How Setting Mortars Bond

How Setting Materials bond and Why Resin Back Tiles Fail

Frederick M. Hueston, Stoneforensics.com

If you search the internet and try to find an explanation of how thin sets and other setting materials form a bond to a tile you will not find much. The manufacturers will give you a slew of marketing jibber jabber siting bonding agents as well their secrete formulas. The following article will break down the basics of thin set and other setting materials chemistry. With this information you can begin to understand why some setting materials do not bond properly or at all.

In order to understand the basics, we must first look at what thin sets and setting material are made of. The main component is Portland cement. Of course, there are other ingredients such as bonding agents but in the largest component is Portland Cement.  So all one needs is a basic understanding of how Portland cement cures to understand how these setting material can bond to tile.

Understanding how Portland Cement Cures

Portland Cement is a combination of aggregate and a cement mixture made from limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. Limestone is heated at high temperatures to form a solid material. This solid is than ground to form a fine powder that we call cement.

Cement is not Concrete

In order to form concrete that we are familiar with an aggregate need to be mixed with cement. The cement and aggregate are added together and mixed.  This is typical how you buy a bag of concrete. The bag you purchase at your local home center or building supply is a combination of dry cement and an aggregate. When you add water to this mix a reaction occurs that causes the concrete to harden(cure).

How Concrete Cures

When water is added to the concrete mix a reaction starts to take place called hydration. In other words the cement paste reacts with the water forming microscope crystals and branches(see photo #1). Without water the hydration process cannot happen, and the concrete will not cure.

How Setting Materials form a bond

Now, keep in mind that tile setting materials are basically concrete. In order to form a bond to the back of the tile there has to be moisture exchange. In other words, the back of the tile must be absorbent. As the concrete cures these little crystals and branches enter the pores of the tile and help create a bond. This bond can be compromised by numerous factors.

Lack of Bond

If the tile is nonabsorbent it will not bond since the crystals and branches cannot grab the tile. This is why resin backed tiles will not bond properly. The resin forms a nonabsorbent coating on the back of the tile.

Bonding Agents Play a Role

Bonding agents such as acrylics, latex etc are added to many setting materials to create a strong bond, However in most cases the back of the tile will still need to have some absorbency. This is why most thin sets and setting materials will have limitations listed on them. In many cases manufacturers recommend using an epoxy setting material on resin back tile. Epoxy setting material create a chemical bond and do not require absorbency of the tile.

Figure 1- Concrete hydration forming crystals and branches