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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.

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 extension 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.

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