Mortar types vs categories: What’s the difference?

By Michael Smith, RA, NCARB, RRC, IIBEC, CSI

Most people in the construction industry know the basic differences between mortar types. They likely have a rough idea of the compressive strength between M, S, N, O, and K. But did you know there are also categories of mortar?

Three major categories are:

• Masonry Cement,
• Cement-lime Mortar,
• and Mortar Cement

When mortar is applied, the category of mortar applied can have a huge impact on the longevity of the wall and bricks. Choose the wrong mortar, and you may inadvertently cause the bricks to crack. Choose the right mortar, and you can extend the life of the wall.

That is why it’s important to get both the type and category of mortar right.

What is mortar?

Mortar is the bonding agent that integrates unit masonry into a wall.

Think of it as not only the glue that holds bricks, stones, or blocks together but also as the element that holds them apart. Mortar is the workhorse and sometimes the sacrificial element. Selecting the right mortar will help the brick and stone last longer and stay stronger.

To serve this purpose, it must be durable enough to keep the wall together and built to withstand the forces of wind-driven rain, temperature extremes, and other weather conditions.

Evolution of Mortar

Early mortars were probably stone and mud. Since then, builders have used clays, gypsum, bitumen, volcanic ash, and ground pottery as mortar ingredients.

Around 550 BC, the Greeks and Romans added lime to their mortar to create lime-sand mortar. In the mid-1800s, builders started adding cement.  Although the ratio of lime-to-cement has evolved, these cement-lime mortars have remained basically the same.

In 1951, ASTM C 270 Standard Specification for Mortar for Unit Masonry was first published. It specifies proportions of material for cement-lime and masonry cement as well as defining properties and “types” based on compressive strength.

Masonry Cement vs. cement-lime Mortar vs. mortar cement

There are three categories of mortar used in most buildings today.

Masonry Cement

In most modern-day buildings, contractors will typically use masonry cement. It is usually bought in a bag, mixed with sand and water, and ready to be applied.

Masonry Cement ingredients:

Masonry cement products are mill formulated, proprietary mixtures. Although the exact formulas are not published, they typically contain one or more of the following:

• Portland cement or blended hydraulic cement
• A plasticizer material to improve workability – usually ground limestone, sometimes hydraulic lime (inert) clays, or shales
• Air-entrainment
• Sometimes water repellents
• Mineral oxide pigments
• Sometimes fillers

A number of brands produce and package masonry cement under source quality-controlled conditions to ensure volume stable mixtures. The pre-blended products reduce the concern for consistent field batching and comply with performance requirements established by ASTM C 91 and ASTM C 270. Specifically, masonry cement meets requirements for compressive strength, set time, water retention, and air content.

Masonry cement uses

Masonry Cement mortars are engineered products that are designed for modern engineered structures with brick veneer walls designed to be laterally stiff while allowing for plane-of-wall expansion and contraction.

For engineered brick veneer masonry walls, ledge angles (vertical support) and structural backup support the masonry.  It is anchored with closely spaced wall ties that provide stiff (maximum of L/600 deflection) lateral support. Sections of wall areas are panelized and separated by control joints to accommodate expansion and contraction.

Cement-Lime Mortar

By contrast, cement-lime mortars are often found in older buildings and are still used for historical preservation.

Cement-lime mortar ingredients:

Contractors create cement-lime mortar in field-produced batches, using proportions by volume.  Each component contributes to the overall recipe.

• Portland cement serves as the binder and contributes to the compressive strength and setting rate of the mortar.
• Hydrated lime helps wet mortar by improving workability and extending board life by increasing water retention. Lime also contributes to the flexural bond strength of the set mortar.
• Water serves as a vehicle for workability and provides the moisture needed for cement hydration.
• The aggregate, usually fine sand, serves as a filler and also reduces shrinkage.

The ratio of cement to lime determines the hardness vs softness scale of the mortar. In order to ensure consistency between batches, contractors must use batching control measures.

Cement-lime mortar uses

Cement-lime mortars were developed to work with mass masonry buildings and are found in most historical brick buildings.

Mass masonry buildings (also known as multiple wythe masonry) typically do not have built-in control or expansion joints, like many modern buildings with a brick veneer. Instead, the mortar between the brick accommodates stresses that occur in the masonry, with mortar as the key element.

Because the mortar is softer than the brick, it flexes to accommodate stresses. In particular, cement-lime mortar will develop minor (sometimes microscopic) cracking to accommodate and relieve wall stresses. Later, as the lime in the mortar absorbs rainwater or moisture, the softened lime will re-emulsify and reseal itself.

As buildings age, the sacrificial mortar will slowly erode to the point where repointing the mortar is needed, usually after 75-100 years. But while the mortar absorbs stresses and will eventually need to be repointed, the harder brick has been protected.

In addition to not having control joints, most mass masonry buildings also lack weep holes and flashings as part of the wall panel. Although water is absorbed in the outer portion of the mass masonry walls, it typically does not get beyond the outer wythe of brick. Eventually, the absorbed water simply evaporates out of the wall.  This contrasts with engineered masonry veneers which actively diverts absorbed water out of the wall via flashing and weeps.

Contractors should use cement-lime mix for repointing mass masonry buildings. Types O and N mortar are most commonly used, but Type K may be needed if the brick is very soft and compressive strength is not a determining factor.  The mortar must have a lower compressive strength (be softer) than the adjacent brick units.  Other factors include current project conditions, condition of the masonry, loading conditions (weight on mortar) and weather exposure.

Mortar Cement

Lastly, mortar cement was developed by the Masonry Industry Code Committee (MICC) and incorporated by the Uniform Building Code (UBC) in 1991.

In general practice, mortar cement is only applicable for certain regions of the United States where earthquakes or seismic events are a concern. It was developed to accommodate seismic activity.

Mortar Cement ingredients and uses

Mortar cement is typically a packaged product, like masonry cement, but mortar cement has less air content which increases bond strength.

In an earthquake, air in the mortar creates less contact between the mortar and the brick. As the building shakes, the bond between the brick and mortar could break, causing damage to the building.

In 1985, the International Conference of Building Officials (ICBO) adopted seismic provisions that prohibited the use of masonry cements in structural masonry in seismic zones 2, 3, and 4.  ASTM C 1329 Standard Specification for Mortar Cement currently specifies increased bond strength by reducing the allowable air-content in the Mortar Cement to 16 percent for Type N and 14 percent for Types S and M. By contrast, Masonry Cement allows 19 to 21 percent.

The minimum flexural bond strength values for mortar cement are:

• 70 psi for Type N
• 100 psi for Type S
• 115 psi for Type M 

Proper selection of mortar is crucial to the strength of your building

Choosing the proper mortar for your project is crucial to the integrity of the building.

Recently, a contractor working on a CTL project incorrectly applied masonry cement to a historical, mass masonry building. CTL directed the work to be corrected using the proper mortar.

Because masonry cement does not absorb water in the same way the original cement-lime mortar would have, it is more likely that the bricks will absorb the water instead. When the cold Ohio winter weather hits, bricks holding water are more likely to freeze and crack. The correct mortar could have prevented this.

Mortar must have the proper attributes to contribute and work in harmony with the overall masonry wall.  It is crucial that the correct mortar is specified based on the project.

To learn more about choosing the correct mortar, particularly in historical preservation, contact CTL Engineering today.

Michael Smith is an experienced senior architect, roof consultant and specifier with extensive experience in building envelope restoration, assessments, and forensics.  Michael has more than 30-years’ experience in the roofing-building envelope assessment/design/bid/ construction observation of large and small facilities.  Michael brings his experience in managing complex projects, particularly when multiple buildings are involved at the same time.