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Unread 11-26-2020, 04:16 PM   #22
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,233
The issue with any mortar mix is that during shipping, it is possible that materials will stratify, meaning that things like the sand and cement could separate. That doesn't happen all that often, but can be critical with stuff like grout where the pigments might cause color variations if you mixed it in batches. But, that doesn't happen often, and if you're worried about it, you can dry mix the bag up first, then use what you need to only have enough before it can become unusable. What you usually notice is that the stuff becomes more plastic and doesn't spread as easily. Depending on how close to the end is that you can sometimes remix it to loosen it up a bit. Keep in mind, what's really happening then is that you're breaking apart the cement crystals that have formed, making the now cured bits smaller, similar to the sand or other aggregate (inert) stuff that was in the original mix. Once cement has cured, you cannot convert it back to active material.

When cement cures, it grows crystals whose spikes interlock, which gives it structure and strength and the ability to bond by sticking into minor crevasses in the assembly (like the minor imperfections in the tile or subsurface, or, in the case of Kerdi, around the fleece.

Without inhibitors in the cement mix, it would fire off quite quickly. Take hydraulic cement as an usually cures in minutes - it has essentially no inhibitors in there to slow it down as the goal is speed.

The chemical reaction speed will vary by temperature and the amount of inhibitors in the mix. Curing cement is an exothermic reaction meaning that it gives off heat when it happens...adding heat, means it happens faster. Using really cold water when mixing and keeping the cement where it's cold could give you a bit more working time. It's important to keep the actual working temp of the assembly within range, though (usually above 50-degrees F), so you can't tile in the winter outside or unheated new construction many places.
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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