Performance and Productivity with Engineered Mortar Selection  

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A high-quality masonry exterior reflects how the properly specified engineered mortar supports mason productivity while ensuring lasting durability, strong adhesion, and a clean, consistent finish that elevates the design.

The ideal masonry mortar selection meets the architect’s vision while allowing the craftsman to maximize on-site productivity. The selection of masonry mortars is more complicated than simply picking a mortar type from a table in ASTM C270, the Standard Specification for Mortar for Masonry. The properties of the masonry unit and the mortar will influence the direct result of the wall’s appearance, the wall system’s hardened properties, and the selected mortar’s ability to optimize productivity for a mason. Success depends on a strategy in which the contractor, designer, and project engineer review the combination of masonry unit and mortar properties. This course explains the importance of selecting laboratory-tested property-based mortars with performance characteristics known prior to construction. A deep dive into mortars for brick, standard and architectural concrete masonry units (CMU), building stone, and adhered veneer cladding illustrates the significance of specifying factory-produced, pretested mortar materials.

Evolution of Masonry

Masonry units and mortar go together like bread and butter. For thousands of years, engineers, architects, and tradesmen have married these two components to form a system used to build everything from the ancient Egyptian pyramids to local schools. Although masonry is one of the oldest and most proven methods of construction, the past few decades have brought major technological innovations. These innovations have improved not only labor practices but also the integrity and aesthetic value of the building materials themselves.

Prior to the advent of factory-preblended mortars in the late 20th century, mason contractors required a specified formula for measuring the proper ratios of cement and aggregates to be mixed on a jobsite, based on ASTM C270 Table 1 Proportion Specification Requirement, written by architects and engineers. The Proportion Specification, commonly referred to as the “recipe method,” allowed an on-site laborer to measure each mortar ingredient based on a cubic foot measurement system. This approach does not allow for regional variance in raw materials and severely challenges the ability of specifiers and mason contractors to achieve prescription mortars that align with the performance and characteristics of the CMUs.

That all changed with the introduction of preblended masonry mortar, which replaces the guesswork of field mixing with pre-determined formulations based on clearly defined raw material weights, measured according to strict performance and quality control tolerances.

As will be discussed in more detail later in the course, producing mortars in a controlled environment, utilizing computerized batching equipment, allows for customized formulations that complement the unique properties of the masonry units specified and the controlled addition of performance-enhancing admixtures. Moreover, from the perspective of a project designer, specifier, or engineer responsible for selecting the best mortar for a project, preblended mortar offers endless customizable mix possibilities that match masonry unit properties while supporting the mason’s need to increase productivity.

The Masonry Process

During construction, a designer’s vision comes to life, and a mason makes money when masonry units are laid. To this end, and to meet or beat deadlines, ample time and money have been spent on improving processes to allow the mason to lay more units per day and meet deadlines. The following are examples of innovations driving units laid:

• Hydraulic scaffold systems
• Telescopic forklifts
• Silo mortar and grout delivery systems coupled with larger hydraulic mixers
• Unit lifting assist equipment, leading to larger masonry unit configurations

These improvements are 100 percent process-focused. However, little has been done to better understand the impact of cementitious mortars and the masonry unit on the craftsman’s productivity and, thus, ability to carry out the architect’s objectives.

Densities and absorption rates in masonry units substantially impact mortar set times, capillary bond interaction, and tooling times. Post construction, the final aesthetic of a beautiful masonry wall is affected by cleaning procedures that can contribute to unattractive color variation in mortar joints. What if there were ways to better understand and avoid appearance and productivity issues caused by these undesired interactions and less-than-optimal processes?

Consequences of Improper Mortar Specification

When the mortar type is adequately matched to the masonry unit, and the mortar is the perfect consistency to enhance the craftsmen’s productivity for smooth and trouble-free installation, the project achieves the contractor’s timeline and the architect’s goals.

However, if the mortar and masonry unit are not compatible, several problems can arise:

If the mortar is stronger than the masonry unit and there is movement, the masonry unit will crack first, which is the opposite of what should happen. Ideally, if there is movement causing the mortar joint to crack because it is weaker than the masonry unit, once settled, the mortar joint can be easily repaired, versus removing masonry units from the assembly.

If the masonry unit has a low water absorption rate and the mortar has a high water retention rate, the masonry unit will “swim” in it and prevent the mason from properly tooling it. That adds time to the mason’s process, and the finished product could look sloppy.

If the unit has a high water absorption rate and the mortar has a low water retention rate, the masonry mortar will set up too quickly, and the mason’s efficiency will slow down as each few units need to be tooled rapidly. When mortar is properly selected to match the properties of the masonry units, the mason can achieve consistent production rates and tooling times that match their pace of work.

If the mortar contains too much sand, it will not achieve the necessary compressive strength, threatening the project’s integrity.

To assist in mortar selection and improve aesthetics and productivity, it’s necessary to fully understand a few basics about the function of masonry mortars and their interaction with masonry unit types.

How Mortar is Specified by ASTM (Proportion vs. Property)

ASTM C270 specifies four types of mortars in two distinct ways. The four types of mortar are M, S, N, and O. M is the strongest, and in descending order, O is the weakest. Strength is derived from the volume of cement and other ingredients. Mortar is specified by proportion or properties. The first method is based purely on proportions, whereas the second is based on physical properties.

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Proportions specification requirements of ASTM C270.

Specification of mortar by proportion of ingredients: The proportion table gives a “recipe” for the mason to mix mortar in the field. For instance, according to the type of mortar desired, the recipe might include a cubic foot or bag of cement, a part or cubic foot of lime or some variation thereof, and so many cubic feet of sand. It gives the mason the formula for how to make an M, S, N, or O mortar.

While such a guide is helpful to a mason when field mixing mortar, it is not easy for the contractor, specifier, or inspector to determine the critical properties of the masonry mortar. That’s because cement properties and aggregate gradations change geographically and from supplier to supplier. On-site field testing typically only consists of compressive strength data and is not set up to provide other pertinent mortar data that could be useful in selecting mortars.

Specification of mortar by the ASTM C270 Property Specification Table: The second way mortar is specified is through properties, as illustrated in this property table. The property table is a prescriptive mortar specification, not a standard recipe. It lays out the performance criteria for each mortar type. It shows compressive strength, water retention, and air content. These are laboratory-tested properties in premixed mortar blends.

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ASTM C270 Property Specification Table.

The property specification gives minimum water retention guidelines. Water retention is a measure of the mortar’s ability to retain water and is allowable to as low as 75 percent. Water retention is a vital characteristic of the mortar, not only as a measure of board life and workability for the mason but also as an indicator of how the mortar will react to the absorption rate of the masonry unit itself. More on water retention will be explained later in the course.

The property specification also limits the total amount of air content allowed in a masonry mortar. Air produces a ball-bearing effect in the mortar that enhances workability for the mason and, within reason, can also add to freeze-thaw resistance. Increased air content is limited in the property table, as increased air content comes at the cost of reduced bond strength.

Finally, the specification sets the minimum and maximum sand contents to ensure the mortar itself is not so rich that shrinkage and excessive strength are a concern, nor so sandy that minimum strengths and cohesiveness of the mortar are not achievable.

Mortars certified to meet the property table must be pretested in a laboratory and certified to meet ASTM C270 property specifications before use in construction. As such, it is more likely that preblended mortars can be specified via the ASTM C270 Property Table.

The Importance of Mortar to the Masonry Unit Bond

Bond is one of the most important physical properties of hardened mortar. It has three facets: extent, strength, and durability.

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Mortar selection based on brick properties.

Extent

The extent of the bond is the ability of the mortar to migrate into the open capillaries of the masonry unit through absorption and suction upon application, thus holding the masonry units together. Typically, higher water retention mortars will allow for a deeper extent of bond. All masonry units have capillaries and absorption rates, though they vary by masonry unit type.

Strength

The strength of the mortar paste itself determines the strength of the bond. All things being equal, the stronger the compressive strength of the mortar, the stronger the tensile bond strength will be (assuming the mortar has a good extent of bond into the masonry unit).

Durability

The durability of the bond is the ability of the mortar unit interface to resist cracking and failure under the stress of movement. It is a relative measure of flexibility.

Selecting Mortars 

Selecting the most appropriate mortar is a balancing act requiring the applicator and specifier to apply knowledge of the structural requirements for the wall with production-related properties for the applicator. Before construction, the first step is reviewing technical data on the selected masonry units—compressive strengths, absorption rates, densities, and water-repellent characteristics. Then, one must verify these known properties during sample panel construction. Once verified, the applicator and designer can optimize the mortar selection to improve workmanship, production speed, and the masonry wall’s hardened properties. Following is a breakdown of mortar selection based on the properties of the masonry units.

Brick

Mortar selection for brick masonry construction starts with investigating the brick properties. A brick masonry unit’s most significant property is its initial absorption rate (IRA). IRA measures how quickly a brick absorbs water, measured in grams per minute. Typically, as IRA increases, brick strength and density decrease. Low IRA brick is generally hard and non-absorptive in nature.

Mortar selection for low IRA brick (which absorbs 0-10 grams of water per minute) that is higher in strength and lower in water absorption will benefit from selecting a Type M property mortar. Tooling times normalize, allowing the mortar to achieve “thumbprint hard” at the normal pace of work. Brick will not “swim” or “float” when laid. Set times are not extended, causing slower internal curing times than the surface of the joint, which can cause surface cement hydration issues and lead to joint discoloration. The mason will experience a mortar that more appropriately grabs and grips the brick upon application, allowing for more consistent placement. Additionally, mortar for low IRA brick units may benefit from including a water repellent to help combat premature weathering during the cleaning process, as the brick will shed the cleaning compound water mix. In contrast, the mortar tends to draw it in, an unwanted effect the repellent helps mitigate.

Mid-range IRA brick (10-30 grams per minute) is the most productivity-friendly for the craftsman. Middle-range IRA brick lends itself to the most standard masonry mortar selection for maximum productivity, such as Type S and Type N property mortar. That being said, the craftsman can improve productivity by selecting a Type S property mortar for brick in the lower mid-range IRA and moving to Type N property mortar when constructing with higher mid-range IRA brick in this range to normalize tooling times to the pace of the work.

High IRA brick (30-plus grams per minute) requires the use of Type N property mortar specifically as the mortar’s higher water retention properties are necessary to keep the mortar from drying out too quickly, interrupting suction into the masonry unit, and shortening the extent of the capillary bond. Additionally, utilizing a high-water retention mortar will help the mason during the tooling process as the mortar joints won’t set prematurely, interrupting the normal pace of work. When the mortar joints are tooled past “thumbprint hard,” discoloration of the mortar joint can occur as the hydration of the cement paste is not fully achieved.

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Mortar selection based on brick properties.

Tuckpointing

Tuckpointing mortar requires removing and replacing deteriorated joints to a nominal depth of 3⁄4 inch. Tuckpointing mortar should generally be accomplished with Type O property mortar and occasionally Type N when the masonry unit has a lower-range IRA. Mortar application is generally best accomplished in lifts, and the mortar (while higher in water retention) is mixed at a lower water content to help combat shrinkage. This ensures the joint is filled completely and compacted properly, creating adequate bond and weather resistance.

The water retention of tuckpoint mortar is even more critical. When tuckpointing, water from the mortar is absorbed not only into the masonry unit above and below but also into the mortar joint behind the repair. This increases the chances of the mortar losing water to absorption before full hydration can take place. If water is lost too quickly in tuckpointing, it can lead to weaker bonds, shrinkage cracking, and surface discoloration.

Standard CMU

Mortar selection for standard CMUs is straightforward. Match the mortar strength to the closest mortar type just below the masonry unit’s strength. A standard CMU is specified at 2,000 PSI. The most appropriate mortar for the application is typically Type S property. Consideration should be made for high-strength units and seismic zones, as well as if the concrete masonry is below grade. In these instances, Type M property mortar is more appropriate.

Architectural CMU

The term architectural CMU refers to split-face, burnished, or honed CMUs. These units are typically water-repellent and often have specific aggregates that are harder in nature than standard CMUs to achieve the desired surface appearance. Architectural CMUs are generally higher in strength/density than standard CMUs and often have surface-applied sealants or sacrificial coatings to reduce the chances of surface deterioration during the cleaning process. As such, the mason can experience the unit swimming or floating in the mortar joint when a higher water retention mortar is selected to construct the masonry wall.

Additionally, if a high-water retention mortar is used in combination with architectural CMUs, the mortar will stay wet for an extended period in the joint, potentially leading to a condition called “surface mottling.” This condition indicates a disruption in the cement hydration at the face of the unit, resulting in discoloration. High water retention mortars make it harder for the mason to keep an architectural CMU level when constructing and extending tooling times.

When cleaning an architectural CMU masonry wall (which should occur within the first three to 14 days), the mortar joints are more susceptible to the cleaning process. The architectural CMU sheds the cleaning solution while the mortar joint tends to 

absorb it. This absorption can lead to mortar joint deterioration of the cementitious paste, exposing the aggregates.

The most appropriate mortar type for this application is Type M property mortar. When selecting a mortar that more appropriately matches the properties of an architectural CMU, the mason’s productivity will increase as the mortar will grip and hold the block up better allowing for more uniform placement and will also create uniform tooling times at the mason’s normal pace of construction.  Mortar joint durability can further benefit from including a water repellent so the properties of the mortar more closely match the CMU, protecting it from degradation during the cleaning process.

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Mortar selection for CMUs based on CMU properties.

Building Stone

Special attention to the stone properties can aid in laying more square feet per day when constructing with full-depth building stone. Stone masonry units can be broadly classified into two categories: hard/dense stone and soft/porous stone masonry units. Stone masonry units also tend to be heavier than standard CMUs and brick. As such, mortars for full-depth building stone masonry construction tend to require lower sand and water contents to support the unit’s weight when laid, increasing the speed of construction.

Hard/dense stone has a low absorption rate and high compressive strength. Therefore, this type of stone requires a lower-water-retention and lower-sand-content Type M property mortar. Conversely, soft/porous stone has a higher absorption rate and lower compressive strength (though still higher than most CMUs). Soft/porous stone benefits from a higher water retention mortar that still exhibits good compressive strength and has enough body to support the weight of the stone. Soft/Porous stone typically requires a low sand content Type S property mortar to optimize production.

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Mortar selection for full-depth building stone based on stone properties.

Adhered Veneer

Adhered veneer systems rely on high-performance mortars to ensure long-term stability, aesthetic appeal, and ease of installation. Two widely used types—Standard Adhered Veneer Mortar (AVM) and Polymer Modified Adhered Veneer Mortar (PMAVM)—are engineered for specific performance characteristics and substrate compatibility.

Standard Adhered Veneer Mortar is a dry, pre-blended mortar designed specifically for bonding lightweight manufactured thin veneers such as stone, brick, or tile over wood-framed or steel-stud wall assemblies. AVM combines cementitious binders, graded masonry sand, and performance-enhancing admixtures to deliver a versatile and durable mortar system. It is intended for use as a scratch coat, bond coat, and grout joint material.

When properly mixed and applied, AVM offers consistent workability, high water retention, anti-sag performance, and reliable bond strength. Applied in layers typically around ½ inch thick, AVM creates a non-load-bearing yet durable finish suitable for both interior and exterior walls in residential and light commercial construction. Color options—both standard and custom—are available to match or enhance the appearance of veneer joints. AVM mortars are formulated to meet key performance standards, including ASTM C270, ASTM C1714, and CSA A179.

Polymer Modified Adhered Veneer Mortar represents an advancement in mortar technology. By incorporating polymers into the mix, PMAVM significantly enhances bond strength, flexibility, water resistance, and durability. This makes it ideal for installing heavier or larger veneer materials such as natural thin-cut stone or full-sized thin brick over cementitious substrates like concrete masonry units (CMU), concrete, or cured scratch coats.

PMAVM also provides anti-slip properties that are critical for setting larger veneer units, allowing installers to place and adjust materials more easily without immediate slippage. As a thick-bed mortar, PMAVM can also be used in all stages of veneer installation—scratch, bond, and joint. It outperforms standard mortars in flexural bond strength and adhesion and complies with ASTM C270, ASTM C1714, CSA A179, and ANSI A118.4 when pre-blended.

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Performance mortar selection for adhered veneer systems.

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Using Water Repellents to Achieve Masonry Assemblage Durability with Engineered Mortars

Water repellents have been successfully used in the manufacture of CMUs and mortars for years. Their inclusion helps otherwise porous masonry materials shed water, resulting in water-repellent masonry wall systems.

The function of water repellent in a wet mortar and how it interacts with a masonry unit is often misunderstood. Adding a water repellent to a masonry mortar simply makes the mortar system less susceptible to water penetration. The inclusion or lack of a water repellent in a mortar does not change how the mortar bonds to the masonry unit itself. Bond is still a function of water absorption, suction, and compression when laying the unit. Matching the chemistry of the water repellent by the brand to the brand of water repellent in the masonry unit is unnecessary. Understanding the properties of a water-repellent mortar and the effect of the mortar type utilized in construction on the repellency characteristics of the hardened masonry wall is essential.

Water repellents are required to reduce the permeability of a masonry mortar by 50 percent when compared to the same mortar without the additive. Including a water repellent in the mortar does not “waterproof a wall,” but it makes the mortar less pervious to water and less susceptible to efflorescence.

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The impacts of water repellents.

Mortar Durability

Finally, let’s focus on mortar durability, which is its ability to resist weathering. Generally, mortars with higher compressive strengths are more durable regarding weathering. However, increasing strength alone in masonry mortar can come at a cost. Higher-strength mortars tend to have a lower extent of the bond into masonry units. This can lead to shrinkage cracking, specifically if the mortar dries out too quickly when placed in contact with a masonry unit that may have high absorption characteristics.

Selecting an appropriately durable mortar is important to help protect the mortar joint in the cleaning process. If the masonry unit has a low IRA and is dense, a high compressive strength mortar, including one with a water repellent, is more durable. In these cases, the masonry units act more like panes of glass, and the mortar acts like a sponge when a cleaning solution is applied to the wall. The mortar joint will pull the cleaning solution deep into the joint, deteriorating the cementitious paste, exposing the aggregate, and prematurely weathering the joints. Adding a water repellent to a mortar when laying low absorption masonry units can help protect the mortar during the cleaning process, as the mortar will act more like the masonry units in terms of water absorption.

Specialty Premixed Mortars

Premixed mortars are the most successful way to end up with the precise masonry mortar needed/required for each project. For best results, consult the manufacturer’s representative for recommendations. Here are some advanced premixed mortar options.

Set Accelerated Masonry Mortar: While there’s no true “anti-freeze” for mortar, set accelerators enable masons to work in cold weather by speeding hydration before freezing can occur. This dry, preblended mortar mix is formulated with Portland cement and hydrated lime, masonry cement, or mortar cement, combined with dried masonry sand and a non-chloride set accelerator. It’s produced using computerized batching to ensure consistency and is tested for bond strength, water retention, and board life.

It is compatible with most masonry units. Set-accelerated mortar is available in custom or standard formulations for use above or below grade. It meets ASTM C 270, ASTM C 1714, and CSA A179 standards. Packaged in 80 lb. bags or 3,000 lb. bulk bags for silo systems.

Key Features:

• Accelerates set time in cold weather
• Blended with non-chloride accelerator and masonry sand
• Lab-tested for strength and workability
• Compatible with specified masonry units
• Produced under strict quality controls

Set-Delayed Masonry Mortar: Designed for hot weather construction, set-delayed mortar slows cement hydration to prevent premature drying. This dry, preblended mix includes Portland cement and hydrated lime, masonry cement, or mortar cement, along with dried sand and proprietary set-delaying admixtures. It’s engineered for optimal board life, bond strength, and water retention. It is compatible with CMU, brick, and stone, it meets ASTM C 270, C 1714, C 1384, and CSA A179 requirements.

Key Features:

• Slows set time in hot weather
• Maintains board life and bond
• Meets ASTM and CSA standards
• Compatible with various masonry units
• Strict quality control

Colored Mortar: Colored mortar is produced using precise color canisters containing synthetic iron oxide pigments. Following a one-can-to-one-bag mixing method with standard 80 lb. bags of gray mortar ensures consistent color across batches. For best results, use a matching color system and mortar from the same manufacturer.

Tuckpoint Mortar: Tuckpoint mortar is a preblended mix with Portland cement, hydrated lime, and sand, formulated for strong bond, flexibility, and reduced cracking. The high lime content helps accommodate wall movement and minimizes spalling. It meets ASTM C 270, C 1714, and CSA A179 standards and is available in custom or standard colors.

Integral Water-Repellent (IWR) Mortar: IWR mortar includes a dry, polymeric admixture that enhances water resistance and minimizes efflorescence. It is preblended with cement, lime, or masonry cement, and dried sand. Compared to field-added liquid admixtures, IWR mortar shows a 46% bond strength increase (ASTM C 1357).

It is available in Types M, S, and N, meets ASTM C 270 and CSA A179, and comes in standard or custom colors. IWR mortar improves worksite efficiency and ensures consistent dosage, enhancing wall durability and moisture control.

Potential Leed Credits for Preblended Mortar Products

Climate is a critical challenge of our time. For some mortar distributors, environmental stewardship and sustainability are the keystones to developing and implementing cement-based building materials and mixing systems for the construction industry. Specifiers, architects, and contractors should seek out companies whose ongoing goal is to support enhanced environmental and biodiversity management, strengthen regional economies, and support local communities in reducing fuel consumption and air pollution caused by transporting building materials long distances.

The attributes mentioned above may help projects earn credits in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating program:

MR 2.1 - Construction Waste Management: For building projects today, contractors are asked to operate sites with minimal environmental impact in addition to careful product selection. Material delivery systems, commonly referred to as silos, help contractors reduce overall job site pollution and waste by controlling the product production based on material needed at any given time during a project’s production schedule. This system minimizes the need to dispose of unused product or discard empty bag waste that ultimately impacts the environment.

Specifically, silo systems are compact and portable allowing for minimal site disturbance during construction. The bulk bag system and nature of using preblended materials eliminate the need for sand and other aggregate piles that could leach into the groundwater, discharge into stormwater systems, or blow into the atmosphere. This contributes to the health and safety of jobsite workers and the community.

For every recyclable bulk bag of premixed construction product distributed to a project site, it is estimated that as many as 14 paper bags will never be produced or disposed of in landfills. In a given year, this saves approximately 10 million pounds of paper waste from ever hitting a waste disposal site.

MR 4.1 – Recycled Content: Mortar, masonry grout, stucco, and shotcrete products are all manufactured to meet specifications promoting the use of pre-consumer recycled content such as fly ash and/or supplementary cementitious materials. Percentages of recycled content vary for each product, and the properties are well measured by weight to meet rigorous standards of ISO14021 (International Standardization Organization). The manufacturer or distributor will provide information regarding mixes that meet the prerequisites for acquiring LEED credits.

MR 5.1 – Regional Resources: Mortar companies should strive to source raw materials cost-effectively while minimizing the carbon footprint of trucking these goods. Look for North American production facilities that are strategically located close to customer bases to extract, process, and package regionally, if not locally. The ongoing goal should be to support enhanced environmental and biodiversity management. For LEED certification, manufacturing plants and extraction points should be located within a 500-mile radius of jobsites required for this Materials & Resources prerequisite.

Conclusion

As we have learned, mortar is more than a mere detail in a masonry wall. It is quite literally what holds the entire assembly together. A material of such significance to the project’s success deserves close attention. In the past, in a simpler world with fewer masonry unit choices, the process of splitting open bags of Portland cement and lime and mixing with shovelfuls of sand and other aggregates may have been sufficient. But today’s complex, contemporary buildings and building methods benefit from a more scientific specification of mortar materials in factory-tested blends. The critical factor is specifying a mortar blend compatible with the masonry unit that it will be bonding into place. The correct blend keeps the mason placing masonry units at a smooth and continuous pace, leading to the architect’s vision coming to fruition. When all factors are considered (including compressive strength, the extent of the bond, water absorption rates, and workability by the mason), taking a scientific approach to mortar specification according to ASTM C270 is more likely to result in a successful project.

Kathy Price-Robinson writes about building and design. Her remodeling series "Pardon Our Dust" ran for 12 years in the Los Angeles Times. She specializes in writing about buildings that are durable and resilient to climate disruptions, as well as products and designs that provide shade in hot climates. www.kathyprice.com