement, salts, weather exposure, and other defects may contribute.

Before repointing an old wall:

1. Investigate the cause of deterioration. Look for roof and flashing leaks, defective drainage, settlement, rising damp, open copings, and severe weather exposure.
2. Identify the masonry units. Soft handmade brick, dense fired brick, limestone, sandstone, terra-cotta, and concrete block do not impose identical requirements.
3. Examine the existing mortar. Consider its binder, aggregate, condition, strength, and permeability—not merely its surface color.
4. Compare the repair mortar with the wall. It must be compatible with both the masonry units and the surviving historic joints.
5. Match the visible character. Sand color and grain size, finished color, texture, joint profile, and tooling all affect whether the repair belongs visually.
6. Consider laboratory analysis. On significant historic work, analysis can help establish original ingredients and proportions. The GSA’s historic masonry repointing specification calls for project-specific professional oversight and matching characteristics that include composition, strength, aggregate, color, texture, and tooling.

Color alone is not a specification. Apparent hardness is not enough either: an old mortar may feel hard at the surface yet remain relatively vapor-permeable. A sound repair decision comes from understanding the wall’s materials and moisture behavior, not from choosing the bag that looks closest or carries the highest strength label.

Repointing replaces deteriorated joint material. It does not stop settlement, repair failed flashing, remove a source of rising damp, or correct continuing structural movement. Those causes should be addressed first, or the new joints may deteriorate while the underlying problem remains.

Mortar is part of a masonry system, not a generic glue. The appropriate mortar beds and bonds the units, cushions minor irregularities, manages moisture in a way the wall can tolerate, and remains compatible with the surrounding materials. For new structural masonry, follow the governing specification and code. For old masonry, investigate why the joints failed and understand the existing brick, stone, and mortar before repointing.