Stone Veneer Mortar: More Than Meets The Eye

Words: Garen Graves

One of the greatest pleasures of being in the masonry industry is that we are the privileged few who never have to give up our Legos. The only downside is that our bigger and heavier Legos do not just snap together. It’s a bit more complicated than that for us. The skill of bonding one masonry unit to another is the heart of our craft. The mortar we use to make that bond has become more interesting, more technically complex, and more specialized—especially in veneer applications.

Given the recent changes to TMS Codes 402 and 602, where polymer-modified mortars are now mandatory under the prescribed method, understanding the nuances of these specialized products is more important than ever. The upside is that these higher-performance products now give us more flexibility for thin veneer applications, including increased unit weight (from 15 lb/sq.ft. to 30 lb/sq.ft.) and higher application ratings up to 60 feet.

Though those numbers don’t seem like major changes, they’re extremely exciting to masons—if a little daunting. To take advantage of these new opportunities, pros will need to get a handle on today’s advanced stone veneer mortars. Let’s dig in and find out what they are all about.

What Does “Polymer-Modified” Actually Mean?

Put simply, a polymer-modified stone veneer mortar has an adhesive added to it, like a glue, which keeps it adhered to the units more strongly than cement alone. Although polymer additives do give the mortar some interesting workability enhancements, such as less water demand and more fluidity compared to traditional mortar, the long-term adhesion benefits are the biggest point.

Understanding the “why” behind those benefits requires a bit of chemistry, so get out your Bunsen burners and safety goggles.

Polymer molecules are HUGE, as far as molecules are concerned, and are made up of smaller bits called monomers. If you were to shrink yourself down small enough, you would see these long gangly chain molecules in the mortar tangled up like a bowl of tiny noodles. When your wet polymer-modified mortar contacts a masonry unit or the substrate backup, those polymer chains flow into the pores of whatever you are laying, and bring some additional cement along for the ride as well.

Here, two things are happening:
  1. When in contact with hardened cement (like a unit or concrete panel), those polymer “noodles” clog up the pores in the crystalline structure of the cured cement and prevent water from being sucked out of the mortar or evaporating. That’s especially important in thin veneer applications, where the surface area of the mortar is higher and allows a higher rate of evaporation. Because those polymer chains hold on to water longer, you get a better cure and ultimately a better bond.

  2. When in contact with something that is not cement, the polymers’ “noodliness” works a bit like a ship’s anchor, holding fast to the microscopic imperfections in the substrate. The long polymers also bond with each other across the entire mortar bed, binding the unit to the substrate. The “noodlier” the polymer, the better the anchoring characteristics. The fancy term for this is “ionic bonding,” but noodles are obviously a more fun visual.
That said, with great power comes great responsibility. Polymer-modified mortar does require a bit more diligence in its use than standard masonry mortars.

These are a few notes to keep in mind on your next thin veneer project with polymer-modified mortar:

Surface Prep
Before applying a polymer-modified mortar, the substrate and the units themselves must be prepared according to ANSI A108.01/A118.4 standards. This includes ensuring that the substrate is clean, structurally sound, and free of any contaminants that could affect adhesion. I have been called out on many projects where units are popping off the wall, only to find out that a form release or other oil had been applied to the substrate, preventing a proper bond.

If you are adhering directly to a block wall or concrete, you do not have to use metal lath. However, if you are adhering thin stone on an OSB or a similar substrate, you will need to ensure you follow standards for vapor barrier, lath, and a scratch coat for the units to bond to.

Slake It Up, Baby
Depending on the material and an individual manufacturer’s recommendations, you may need to “slake” the mortar for 5-15 minutes after mixing. Allowing the mortar to slake (aka rest) after the initial mixing allows the various additives to react and activate. Often, immediately after slaking, the mortar may look stiff, dry, and even unworkable. Don’t let this fool you! Simply mix the mortar again WITHOUT adding more water, and you will see the material loosen up back to a creamier, workable texture. Proper slaking is vital to polymer-modified mortar performing as designed.

Apply Pressure
Understanding the key to ensuring proper adhesion is remembering how the polymers work: they hold the water. Water is key! This is why when applying a veneer, you first wet the stone before applying a ½” layer of the veneer mortar. Before pressing the unit into place, you may even give the mortar an extra spritz of water to ensure that the water is not pulled from the stone too quickly and also to help the polymers and cement bond tightly to the substrate.

Press the stone to the wall and give it a wiggle to be sure that as much of the polymer mortar is in contact with the substrate as possible. Now you’re all set.

Curing and Cleaning
To get that tight bond, it is important to damp cure the wall initially, especially in hot weather conditions. Make sure the mortar has time to fully hydrate and make the most use of the polymers. This typically involves keeping the mortar damp for a specified period to ensure proper hydration and strength development. As always, curing times will vary depending on the specific product used and environmental conditions.

After all that work to make sure you have a strong bond, when it comes time to clean the wall, it may take a bit more effort. The main thing is this: as much as is humanly possible, excess mortar should be cleaned from the surface of the stones while it is still fresh. You can use a damp sponge or brush, but take care not to disturb the joints or scratch the coat during cleaning. Any mortar that has hardened on the surface of the stones can be removed using a suitable cleaning agent and a stiff brush, but as always when cleaning masonry, use the gentlest method you can to get the job done.

The evolution of mortar in masonry, particularly in veneer applications, has been remarkable. Polymer-modified mortars, with their adhesive properties, offer a new level of performance and durability for those projects where our word is our literal bond. By being diligent in the details, we can ensure that these advanced mortars deliver on their promise of superior adhesion and longevity, enhancing the integrity and beauty of our masonry work for years to come.
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