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Unread 06-15-2020, 11:16 PM   #7
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,280
Cement based thinsets will grow their crystals and cure, regardless of the moisture as long as it has enough to fully activate their growth. You can think of them as fine glass fibers. They'll shatter when they're bent more than their limits. The modifier coats them, can act like a glue, and helps limit how far they can move before they break. So, ultimately, the bond can be stronger with a modifier since it's using both the interlocking spires of the crystals and the glue with its protective cover. That full functionality doesn't happen until the modifier has cured. On some, that just takes time. On others, it needs to dry. The ratio of the modifier versus the cement will vary, and that can be critical. A modified thinset may have less actual cement in it than a premium one without it to compensate. It's the magic mix that does its thing, tailored to the specific needs of the application. That's the difference. That happens over time, not necessarily the moisture content. Thinset is mixed with more water than it needs so that it can be spread and will provide good coverage. Those crystals literally grow, crosslink, and get into microscopic pockets in the mating surfaces. Industry rates the ultimate strength at 28-days, but in reality, it continues to get stronger for decades and probably longer, but the majority happens fairly quickly as you might notice since you can usually walk on a tiled surface overnight, if not hours (some of the fast setting types are good to go very quickly). You wouldn't want to roll a safe across it then, but a smaller load would work.

If you bend a steel coat hanger back and forth enough, it will break...exceed the limits on the cement crystals, and they too will break. The modifier helps cushion them, and to do its work, it needs to fully cure. With the random structure of those crystals, it may not fail for a long time but will get weaker as some of them break. So, some of it is how stressed is the application and how long can you wait? Schluter did a demo with a large plate of glass over a modified thinset. It took something like 82-days before the middle was visibly dried out when installed over Ditra. If it was one that needed to dry to reach full strength, that could be an issue.

If you follow the manufacturer's recommendations for the materials you use, you should get a good result. That's still possible when you substitute, but you need to know what you're using. You can get an idea from the materials data sheet, but for product protection purposes, as allowed by the regulations, they don't have to tell you everything, otherwise, anyone could copy their hard earned research results.

So, yes, some modified thinsets will work quite fine between a tile and a waterproof membrane...some won't. It's not about the cement curing, it's what those modifiers provide in the overall mix for that product, and how fast and under what conditions.

If a manufacturer has a waterproofing system (most do) and you see what thinsets they recommend, they will likely work with other similar membranes.

Schluter, until recently, did not make its own versions of thinset, so relied on what they knew would work under the ANSI specs. They now have some modified thinsets that they know will work with their products, and other manufacturers with similar products do, too. There are differences, but...
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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