Is there anyone out there who has worked with SpecMix Type S (PortlandLimeSand) and cultured stone? My experience so far has been terrible and very frustrating. I started my project last fall and found that the SpecMix Type S did not appear to be holding very well. I tried SpecMix AVM and also tried VersaBond LFT. At that time I made myself a test board using the same cement board and mesh that I had on my fireplace and attached three pieces using the three mortars mentioned. As summer came I left that project and my test board to sit over the summer. I recently came back to that project and checked my test board. All three stones seemed to be attached pretty well which left me to conclude that the Type S SpecMix was OK if left long enough to cure.

Since starting back on the project I have layed several rows 5 or 6 pieces at a time with a day to set. A couple of weeks later as I go over the stones with a bristle brush I find several are loose and not bonded. My conclusion is that after laying several stones and then coming back to lay a few more, as I tap the new stones with a rubber mallet to bed them in, the vibration breaks the bond on the partially set stones. Perhaps my technique is wrong and I should simply set the stone in place and not try and 'bed' it with the mallet or maybe I'm doing something else wrong. Is it normal that a strong adhesion should not occur until after a week or so? I find that anytime before that I can break the bond with my bare hands using progressively more force as the days go by.

Any comments will be appreciated. Thanks

 

Without seeing your stones or what you are doing I think your problem is the test boards. I have never used a mesh and just apply directly to cement board, Hardie backer or concrete. What is the mesh you are using and how is it attached to the backer? Is that mesh recommended by your cultured stone manufacturer? Also, how sturdy are your test boards? Masonry doesn't flex so any flexing or movement of your backer could cause problems.

I've done a lot of real stone veneer. Even a 2" thick 60+ pound stones will stick to a vertical wall without support. An entire wall can be veneered without having to wait for mortar to harden. I've never had a stone come off even when bumped by a tractor or car so it can be made to stick very well.

 

Maybe I didn't express myself clearly. I experienced problems on the actual fireplace not the test board. I created the small test board afterwards to get an idea on how the three mortars were performing.

The mesh I am using is that expanded diamond type mesh and is suggested by the manufacture. The mesh is attached to the cement board at the studs with galvanized staples. Again, my problems are on the actual fireplace wall and in an area that has been set within the past couple of weeks. My test board was set some 6 monthes ago and all three mortars have stood up to some rather rigourous attempts to dislodge them. My suspicion is that SpecMix Type S requires a fair amount of time to fully set and any vibrations can break the bond between the partially set mortar and the stone. But then again maybe it's something else. Your further comments will be appreciated.

 

I've never seen a mortar that takes unusually long to cure. They are temperature based but all withstood wire brushing down the joints 6 hours after installation. I often brush down the joints when the mortar is dry to the touch and doesn't stick to the brush but it still soft enough to be easily brushed down. I wonder if you've got a bad batch of mortar. Was the mortar dry, loose and powdery/dusty when it came out of the bag or was it in sorta hard lumps?

 

I appreciate what you are saying. I too was very surprised. I have had a lot of experience with tile and was used to things being pretty well set after 24 hrs. This is my first experience with cultured stone and mortar, specifically SpecMix Type S. I have no problems with the stones sticking to the wall and after 24hrs a gentle nudge suggests they are set in place but if I apply a bit of force I can pull the stone free. No the one caveat here is when the stone comes free it is pretty clean on the back i.e. the mortar does not appear to have bonded to the back of the stone. With each successive day of curing it takes a greater amount of force to get the stone free. As i daid earlier, after several months the Specmix stone on my test board appears to be pretty solid. I am trying to get in touch with the manufacturer to get their comments.

 

Oh, that's interesting. What does the manufacturer recommend for installation? Were the back of the stones dirty or chalky when you installed?

How did you install the stone? Did you butter the back of the stone before applying it to the wall or did you stick the dry stone into the mortar?

 

I spoke with the manufacture and they have suggested that I should not be applying too much pressure as I set each stone; definitely no tapping with the mallet. They didn't elaborate but I wonder if too much pressure squeezes to much moisture out of the mortar? I thought about the back of the stone and although it doesn't 'seem dirty I'm thinking a quick rinse of each remaining stone can't hurt. Each stone is buttered with fairly wet mortar and wiggled into position as per the manufacturers suggestion. Hopefully I can get the rest of the stone in place and leave it set for a couple of weeks. Again, thanks for your comments.