Frederick M. Hueston, PhD

Stone Forensics

Educational information that adequately explains how a bond forms between tile and thinset or other setting materials can be hard to come by, despite the abundance of general information available online. Manufacturers do provide some information, but it is mostly marketing jibber-jabber that cites bonding agents and secret formulas, without providing details into exactly how the bonding process happens. This article examines the basic chemistry of thinset and other setting materials and explains why some setting materials do not bond properly or at all. Armed with this information, installers may be able to avoid bonding failures.

Setting Material Ingredients

Thinset and other setting materials are made of Portland cement. Although setting materials contain other ingredients including bonding agents, the main ingredient is Portland cement. So, all one needs is a basic understanding of how Portland cement cures to understand how these setting materials bond to tile.

Figure 1 Crystals and  Branches formed during hydrationWhat Is Portland Cement?

It’s commonly misunderstood that concrete is the same material as cement. They are actually two different materials. 

A typical bag of concrete, or Portland cement, is a combination of aggregate and a cement mixture. The cement mixture is made from either a combination of limestone, shells, and chalk or a combination of marble, shale, clay, slate, blast furnace slag, silica sand, and iron ore. The Limestone and other ingredients are heated at high temperatures to form a solid material, which is ground to form a fine powder that we call cement.

How Concrete Cures

Adding water to a mixture of cement and aggregates will cause a reaction called hydration. The cement paste reacts with the water, forming microscopic crystals and branches (see Figure 1 – Crystals and Branches Formed During Hydration). Plainly stated, the concrete cures. Without water, the hydration process cannot happen, and the concrete will not cure.

How Setting Materials Form a Bond

Tile setting materials are basically concrete. In order for these materials to form a bond with the back of the tile, there has to be some moisture exchange. In other words, the back of the tile must be absorbent. As the concrete cures, the little crystals and branches enter the pores of the tile, creating a bond.

Causes For Lack of Bond

Setting materials, in and of themselves, will not bond to non-absorbent tile, because non-absorbent tile lacks pores. Remember, bonding takes place when the crystals and branches enter the pores of the tile. Resin-backed tiles (whether absorbent or not) will not bond properly, because the resin forms a non-absorbent coating on the back of the tile.

Bonding Agents

Bonding agents, such as acrylics, latex, etcetera, are added to many setting materials to help create a strong bond.

However, in most cases the back of the tile will still need to have some absorbency. Manufacturers often recommend using an epoxy setting material on resin-backed tile. Epoxy setting material creates a chemical bond that does not require tile absorbency

Most thinsets and setting materials have limitations listed on the bag or information data sheet. Review these limitations carefully and take the necessary precautions to avoid bonding failures.

Dr. Frederick M Hueston has been teaching about tile and stone installation failures for decades.
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