DIY Castable Refractory Version 2.0

Prototype Gasifier Build
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Castable Refractory Lessons Learned

Larry Dobson has brought to my attention that the formula I used (Portland Cement, Fire Clay, Silica, and Perlite) may not withstand the 1500-1700 deg. F temperatures of the Gasification zone.   I researched online looking for others that may have used portland cement in a high temperature forge, kiln, or oven.  I found people that had successfully used “some” portland cement mixed with the fire clay as a binder.  Others had bad experiences with the portland cement breaking down during operation over 1000 deg. F.

It seemed that the successfully users of the portland cement used a little as was required to bind the other higher temperature components together.    I saw a suggestion that you can purchase the cheap beninite kitty litter and crush it into a powder.  It was said this beninite clay would be stable at higher temperatures then the portland cement.

The third  (highest temperature) recipe just eliminates the portland cement / clay all together.  Apparently the Fire clay is sufficient to bind the silica and perlite together.  This mixture will probably be the most brittle, so the user will need to be careful  when loading the fuel hopper.

So which recipe will I test next?

For my second refractory hearth I will probably use a small of amount of bentonite clay clay along with the fire clay, silica, and perlite.   Some Forge builders cautioned against using perlite above 1500 deg Celsius (2732 deg. F) , but this is much higher then the average gasification temperature, so it should be fine for this application.

I am going to cut the sheet metal form off of the first prototype refractory, so I do not need to re-weld a new concrete form.  I will re-close the mold by welding flange tabs to the cut seam.

You can see what happened when I removed the metal form.  I couldn’t get the thick cement to pack down in the form.  Next time I am going to press the cement on the inner coneand then will force the outer cone over the outside.  This will allow me to ensure the cement is firmly packed to create a stronger cement casting.

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2 comments… add one
  • John, didn’t you mean to say, “…the formula I used (Portland Cement, Fire Clay, Silica, and Perlite) may (NOT) withstand the 1500-1700 deg. F temperatures of the Gasification zone.” ?
    Since the hearth region is the most susceptible to breakage from falling logs, etc., I strongly recommend using the strongest ready-mixed commercial castable you can get, rather than fooling around with a much weaker DIY mix. Pearlite is used for much weaker insulating castable, and not, in my mind, suitable for this application. I have experimented with many different castable ceramic mixes, and now only use low-water, low-cement (high-temperature Alumina-Silicate, not Portland cement, bentonite or fireclay) refractory ceramic. Sparcast LC 32-AL, for example, is a high-alumina castable with all ingredients precisely sized to create a low-porosity, high-strength (modulus of rupture around 3,000psi at 1500F) casting. Despite the addition of wetting- & flow-agents , with only 5-6% water used, it must be vibrated into the mold. I have described the vibrator design in an earlier comment here. You are not trying to substitute mild steel for stainless, why reinvent the tough ceramic that industry has improved over many generations? A 55lb sack will cost you about $50. Go for it!

    • Jfedock

      Ok then… I don’t have much to add to larry’s comment. I’ll just buy their cement more details to follow.

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