From memory we used to use a jam jar full of the stuff for 1 bag of cement (50Kg). That seemed about right. The mix we used to use was 3:1.5:1 by volume of 15 mm granite chips, sand and portland cement. We kept water to a minimum. I think it was around 20 litres maximum for that, but with all such mixes, that assumes your sand to be dry, which it rarely is. The minimum water was to ensure maximum density. Of course with fire resisting material you don’t need density, quite the reverse.
One problem is to make sure that the material you are using won’t react with the steel and destroy it through corrosion. How you protect against that, I have no idea. I don’t think plaster is particularly aggressive.
