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  • mortar problems??

    Help! I just finished motaring a 36 inch pompeii dome yesterday built over 7 days using the calcium aluminate, sand, lime and fire clay in 1-3-1-1 formula found in the pompeii oven plans. Today when I went to see what I could do to clean up smears of mortar inside I found it very easily removed. Furthermore the arch mortared a week ago has mortar that I can easily stick my thumb nail into and indeed excavate without difficulty. I mixed only small quantities using sufficient water for it to remain workable for a maximum of 5 minutes per mix so it was pretty laborious. The cement I used is called A500 in Australia

    Now I'm worried.
    Is this mortar ever going to harden?
    Will heat change it's character?
    I feel that if it remains as it is then at the very least I will be eating the stuff with my pizzas for a long time to come

    Can anyone give me some reassurance or should I just pull it all apart and start again? [not for some time that's for sure]

    Thanks for your time
    Stuart Moore

  • #2
    Re: mortar problems??

    Calcium aluminate based mortar should set hard and fast. Hmmm.

    Can you give us any more details on A500. In the U.S., it is typically branded Lumnite.
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    • #3
      Re: mortar problems??

      Thanks for your prompt reply James. I bought the bag of cement from a local pottery supplies place in Aus who told me that an effective mortar would be 3:1 sand and cement. I chose to use the mix specified in the pompeii plans.

      The bag has no info on it other than stating it to be "A500 cement" followed by a batch number. When I google A500 cement I find it to be in common use for Chinese cements, one site gives this info;

      Features Specifications: High Alumina Cement A500
      1. A500:
      1>Chemical composition:
      Al2O3: 52. 0% min. Cao: 33. 5% min.
      2> physical properties: Fineness (0. 08mm residue) : 8% max.
      3>setting time (h: Min) : Initial: 0: 40min. Final: 6: 00max.
      4>Flexural strength (Mpa) : 1day: 5. 0min. 3days: 6. 0min.
      5>compressive strength (Mpa) : 1day: 50. 0min. 3days: 60. 0min.

      This is probably not the manufacturers site of the product I have but I thought would indicate the general specifications of this term.

      The mortar seemed to get unworkable very fast and pretty hard by the end of a days work so I was happy with the way things were going apart from the difficulty of having to be fast and constantly mixing. On completion of the dome I hosed the whole thing down as the day was warming up and I wanted to slow down curing and I am wondering whether the mortar may be reabsorbing moisture from surrounding masonry and becoming softer and more plastic but if it is that itself would be worrying.

      The dome seems to be holding together well with no sign of movement but I am reluctant to proceed to the next stages with the mortar looking like this. What do you think? should I pull it all apart?

      Thanks again Stuart


      • #4
        Re: mortar problems??

        Stu, yes A500 is ok, dozens of chinese refractory manufacturers make it.


        • #5
          Re: mortar problems??

          Does it feel as though the mortar has set and is continuing to cure? Is it getting harder over time?

          You should not try to re-hydrate aluminate mortar after it has started setting, but if is is curing and becoming hard, it should be OK.

          What do you think?
          Last edited by james; 09-10-2008, 07:50 PM.
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          • #6
            Re: mortar problems??

            Hi Stuart,
            I am currently constructing a pompeii oven in cambodia and i was only able to find CA50 cement, which also comes from china. People have told me that this is similar to A500. The composition of this is AL2O3 50%, SiO2 8% and another substance labelled in vietnamese as Pha Nuoc at 26%, (i have been unable to find out what this is). I was also given a white powder to mix to make this mixture stronger??. When i first mixed this is set like clay and i was able to push it out of joints after 24hrs. Then i was told to mix this with normal portland cement. Having mixed the Ca50, white powder, sand and cement at 3:1:1:1, it begain to set be remained very crumbly and when it was rained on (monsoon rains) it washed it all away. At the same time i tried mixing it excluding the mystery white powder it sets very well. I have tried heating and cooling it over the past few days and seems to remain strong. I have decided to leave out this white powder in construction, but i wont know if this is a good plan till too late. I am wondering if this powder is Lime as that should slow down the curing process, but i think other people would have had this problem if this was the case. But maybe you could try mixing without the lime and see if this is different, or try mixing some normal portland with this and see if it sets as my Ca50 has. I am trying to find out what the white powder is, it could be Cao that is a part of your mix but that i have to add myself which is about 33.3%. Having googled this it is calcium oxide (brunt lime), is it possible that this is the same as lime and thus too much is going into the mix if you add normal; lime aswell.If i find out any more i will let you know. sorry i couldnt be more helpful.


            • #7
              Re: mortar problems??

              Thanks everyone,
              I am now reconciled to demolishing my dome and starting again. The mortar remains powdery where dry and pasty when wet and appears easily removed. To benefit future would be oven masons I will detail what I did. The mortar I used was:
              1 part calcium aluminate cement [A500]
              1 part fireclay
              1 part hydrated lime
              3 parts fine crushed sand
              The fireclay is stated to be; ?A sandy mixture of fine and coarse solid particles with a melting point of 1200 degrees C and zero solubility with a chemical composition of Kaolinite 30-60%, Quartz 30-60%, and Illite 0 - 1%

              The mixture was made up in batches of less than 1 litre, mixed with about 2 parts water for about 2 minutes when it reached a consistency capable of being used then used for about five minutes to joint 2-3 half bricks at which time the remainder became too stiff and was used to pack the back of the joints. I made no attempt to rehydrate the mix.

              Now I have determined to start again I need to decide on the best mix of materials at my disposal. I have now belatedly mixed trial batches using all possible combinations of these ingredients.
              The weakest by far is that using all 4 of the above. This would reabsorb water after 24 hours and could easily be reduced to mush by pressing a small piece between two fingers.
              Next weakest was sand, cement and lime which required slightly more pressure to crush
              Next came sand, cement and fireclay which could not be crushed by [my] fingers but could be broken by twisting by hand
              Sand and cement was the strongest.
              The lime free mixes remain workable for far longer ? in excess of 1 hour

              Clearly this type of strength is not the only criteria for assessing the best type of mortar. Please give me feedback on this. It seems I have to eliminate lime from my mix ? it just doesn?t seem to work well with this A500 cement. Would that be OK? What is the purpose of including the lime? I guess I think of it as making the finished joint more pliable and thus not so liable to fracture ? is that correct? I could try reducing the quantity of lime

              Sorry about the length of this message. Any further suggestions gratefully received



              • #8
                Re: mortar problems??


                I think you want the lime - it gives the mortar a "plastic" nature when you are working it. Everything you are saying comes down to the Portland cement. With the whole world building, and the cost is going up - it's not inconceivable that someone is selling crap. The fire clay, although fairly important, should not cause the final product to turn to dust. It's got to be the cement.

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                • #9
                  Re: mortar problems??

                  Ah Les, if only I were trying to use portland cement! That really is another massive temptation - to abandon this calcium aluminate and go for plain old cement. I guess my feeling is that not all calcium aluminate cements are the same and that this popular formula from China, A500 just doesn't work well with lime. Maybe I will just go for Portland. Thanks for your input



                  • #10
                    Re: mortar problems??

                    How about performing a test? Mortar a couple of bricks together with different mixes and then grill them for a few hours and see which ones hold together?

                    Some refractory products don't get cured until you heat them...perhaps that is the case with the A500?

                    (disclaimer: I am no mortar expert)

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                    • #11
                      Re: mortar problems??

                      Woodsman try "cement fondue" made by Kerneos [ used to be made by Lafarge] very good c/ aluminate cement.


                      • #12
                        Re: mortar problems??

                        If you can find it, the Lafarge-like product will do it. It's called Lumnite in the U.S.

                        If not, and you decide to go Portland cement, make sure you add the fireclay. That is what will hold your mortart together over the years, and the Portland eventually starts to give up from the expansion and contraction.
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