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I am installing larger format Porcelain Tile (18"x18") over ditra on a plywood subfloor. I do not want to use unmodified thinset like they recommend. The TCNA says modified in their installation handbook and with my experience modified is best for tiles and that's how I do it, but I am looking for a better way of cutting the setup time of modified thinset on top of a vapor barrier type product. Rapidset has been suggested but as many of you know it can be a really big pain to work with.

I am looking into a cement mix accelerator from homedepot and I am wondering if cement accelerators would be a good way to cut the setup time so I can grout within about 12 hours.

  • The accelerator will decrease the time. I think you know this but not what title says. Accelerators do work I use them especially when doing outdoor work in cold weather. – Ed Beal Mar 2 '17 at 19:39
  • Ah yes, saw that in the title and I fixed that thank you! But it will work with modified thinset? I know that the setup time will depend on the tile size but do you think it will set up in about 12 hours with an 18"x18" tile? Also this is for an indoor job. – Matt Kriser Mar 2 '17 at 19:43
  • With your experience Ed Beal, have you found a good ratio that works best? – Matt Kriser Mar 2 '17 at 19:47
  • The stuff I used was from HD and measured to the instructions. Accelerators speed up the set but not the full cure time as they react with the cement part of the mix. I have been told that a fan will also help but have not ever tried this because I was afraid it would cause more problems. – Ed Beal Mar 2 '17 at 19:55
  • Thanks! So that answers that will work. Now, what about the drying times. That is my main concern. Does it cut the time from 7 days to 3 or can it actually get down to 12 hours. I took a quick look at the installation and it didn't show are setup or dry time at all. So do you have any rough data on dry times with mortar? – Matt Kriser Mar 2 '17 at 20:06
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It is obvious that there is a large concern about getting air to your thinset. The problem that you have is that there is too much variance in the equation to properly answer this. Porcelain plus Ditra will cause quite a barrier but there is also airflow of the room and temperature.

The problem isn't the use of the accelerator, it is the effectiveness of it and how it might effect the outcome. I have used accelerators for thinset before and have had mixed results. Well since I didn't live in the houses I am not sure about the results but on at least 2 jobs my crew used accelerators and the thinset became brittle to the floor upon drying. I would personally never use them and for tile that large I would give it 3-4 days. But I am risk adverse especially when the risk I am taking is mitigated by me doing "nothing" for a couple of days.

I think your concrete accelerator will work - as far as drying faster. However will it cure faster or cause an issue with the cure. I think that will be product/chemical dependent and has to do with the ratios you are using. Someone may have a better answer than mine but they would have to have tested specific products and ratios and give you their experience - not thinking you will get that but who knows.

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  • Thanks for this! I understand that there is an issue overall but that's part of the job. This is also why I'm asking the question to see if there are more experiences out there with this specific situation. Large format tiles is a big trend here and a constant issue. I will continue with this and post my results after a while. Thanks again for the insight! – Matt Kriser Mar 3 '17 at 15:29

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