Picking the right grout
In traditional “thick” set mortar installations, grout refers to the excess mixture of sand and cement that oozes out between tiles, filling the space created at each joint. This grout usually stays moist for some time and cures properly, ending up very hard and non-absorbent.
Grout as we know it today is a more recent invention that has been designed to fill the gaps left when tiles are installed with a thin adhesive (thinset). This grout is still made of sand and cement, but has been modified to enhance moisture content, and is also used differently - installed over the tiles after the thinset adhesive behind the tile has completely set and dried.
Potential problems: the installation method and the grout itself.
We normally apply grout to a tile installation that has been sitting for at least 24 hours. At that point the tiles are very dry and absorbent. So as soon as the grout is applied to the joints, the moisture is immediately absorbed into the porous tiles. Within an hour the grout appears to have set and looks finished.
Here is the problem. Grout, like any cement product, needs to cure for at least 28 days. That process requires moisture, so even though the grout appears to have set after an hour, it is actually starved for moisture and will remain porous, chalky and brittle for the rest of its life. This makes room for mildew and other nasty things to grow in the grout.
The solution: Use a modified grout and maintain moisture content while the grout cures
The glass tile industry usually recommends the use of polymer-modified grouts. Polymers help grout retain moisture and achieve a better cure; they increase bond strength; they are effective at plugging up pores.
It is also important to pay attention to the grout's moisture content. Grouts should not dry too quickly after installation, so cover the wet grout with a plastic sheet to keep moisture in. It takes at least 7 days to achieve minimum strength and 28 days for the grout to approach full strength, so don’t be shy about applying a bit more water every so often. Be sure not to apply any grout sealers for at least 28 days, otherwise you will disturb the curing process. And carefully follow the installation instructions that come with the grout sealer.
What about epoxy grouts?
Epoxy is a two-part resin that together with fillers can be made to look like grout. Epoxy grout has always been appealing from the homeowner's point of view because of the problems with "regular" grout, and its apparent pluses - it is not porous and does not require moisture to set
The glass tile industry has historically discouraged the use of epoxy materials because they tend to be very inflexible, making the entire installation rigid and prone to cracking. There have also been installation difficulties and safety issues. Epoxy-based grouts have had a tendency to be very sticky, making installation and cleaning difficult. They have also had a "toxic" odor.
But not all epoxy grouts are equal. Some chemical companies now claim a more flexible product. For instance, when used in conjunction with spacers and a special sealant, Latricrete recommends its Spectralock grout for small mosaic glass tiles.
Technical information on these pages is offered as a background and general guideline. No warranty is implied. Always consult manufacturer instructions.
