Surface Mottling of Mortar Joints

Words: Dan KamysJune 2014

Mortars, Mixtures and Staining

Understanding the science of why surface mottling of mortar joints happens and solutions to avoid or mitigate the occurrence

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Surface Mottling of Mortar Joints Surface Mottling of Mortar Joints

While masonry mortar makes up only a small percentage of the total surface area of a masonry wall, its finished appearance in a structure complements or contrasts masonry units. When the color of the mortar meets expectations, the resulting assemblage completes the designer’s vision. When the mortar color does not meet expectations, the designer’s vision is compromised.

Leading edge discoloration of a mortar patch – over tooling??????
Extended curing times due to grouted cells??

One undesirable color variation in masonry mortar joints is surface “mottling.”?? Similar to mottling in concrete surfaces, mottling on mortar joints is best described as generally darkened or discolored patches or areas that are adjacent to, or within lighter colored mortar on the surface of mortar joints. Mottling generally appears during the construction phase of the project and may or may not be associated with inconsistent mortar composition. Such variation can be so random in a head or bed joint that the mortar would have to be striped or swirled on the mason’s mortar board or trowel during installation – a consistency issue easily identified by the training of an experienced mason.

Surface mottling of masonry mortar occurs when on-site conditions and construction practices cause the extention or premature termination of the cement hydration. Densifying of the mortar at the surface of the mortar joint during finishing practices also can cause this surface condition. Hydration of portland cement cannot proceed if water is unavailable, making the mortar joint darker than desired. Conversely, if additional water is available during the curing process the cement can over-hydrate, yielding a lighter-than-expected result. Generally, the higher the water content in the mortar, the lighter the mortar. Mottling can manifest itself across the entire surface of the mortar joint or even in small, irregular patches.

This condition of mortar joints is more common when using low initial rate of absorption and/or water repellent masonry units. In general, these units tend to repel water, which forces the water that would normally be absorbed into the masonry unit to stay in or exit through the mortar joint. This can lead to higher water contents in the mortar. To help mitigate these issues, the mortar specified for these types of masonry units should match the density and absorption properties of the masonry units as closely as possible. Mortars used for this application should, generally, have a low water retention and contain a water repellent admixture to properly match the masonry unit.

If the masonry unit is a polished block or glazed brick, great care should be taken in the removal of any fresh?? mortar smears as soon as possible. If the mortar smears are removed by rubbing the units with a rag or towel directly after tooling, mortar material and surface water adhered to the rag can be transferred to the mortar surface. That can contribute to surface mottling. For densified or glazed masonry units, the use of an acrylic finishing tool is generally more successful than using metal finishing tools.

Product Watch
Cresco Concrete Products’ Premocoat Bonder is used as an adhesion promoter for cement based coatings and an admixture to enhance UV and moisture resistance. Premocoat Sealer is a clear, acrylic sealer used to enhance the moisture resistance of finished cementitious surfaces or for making colored surfaces more vibrant and glossy.

Cresco Concrete Products manufacturers Liteblok dry stacked, interlocking green building block; Litecast and Premocrete precast concrete fencing; lightweight air conditioner pads; lightweight pier and wall caps; and fastening systems. Cresco also offers cast stone products including accent blocks, address blocks, balustrades, bollards, columns, crown molding, entry ways, fireplaces, keystones, mantels, planters, pool coping, signs, window surrounds and windowsills.

For more information, visit

Another contributing factor to this condition is how often the mortar joints are addressed during the tooling of the wall. The mortar joints should be tooled when thumbprint hard. If the mortar joints are tooled when they are beyond thumbprint hard, more effort is required to achieve the desired finish, which densifies the mortar at the surface leading to dark discoloration. This dark discoloration can also be noted in mortar overlays that tend to dry out quickly, due to their shallow depth.

The addition of core fill grout in a concrete masonry unit wall can lead to color variance in the mortar joints by increasing the free water available to the mortar during the curing process. Initially, when grout is poured into masonry walls, the mortar and block appear to be wet. The moisture absorption from the grout into the block and mortar helps promote a good strong bond between the block, grout and mortar.

However, the additional water provided by the grout can result in inhomogeneous paste where some areas hydrate more due to the presence of excess water, while other areas dry out. Excess moisture can migrate through the mortar and pick up soluble salts, such as calcium hydroxide, depositing them on the surface of the mortar upon drying. Exposure of calcium hydroxide to CO2 in the atmosphere rapidly converts the calcium hydroxide to calcium carbonate, which leads to light discoloration.

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Removal of mottling by hand abrasion Mechanical removal of mottling
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Surface mottling due to sealing practices Closeup of sealer not soaking into the mortar joints

When the appearance of surface mottling occurs, an effective means of removal can be through abrasion. The manual use of a “rubbing stone” or a stone drill bit, followed with washing the joints lightly with a mild cleaning agent has proved successful in removing surface mottling.

Product Watch??
Grace has introduced a portfolio of value-added concrete admixtures, fiber reinforcement and products for architectural concrete designed to improve the quality, strength, durability and appearance of concrete. These products are backed by Grace’s technical customer service and support.

STRUX Synthetic Macro Fiber Reinforcement is a form of high-strength, high-modulus synthetic macro fiber reinforcement that is distributed throughout the concrete matrix, thus imparting enhanced toughness, impact and fatigue resistance to concrete. The STRUX line includes STRUX 90/40, which is specifically engineered to provide high, post-crack control performance. It also includes STRUX BT50, a patented, engineered design providing superior post-crack control performance with a broad range of applications.

Top-Cast is a water-based, top-surface retarder for poured-in-place flatwork and the top surface of precast panels. In addition, Grace offers Hydrotint liquid pigments for colored concrete, which enables concrete producers to integrally color concrete quickly, safely and reliably through the Chameleon PC-based dispensing system. Reliable dosing of liquid pigment into the concrete mix minimizes color variability in finished products, satisfying owners, specifiers, contractors and producers. (Hydrotint and Chameleon are trademarks of Davis Colors.)

Grace’s products for Self-Consolidating Concrete include its ADVA and ADVA Cast superplasticizers; and V-MAR 3, a high-efficiency liquid admixture designed to enable production of self-consolidating concrete by modifying the rheology of concrete. The company’s technological solutions for concrete also include Verifi, which provides service and technology to ensure consistent quality concrete.

For more information, visit

Chemical cleaning agents can also be useful to remove this condition. When using chemical cleaning agents it is important to choose the right chemical agent. Use the least-aggressive cleaning solution and always follow the cleaning agent manufacturer’s recommendations for use. If the masonry unit is a burnished block, great care needs to be taken as the masonry unit itself is polished and smooth. When a cleaning solution is applied to the wall, the mortar joints take the brunt of the cleaning solution as the masonry unit does not absorb any of the cleaning solution.

It should be noted that many manufacturers of burnished masonry units warn against the use of muriatic acid (HCl) when cleaning the wall system. If the mortar joints are washed too aggressively, the cementitious paste will become etched, exposing the aggregate. To the eye, the mortar joint will take on the color of the aggregate in the mix only, likely changing the the desired color of the joint.

When applying a surface sealer to the mortar joints, as is common for burnished block applications, it is extremely important that the mortar has sufficiently cured and that the wall is dry. It is also important that the wall is above 40??F and below 90??F when applying the surface treatment. If the mortar is saturated during installation, the bonding and penetration of the sealing agent can be inhibited. This can lead to moisture penetration or flaking of membrane-forming sealers upon further cleaning or freezing and thawing. This action can lead to mottling.

In today’s world of fast-paced construction schedules, and with the strict demands put upon mason contractors, it is critically important to understand the basic principles behind the causes of the mottling of mortar joints. Mottling can be eliminated or minimized in most instances, whereas an aesthetically pleasing finished masonry assemblage is always the result.

Take care to ensure that placing core fill grout doesn’t happen too early, before mortar joints have had time to achieve proper cement hydration so as not to over-extend the hydration cycle. Work with the designer on matching the mortar type with the masonry unit. Follow proper and consistent cleaning practices, and use a reputable branded product.

Remember, aggressive cleaning can burn the cement paste off the surface of the mortar joint and change the color. Make certain that consistent tooling procedures and times are put into place. On your next project, take a step back and visualize what you can do to make sure the appearance of the mortar joints will have a true and lasting impact on your project.

Nick Blohowiak is eastern regional sales manager for SPEC MIX Inc. He can be reached at

Case Study

Custom Mortar Color at the Omni Nashville Hotel

Shown is the Omni during construction
?? Brick in the kitchen of the Omni
The latest addition to the Music City’s bustling epicenter, the Omni Nashville Hotel serves as the premier downtown accommodation for everyone from visiting tourists and business travelers to celebrities and local residents. Mason contractor Wasco Inc. incorporated natural brick and stone from the region using QUIKRETE Mortar to give the Omni Nashville Hotel a modern look that still reflects the city’s distinct and authentic character. Located in SoBro (the South of Broadway district) across from the 2.1-million-square-foot Music City Center and attached to the Country Music Hall of Fame and Museum, the Omni Nashville Hotel has 800 rooms. Nearly 80 3,000-pound bulk bags of standard gray QUIKRETE Type S Mortar were used to lay the foundation blocks and 60 3,000-pound bulk bags of QUIKRETE Type N Mortar in a custom dark buff color helped accent interior and exterior stone and brick work. The QUIKRETE Mortars specified on the project are contractor-grade factory blended materials containing the properly graded and proportioned masonry sand, masonry cement and other approved admixtures. In addition to meeting the requirements of ASTM C270 and C1714, the delivery of QUIKRETE Mortars through a bulk mortar silo system improved productivity and reduced labor costs on the project. For more information, visit
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