Q: Repairing and patching concrete surfaces makes up a lot of our company’s work. There seem to be many different types of polymer modifiers for cement and concrete. What do they do exactly, and how do I know which ones to use for various applications?

A: Modifying cement formulations with polymers provides many important properties that make a variety of niche applications possible, including concrete patch and repair, decorative cement overlays, ceramic tile adhesives, and many others. To get the performance desired for any of these applications, it helps to understand some of the basics of why polymers are added to cement formulations.

The first thing to understand is that polymers are primarily used to modify mortars (portland cement + water + sand aggregate), not concrete (mortar + larger aggregates such as gravel or stone), due to the relative cost involved. The main exceptions are polymer-modified concrete for bridge deck overlays and road repair work. Most polymer-modified cement applications involve mortar that is not more than an inch or two thick. But let’s start with the basics.

Polymers improve mortars in four main ways:

1. More extensive cement cure. Cement/concrete strength depends on proper curing, a chemical reaction (hydration) between water and cement that causes crystals to grow and wrap around the mix components. During the early stages of cure (roughly the first five to seven days), there must be enough water to maintain the hydration process or the cement/concrete will not harden properly.

Polymers reduce the rate of water evaporation, allowing the crystal structure to keep growing and building strength during these critical early curing stages. This reduced water evaporation is especially important in thin applications, where the surface area for evaporation is high, relative to the volume of the mortar.

 At left, an unmodified patch is easily dislodged due to a lack of hydration during curing and lack of adhesion. At right, a modified patch at 10% polymer (percent polymer by weight to cement) would not budge.

At left, an unmodified patch is easily dislodged due to a lack of hydration during curing and lack of adhesion. At right, a modified patch at 10% polymer (percent polymer by weight to cement) would not budge.

Credit: Dow Construction Chemicals

2. Improved workability. Polymer modification noticeably improves application characteristics, making the mortar more fluid and easier to handle and apply. Certain polymers also prolong the hydration period, which can increase working time, an important characteristic in hot climates. This means contractors can use less water for workability purposes. The polymer acts as a water reducer, ultimately leading to a stronger mortar with fewer voids, or weak spots.

3. Improved adhesion. Polymer modifiers act as an adhesive to enable the modified mortar overlay to stick to a variety of surfaces such as concrete, masonry, brick, wood, rigid polystyrene and polyurethane foam, glass, and metals. Adhesion is an important property, especially in thin section overlay mortar applications such as spray coatings, stuccos, and underlayments, and applications with excessive vibration and heavy traffic.

4. Improved strength and durability. Cured polymer-modified mortars generally have improved tensile strength, flexural strength, impact and abrasion resistance, water resistance, and chemical resistance versus unmodified mortars. Also, the polymer in the mortar helps restrain micro-crack propagation, which improves the overall toughness of the mortar.

Applications

The combination of improved workability, adhesion, and curing performance allows using polymer-modified mortar in several niche applications that would otherwise be difficult or impossible. One thing most of these applications have in common is a thin layer of mortar, usually no more than 1 or 2 inches thick. All of them require one or more of the chemical and/or mechanical properties mentioned earlier.

Adhesives and Grouts: Wall and floor tiles—ceramic, marble, stone, etc. Adhesion, water/chemical resistance, flexural strength. This is the most common application for polymer-modified mortars.

Patch and Repair Mortars: Grouts for repairing cracks and delamination of concrete structures such as sidewalks, driveways, and walls. Adhesion, abrasion resistance, tensile and flexural strength. New mortar will not stick well to old concrete without polymer modification.

Decorative Overlays: Wall coatings and textured building finishes (polymer-modified stucco). Adhesion, tensile and flexural strength, exterior durability.

  • Toughness test of modified and unmodified mortar patches. Mortar Patches are feathered down to blend in with original substrate. Adhesion is a key property in mortar patches over existing concrete. Without polymer modification (left), the patch flashes off very quickly and breaks like a potato chip. Notice the residual mortar on the concrete substrate. The 10% polymer-modified patch (right) could not be moved. Notice the scallop marks near the bottom where removal was attempted.
    Toughness test of modified and unmodified mortar patches. Mortar Patches are feathered down to blend in with original substrate. Adhesion is a key property in mortar patches over existing concrete. Without polymer modification (left), the patch flashes off very quickly and breaks like a potato chip. Notice the residual mortar on the concrete substrate. The 10% polymer-modified patch (right) could not be moved. Notice the scallop marks near the bottom where removal was attempted.

Flooring and Pavements: Can be used in mortar or heavy use concrete industrial/commercial flooring overlay formulations. Warehouses, factories, hospitals, stairways, garages, railway platforms, airport runways, etc. Abrasion resistance, tensile and flexural strength, water resistance, chemical resistance, and exterior durability.

Waterproofing: Basements, bulk water storage tanks, septic tanks, ship decks, roof decks, and concrete walls. Water resistance for all; chemical resistance, other properties depending on the application.

Types of polymers

Many different types of polymers can be used in mortars, including latex polymers, redispersible dry polymers, and water soluble polymers, far too many for a comprehensive review here. The important things for contractors to look for include:

  • Follow the directions. Using only 5% polymerone-half the recommended amountprovides some adhesion, but not enough to hold the water in for good hydration and proper curing. The result shows a weaker patch that is easily chipped and gouged with a 1/16-inch blade.
    Follow the directions. Using only 5% polymerone-half the recommended amountprovides some adhesion, but not enough to hold the water in for good hydration and proper curing. The result shows a weaker patch that is easily chipped and gouged with a 1/16-inch blade.

  • Choose the right mortar for your job. The label should indicate whether the mortar is appropriate for outdoor uses (UV-resistance), waterproofing (water resistance), ceramic tiles (adhesion, water resistance). Most labels will not tell you that a mortar for outdoor use is modified with a 100% acrylic polymer, but it should indicate that it is appropriate for outdoor use and/or resistant to sunlight. 
  • Likewise, most labels will not indicate that a waterproofing mortar is modified with a styrene-acrylic polymer, but it should indicate whether it is recommended for use as a waterproofing mortar.
  • Choose a liquid or dry redispersible polymer option, depending on your needs and preferences. Liquid polymers usually come in a jug with instructions for how much to add to a bag of dry mix containing sand, cement, and other ingredients. It will also include instructions for avoiding freezing temperatures; this affects the integrity of the polymer. Dry polymers are usually added to the dry mortar mix in advance, so the contractor only needs to add water to a bag of dry mix.
    •  A 10% modified polymer mortar patch can be scratched but is too strong to be gouged. This is a much stronger more durable patch.
      A 10% modified polymer mortar patch can be scratched but is too strong to be gouged. This is a much stronger more durable patch.
  • Follow the instructions on the label: Many contractors try to cut corners by using too little polymer from the jug or using a product in an application for which it was not designed. To get the performance needed for most applications, the polymer content is normally about 3% to 5% polymer solids of the total (~10%-15% polymer solids by weight to cement). Anything less and performance will suffer. 

Contributed by Dow Construction Chemicals. Visit www.dowconstructionchemicals.com. for more information.