Explaining the Problem When Bonding Mortar Adheres to the Substrate But Not to the Tile.
When setting mortar fails to bond to the tile the reaon is obvious. During
the initial setup of the mortar the tiles did not receive full contact
with the mortar. That is, the mortar did not wet out the back of the
tile. This leads to the next question: Why didn't it wet out? Here are the
most common reasons:
1- If the back of the tile is dusty and is not cleaned prior to
setting, this thin dust becomes a barrier between the tile and the mortar.
Tile is, in fact, composed of talc (talcum) and there is often a residue
from production on the back of the tile. People in the molding industries
also use talcum as a release agent to prevent the casting from sticking
to the mold, so it is easy to understand how this dust film prevents adhesion.
There is a simple solution to this problem. Simply dip the tiles in clean
water 15-60 minutes prior to setting them.
2- The mortar was too dry when the tiles were placed. This may
involve a number of important errors. The mortar may have been mixed with
insufficient water, or the mortar may not have been slaked.
Slaking involves re-mixing the mortar 10-20 minutes after the initial
mixing has taken place, and is very important. This allows the chemicals
to become fully active, prevents settling, and actually extends pot life.
It also allows the mixer to adjust the water level at that time. It is a
common mistake to use too little water in the initial mix because the chemicals
in the mortar are not fully active. The nature of the chemistry is such
that the powders need time for water to activate them, and with time they
begin to swell. This swelling is the normal and proper reaction in the mortar.
However, as swelling takes place, installers usually make two common
judgement errors. First, they do not re-mix and adjust the water level.
Usually, water must be added. Second, they mistake the swelling/thickening
process for setup. They mistakenly believe that the mortar is drying or
has set up, and then they rush to trowel it because they think they have
little working time (pot life) left. Actually, all they are doing
is putting down a dry, insufficiently wet mortar.
Further, a dry mortar begins to skin early. As the mortar is troweled,
a thin film begins to form on the mortar. This is known as skinning.
It is a natural drying phenomena since mortar dries from the outside to
the inside. However, it must be accounted for, or bonding will not take
place. The way to overcome skinning is simple but rarely done, and this
leads us to the third reason for bond failure.
3- Beating in. Tile intallers commonly use two types of setting
materials: Thin-set mortar and ready-mixed adhesive. Unfortunately,
tile installers tend to use both products in the same way, and the products
are not the same. Ready-mixed adhesive is a very wet product. After troweling,
it readily accepts placement of the tile. In placement, the tiles are very
easily wet-out. Minimal contact is needed. This can be readily observed,
because if you remove a placed tile, you will notice how the adhesive coats
the back of the tile.
This is not true of thin-set mortar at all and this is the most important
thing that most tile installers forget. The installation instructions
as well as all of the published literature supplied by the Tile Council
of America specify that all tile must be fully beat into the mortar.
This allows the mortar's skin (thin drying film) to be physically broken
and for wet mortar to contact the tile, thereby wetting it out. Further,
beating in assures full contact and coverage of the mortar on the back of
Fully beating in is a requirement of the Tile Council and all mortar
manufacturers. It is not sufficient to simply place tiles on the mortar.
The larger the tile, the more important beating in is to the success of
the job. Placing the tiles with one's hand is not sufficient. A rubber
mallet or beating block with hammer is needed to do the job properly.