From most articles and posts I have read the usual material is Fire Cement and Perlite but I have also seen reference to a Plaster and Sand mix. My construction will uses 44 gal drum so the requirement quantity is quite large. Where I am located cement is about 1$ a Kg and Perlite is about twice that. A Plaster and Sand mix would be much cheaper but is it as effective as Fire Cement and Perlite. I would appreciate any suggestions/advice fellow posters can offer.