Colored mortar can make a big impact on the aesthetics of a project, but many factors can affect how easily this is achieved during construction.  A recent example is  a project that is currently under construction in Minneapolis.  The project is to assist Chicago artist Theaster Gates in the creation of an art piece for the expansion of the sculpture garden at the Walker Arts Center.  The intent was to provide a monolithic black enclosure for a statue at the interior, which the artist salvaged from a demolished church on the South Side of Chicago.

The artist commissioned a brick manufacturer to create custom black radial bricks for the enclosure and we chose a dark black mortar to complement the brick color.  The contractor constructed a mock-up of a section of the wall.  We noted that the mortar was lighter than anticipated and the joints had not been raked (below, left).  In addition, on the portion of the mock-up that was cleaned the mortar appeared even lighter (below, right).

But it was only lighter on the faces where the brick had been cleaned.

The contractor offered several solutions to achieve a closer match to the mortar color we’d selected: 1) Try to obtain darker pigment, sand, or cement for the mortar mix or 2) Use a stain on the mortar to darken it after it has been cleaned.

In order to determine how best to address the issue, I looked to the contractor to describe the process used in constructing the mock-up.  The mortar used was an alternate manufacturer from what was specified, but was Type N and pre-mixed as specified.  Both mock-ups had been created during the winter, but were built in a heated work space.   The mock-ups were washed 3 days after they had been built, and had been cleaned with a standard brick cleaner diluted at a ratio of 4:1.

I then consulted a mortar company to review our mortar choice and get their opinion of what solutions were available.  In reviewing options for darker aggregates and cement, they indicated that dark sand or aggregate is very hard to source, and using typical type N mortar the darkest option for the cement color.  This eliminated a portion of Option 1.

The mortar color we had chosen had a majority artificial pigments, but did contain some amount of carbon black pigment, which is how the manufacturer was able to obtain such a dark color.  Carbon black will fade over time due to UV exposure, but shouldn’t have faded this significantly in such a short time-frame.  The contractor suggested staining the discolored mortar to achieve the color we wanted, but because the stain will also fade over time, I continued to pursue other solutions.  In order to minimize color loss, the manufacturer recommended we switch to a mortar with 100% artificial pigments, eliminating the carbon black pigment.  The change would give us a slightly lighter color, but would maintain it’s color more consistently.  This seemed like a possibility, but I was still curious why the mortar color had faded so quickly.

Next we reviewed the sequence of construction.  Mortar typically requires 7 to 28 days to cure, depending on the weather.  Our mock-up was built in Minneapolis in the winter, originally in a heated shop, but shortly moved outdoors.  Standard curing time for these conditions should have been 14 days, but as the contractor noted, the mock-up was cleaned on day 3.  In addition, the manufacturer indicated that a dilution ratio for the brick cleaner of 10:1, rather than the 4:1 used on the mock-up, would be more appropriate for this kind of application.  The combination of these two factors looked to be the culprit of the discoloration.  Due to applying too strong of a cleaner on uncured mortar, the cement paste, which is what holds the pigment for the mortar, was completely washed away.

Based on this review, our final recommendation to the contractor was to obtain a mortar mix with 100% artificial pigment, to wait 14 days for the mortar to cure, and to wash the wall with a cleaning solution diluted to a ratio of 10:1.

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