When I think about mixing mortars and grouts, I am reminded of my friend’s first encounter with the tile trade. As a “green” helper, Gerald was instructed to quickly mix the thinset for the installers. His equipment consisted of a well-worn five gallon bucket and a mixing paddle or should I say, broomstick. That’s right; his normal mixing process included the handle of an old broom. It is difficult to imagine how effective the mixing action was, let alone the aching muscles that went along with this exercise. The tools available today certainly have made mixing mortars and grouts easier and better.
The person mixing mortars and grouts should always follow the manufacturer’s guidelines which include the speed of the mixing motor—normally about 300 rpm. Speeds lower than 300 can lead to ineffective and incomplete mixing while speeds above this recommendation can whip (entrain) air into the mix. This additional air is excessive, making the mortar or grout light and fluffy. While this does ease the fatigue of the installer’s arm, it also significantly reduces the product’s strength. The loss of strength can cause the thinset mortar to not adequately bond to the tile, or cause soft and powdery grout.
Equally important is the time allotted to mixing: the initial mix, the slake time (allowing the material to rest in the bucket) and remixing time. But beyond that, very little attention is placed on the mixing paddle itself.
Today’s mixing paddles come in numerous configurations. Basically, the assortment available from the tile distributor, box store or the Internet consists of three models as seen in the attached photo—from left to right: the box mixer, the beater, and two styles of the spiral mixer. The beater has been around a long time and looks somewhat like a larger version of the kitchen mixer your mother may have used to mix cookie dough. It does a good job of blending the ingredients while not whipping (entraining) excess air into the mix.
The spiral mixer has gained popularity due to its ability to thoroughly blend the mix from the outside of the bucket to the inside without adding additional air. However, the box mixers are not as effective. Originally, they were designed to quickly mix drywall mud. The problem with the box mixer is that it moves the mud from the inside to the outside of the bucket and in doing so, mixes in a large volume of air.
This is where the double whammy comes into play. As mentioned previously, when the finisher (helper) mixes with a motor that spins faster than the recommended 300 rpm, it whips air into the mortar or grout. Add the results of excess speed to that of the box mixer and you have an extremely weak product. One other feature of this paddle is that not only does it throw the mortar to the outside of the bucket; it does a great job of also throwing it all over your clean blue jeans from the knees down.
When mixing, an installer should consider using a dust containment device that attaches to the rim of the bucket and a HEPA-filtered vacuum. This combination eliminates just about all of the dust involved in the mixing process. This is a wise choice that protects the health of the person doing the mixing, while also providing a clean work space.