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Wet premix mortar - crumble risk - Forno Bravo Forum: The Wood-Fired Oven Community



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Wet premix mortar - crumble risk

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  • Wet premix mortar - crumble risk

    I had three tubs of "premixed refractory mortar" (Clayburn set 50 DC is what they call it) sent up to me on a barge, along with a pallet of 200 fire bricks from BC bricks (an effort to make the most of the shipping cost).
    A couple days ago I opened a tub of mortar and only then realized that it was premixed WET mortar... new to me; I didn't know that was an option...
    I went ahead and mortar-ed my weekend away - I completed the arch and the first FIVE layers of bricks. Used up two of the three tubs. Long days, but looking pretty good.
    I then read on this forum that a wet mix like this is best avoided.
    I emailed BC bricks (where I ordered the mortar from) for more information (they have been most helpful) - they replied right away and said:
    "You are not supposed to exceed ¼” thick. It is actually supposed to be laid 1/8” or 1/16” thick. It is fine for outdoors and it is air setting so you do not need to light a fire to cure it. If the gaps are larger than ¼” the mortar probably won’t dry properly and will start to crumble."

    Many of my gaps are significantly larger than 1/4" thick on the outside (bricks touch on the inside of course). I had put small wedges of firebrick in between the bricks (between the rows), and cut the bricks in to wedge shapes to make horizontal gaps smaller - but not to fit tight - and larger than 1/4' gaps.
    So, do I:
    A) start over with a homebrew mortar like I should have done from the get go (but, this premixed mortar wasn't cheap, I put a lot of time in, and will take considerable effort to deconstruct and clean up all that work)
    B) Keep on plowing ahead with periodic prayers to the goddess of fire.
    C) What if I finished the dome and coated it in a homebrew mortar that would hold the crumbled mortar in place???
    D) Other ideas? (Hint: "Don't worry, it will all be fine" is the answer I seek)

  • #2
    Re: Wet premix mortar - crumble risk

    DB, I'm nearly certain someone here will have had experience with that product and give you good advice. I used a premix "wet" refractory mortar from a company called Onex, not being a refractory mortar expert I can only tell you that after two summers useing my WFO I haven't had any problem. I had many exterior gaps as large as 1/4".

    I would wait to hit the panic button untill you get more input, hopefully from someone familiar with the mortar your useing.

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    • #3
      Re: Wet premix mortar - crumble risk

      Yes - I will hold off on panic. I'm going away for a couple weeks, and will see how things look and feel upon my return - and what the others out there in the great world of WFO makers have to say.
      At this time I still hope to carry on as I am - will try to cut bricks to make smaller gaps from here on in.


      • #4
        Re: Wet premix mortar - crumble risk

        I would use homebrew or something similar on the exterior and proceed. At thicker quantities that type mortar has a large amount of shrinkage (very small aggregate and very wet blend). FYI, I laid the barrel vault of mine with a similar type and other than making sure the joints on the inside were as tight as possible, I did nothing else other than parge with regular refractory cement and then normal mortar.


        • #5
          Re: Wet premix mortar - crumble risk

          I also used a wet premix refractory mortar for my dome. Most all of the bricks on the inside of the dome touch and have tight joints. The outside of the dome I used the brick shims and also have larger gaps similar to yours. I do not recall the brand of the mix but I contacted the manufacturer (not the supplier), who gave me the same story as you were told....1/4 inch max. They also told me their instructions error on the side of safety, so I should not have any problems with 1/4 inch or so joints. I did add a 1/2 inch or so layer of refractory mortar with stainless steel needles over the entire dome. I had one very small crack on the outside which I went over again with the refractory mortar and since have had no problems with the dome. There are others who have a lot more experience than I who may respond with different advise.



          • #6
            Re: Wet premix mortar - crumble risk

            I've run out of the premix I inadvertently bought - and am just finishing up the dome - last three rows. I mixed a "homebrew" with
            3 parts sand - (20/30 "lane mt. silica sand")
            1 part lime (pressure hydrated type S lime)
            1 part Portland cement ("general purpose" - a few months old but not clumpy or anything)
            1 part powder from firebrick cutting
            I mixed this together very well, dry, and then took out a big scoop full, added water to a paste-like consistency, and tried to mortar in a couple bricks...
            BUT - it was not sticky, did not hold the bricks together. I tried a few times with different amounts of water - the last time I let the indispensable tool hold the brick in place for an hour or so to let the mortar harden, but then went to see how it worked out, and the brick was loose and the mortar crumbled away into a sad pile on the bottom of the oven.

            What am I doing wrong?


            • #7
              Re: Wet premix mortar - crumble risk

              It sounds like you are trying to work with it too dry. It needs to be the consistency of brickies mortar. If your bricks are dry as well the bit of moisture in your mix is being sucked up by the brick immediately.

              I have used the home brew mortar quite a bit and have had no problem with it but it is not meant to be used as a paste.
              Cheers ......... Steve

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              • #8
                Re: Wet premix mortar - crumble risk

                Well I've tried and wetter... and wetter still, from paste to runny mashed potatoes to a soup... no luck. Thicker - no luck either. Don't understand what I'm doing wrong - or even what things to try to change. It seems simple enough...
                I tried soaking the bricks, dipping the bricks, dry bricks... I have spread a bunch of the mortar on the outside of the oven to use it up, and it dries hard by the next day... it's just not sticky and crumbles away when I try laying bricks with it.


                • #9
                  Re: Wet premix mortar - crumble risk

                  Ok - so I think I was expecting too much of the mortar - expecting it to stick the bricks together at a near vertical angle, and, the temperatures are getting rather cool - significantly extending the setting time - so - I will build a form, mortar in the last three rows of bricks while they are held in place, and let it set for a few days...


                  • #10
                    Re: Wet premix mortar - crumble risk

                    Well that's sounds like a plan... Build to a form.
                    A simple way is to cut a circle of ply, in through the door and using blocks of wood and a car jack, jack it up till it touches the dome.
                    Build a form on that with damp sand and cover with plastic wrap or damp newspaper. Build to that.
                    As soon as your last brick goes in wait till the mortar get to that first "set " stage and drop the disk of ply and get in with the rags and water and clean up the insides. You'll know when that happens don't wait for days just to be cautious , otherwise your mortar will be to hard to clean up .
                    Trust that a complete dome is a strong structure and the mortar isn't the glue that holds it together but rather what holds the bricks apart once its complete.
                    Regards dave
                    Measure twice
                    Cut once
                    Fit in position with largest hammer

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                    • #11
                      Re: Wet premix mortar - crumble risk

                      Hello DB

                      I used that mix for 3 domes and it worked ok. Beyond my limited experience. Sorry.
                      Dave's notion will work. Once the last brick is in place it should be self supporting after the mortar has gone off.

                      I hope it works for you.
                      Cheers ......... Steve

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                      • #12
                        Re: Wet premix mortar - crumble risk

                        I would like to add that after each course is completed, the platform can be lowered for clean up. I used Karangi Dude's method on mine. His platform and car jack idea worked great for me. I completed the first course with the platform and cleaned up that course. I then placed it back inside and fit a few more discs on top of it. The thickness of each circle will be thinner than the one before it. I think you will get a better idea from the pics below.

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                        I stated that the form can be lowered after each course is completed. That was the plan, but having a stable platform to work to, and precut brick made closing go pretty fast. The last 3 courses and the plug were completed at one time. I was then able to drop the platform and clean up, like Dave described, while the mortar was still green. Here is a link to my dome closing.
                        joe watson

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