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Castable refractory question

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  • Castable refractory question

    casted the front section of my oven today using some 46% alumina castable. Direction says to wet cure it for the first 24 hours so after finishing it, I covered it with plastic and placed a large piece of foam over it. Several hours later I go to check it and it's going through a chemical reaction that has the refractory over 150 degrees in the thicker section. The metal vent will burn your hand if you touch it, since it's in the thickest section.
    Now, within a few hours, the casting looks dry and hard as concrete so how do you wet cure something that's almost to a spontaneous combustible temp if it was flammable?
    The manufactures instructions say nothing about it going through this thermal reaction., so my question is, is this normal?

  • #2
    It is normal for the reaction to give off heat, but I've never experienced it getting that hot. Ordinary concrete also heats up, but because the reaction is much slower the heat is dissipated more easily and the temperature rise less noticeable. As calcium aluminate concretes achieve full strength in 24 hrs under normal conditions, prolonged damp curing is not required.

    In hot weather I used chilled water to add to the mix. This increases working time by slowing down the reaction.
    Last edited by david s; 08-12-2017, 10:12 PM.
    Kindled with zeal and fired with passion.


    • #3
      Thanks. Yea, I knew there would be a small reaction but 152 degrees checking it with a thermocouple was a lot more than I expected. I just checked it 14 hours later and it's still pretty warm.
      So, this afternoon I should be able to see if it's going to come out of the mold or not. There are two small straight sections I forgot to use a couple of degrees of angle so it will turn loose so I have some concerns about getting it to come out. I was hoping to use my mold to cast a second piece and after spending a week making that one, being made of Styrofoam, I'm not sure it's going to survive


      • #4
        I haven't had any experience with calcium aluminate. However, I did have some temps fairly close to that with some 5000 psi concrete that I had pumped into the walls and ceiling of my safe room. About 12 hours after the pour the metal door jams were pretty hot to the touch.
        Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build


        • #5
          Well the mold came out, at least what was left to it. It got hot enough it melted a couple of inches off the whole dome, so to reuse my mold, I've got a whole bunch of repair work to do on it.
          I don't like the way I designed the opening but my wife says she loves the way I make it so I may use this one.


          • #6
            A little gee wiz info for those that might be thinking of using a industrial type castable refractory like calcium aluminate. I find very little info on the actual use of it, like no one has mentioned the heat that may be developed during curing. Now, this may be specific to the one particular one I'm using and in no way am I claiming to be an experienced expert with the stuff. I'm working on my fourth piece for my oven and here's some lesson's learned so far.
            1. It's going to get hot, very hot within several hours, the thicker it is, the hotter it will get.
            2. It needs to be wet cured. As hot as it gets, to do this I've found about the only way to do this is, after it about an hour and it's firmed up, I cover the piece with a wet towel and then some 7 mil viscreen. After several hours, I usually have to wet the towel again so it's almost dripping wet. After that, I check it every several hours but usually it's cooling off enough that the towel stays damp enough to keep the piece moist. I leave it like this for at least 24 hours.
            3. Vibrating is a must if you don't want an extremely rough look surface. I find the easiest way to do this that works well is wrap the bottom of a random orbit sander with plastic and pressing it into the mix, lift it, move to another spot and press it again until I have covered the whole piece. I do this for each inch of castable I apply, so if I'm if doing a three inch thick piece, I've vibrated it three times and then troweling it. I'm only using a molds to form the inside shapes so the outside is exposed to make all this possible.
            4. Moisture content is critical and is extremely easy to over shoot. With a 55 pound bag of denscrete mix, a couple of ounces of water (literally) will take a mix slightly on the dry side to being too wet and will be difficult to apply without it sagging badly. What looks too dry may very well be prefect when applied and vibrated.

            Like I said, this is my only experience with this stuff in thirty years and by no means do you want to take this info as the gospel for all refractories, just things I've run into while doing mine.