The Science of Custom Mortar Mixes

Some masonry jobs require specific mixes for the project at hand. For instance, many restoration projects are managed by local historical societies that require the mortar to be mixed using the same formula as the original construction, matching the specifications exactly.

In other cases, the project architect or engineer, for reasons of performance or bond, may specify mortar additives, such as water repellants, pigments, lime replacements, latex modifiers, accelerators (cold weather) or retarders. The specified additive or pigment can sometimes prove too difficult or impractical to add in the field because of its small dose in relation to the batch size.

When field mixing using a conventional mixer, pigments can sometimes prove to be a challenge for other reasons as well. Aside from measuring the pigment accurately, moisture content in the sand pile varies from dry to wet conditions. Typically, the laborer in charge of mixing will add the same number of shovels full of sand to a batch and adjust as "instructed" by the foreperson or masons on the wall. Inherently, there is usually less moisture at the top of the sand pile than at the bottom. Many color pigments are temperamental to the varying sand dosages - deep reds and darker colors generally require more care in the sand/cement/water ratio to ensure consistent color.

For such instances, creating a custom mortar mix can often be a practical solution.

When completed as a custom blend, finished mixes can be supplied in any size, but are typically packaged in small (80 lbs. or less) and bulk (3,000 lbs.) packaging. When using the bulk package, as supplied for commercial and large residential contractors, a silo system can be used, dispensing dry product into a standard mortar or concrete mixer.

Case Study: Matching Mortar
Tony Fox, president of Masonry Specialists of Union Grove, Wis., was awarded a job on the Milwaukee County Botanical Gardens in Greendale, Wis., which required a latex additive to increase bond and eliminate the penetration of water. The project was the New Visitors Center and Café for Milwaukee County. The face of the building consisted of large fieldstones that were mostly encased in mortar, with only about a third of each fieldstone exposed.

In addition to a latex additive, the mortar had to match existing structures on the grounds that were built in the early 1900s. To obtain the perfect match, many parts of the existing structures had to be appraised and analyzed in a lab. It was ascertained that, to obtain a match, the mortar also needed a pigment. Furthermore, matching the texture of the original mortar increased the complexity of the mixture. Ultimately, the job required a combination of 3/8-inch pea stone, course and fine sand, pigment and the latex additive within the mortar.

"That would have been pretty hard to do in the field and get every batch right," Fox said. "We also had to strike the joints with a bucket trowel to match the original building."

"One man was able to do 80 to 120 square feet a day with this fieldstone - I can't imagine having to measure all the different materials that ended up in that mortar," he added.

Case Study: The Right Amounts

Quick Set Panels had their own concoction of mortar used in the production of pre-assembled Pittsburgh Corning glass block panels made for installations in residential and commercial work. The mortar, designed by Bundy, was a blend that had a rapid initial set that allowed for a more efficient process in the plant when assembling glass block panels.

"After obtaining field measurements, or measurements from plans, we pre-fab the [glass bock] panels in our shop. Our process, set up in an assembly line format, cuts down on field time in the installation process," Bundy said.

Quick Set Panel employees typically measured the components by volume; however, one employee's idea of a half pail of this and a quarter pail of that varied slightly.

"The problem with mixing the way we were is that you didn't really know if the mix was bad until you started using the material on the panels," Bundy said. "If something wasn't right, we had to disassemble [the panels], clean them, and mix another batch.

"Also, some of the components were not readily available locally and had to be obtained through distribution," he added. "To get a good price on a few of these components, it was necessary to buy in pallet quantities. Certain products last quite a long time."

One of the components, specialized Fondue Cement, used as an accelerant, was not readily available. "To get this particular grade of Fondue, it was necessary for us to arrange for a pickup in Indiana. I also had to buy in bulk quantities and the product had a shelf life. Also, buying some of these raw materials in pallet quantities was not something I liked to do because it was an inventory headache."

The next time a job requires mortar additives, consult with a vendor to see if a custom-mixed mortar mix might be right for you.