Epoxy application - the secrets to a “magic” mortar mix

After delving into a lot of heavy, “big picture” stuff in the last few months, I felt it was time to get back to some good old-fashioned tech support for the contractors out there.

A topic that regularly bobs up and has always held certain intrigue for me personally is mortar mixes. As a manufacturer, I’ve found it to be one area that’s very hard to service because it seems every contractor out there has their own “magic” mix they swear by for patching, coving, levelling, falls-to-drains, and everything in between.

Despite this unwavering trust in their own products, I’ve had many discussions over the years about the problems that can show up when these mixes aren’t quite put together as well as they possibly could. The most recent involved a widespread issue in trowel-down leveller applications and I thought it was worth going over in greater detail here.

A common mortar problem

In this particular case, the contractor had applied his own recipe for a sand-filled epoxy leveller over the floor and was worried by the “drummy” feel it had when he got back on it the next day. It just didn’t sound solid enough underfoot and he also reported some kind of “blistering” when the film was inspected closely.

After talking through it all a little more, we worked out that what he was describing was probably due to a very common, undesirable trait in many homemade mortar mixes: voids

At risk of stating the obvious, voids are caused by either incorrect application or incorrect formulation. It didn’t sound like he was doing much wrong on the application side of things – he was applying good pressure on the trowel (to pack the aggregate tightly), and slicking off the surface without over-working it (which can sometimes dry the product out). It became apparent the problem was the mix itself and the fact it wasn’t doing what a mortar mix ideally should in this type of application.

The ideal mortar properties

Taking a step back, what should a mortar mix used for levelling actually look like? What properties should you aim for?

In my opinion, you want a mortar mix that is –

  • Easy-to-use – with mortars in general you find areas are often small, awkward and don’t allow for machinery or bulky equipment to be brought in. With this in mind, you want a product that can be mixed, applied and slicked off by hand without busting your gut. You also want something that is reliable and consistent.
  • Versatile – if you want to tackle a wide range of applications then your mix must be suitable for a wide range of film thicknesses.
  • High-performance – ideally your product can be applied without a primer, so it must have good adhesion to concrete. Something that is resistant to both chemicals and impact is also a must in the industrial environments these products are often faced with.

How to get your mortar mix to work

So, how do you get a product that can even come close to ticking those ideal boxes?

Well, the first thing I believe you need to get right is the aggregate blend. The goal here is to find a combination of particles that can “self pack” and therefore doesn’t need a ton of force on the trowel. Unfortunately it’s not as easy as randomly choosing a small and large grade of sand. In my case, I ended up using four particle sizes and went to the extra length of sourcing a rounded particle rather than “jagged” or “crushed”. Once you’ve got your blend sorted, I always believe you’re better off putting it in pre-measured kits rather than tossing it together on site to help with the consistency I mentioned earlier.

The next area of focus is the binding resin. The natural tendency here is to think the lower the better as far as viscosity is concerned and many make the mistake of turning to water-thin GP-style epoxy resins. In contrast, you actually want a bit of body in the resin so it can cling to the particles and hold the film together evenly, rather than drop out and leave “bony” areas.

If you can get both of those things ticked off, the result should be something similar to the leveller I swear by myself.

It’s a product that looks and feels resin rich, so I can easily position and pack without requiring too much pressure on the float. Being resin rich also means it’s primerless and I don’t have to put up with the hassle of working on tacky primer coats. The four aggregate sizes I use slot together nicely and push the air out of the voids, which helps bring an even layer of resin to the surface and allow for it to be slicked off without solvent. Finally, it can go from 4mm/160 mils to 50mm/2” without resin pooling on the surface, and has the very neat feature of allowing extra mix to be added without showing any lumps or bumps in the finish. The result is a tightly packed, fully closed off/sealed film that is both strong and impervious to chemicals.

A close up of a trowel-down mortar mix used for levelling applications.

Overall it’s a pretty impressive product – even if I do say so myself – however, there’s one thing it can’t do and that’s feather to an edge. With trowel-down levelling products, your minimum film thickness is linked to the largest particle in your mix; as a rule of thumb, you won’t be able to go any thinner than roughly twice the largest particle size. In my case, the largest particle is 2mm and that’s why the natural range of my leveller is 4mm and above.  

Take care and keep smiling,