Ah, I’ve almost had an important oversight. Plaster is another good material that does not burn. Two formulations, clay-based and gypsum-based, do not contain any carbon. The limestone-based formulation does contain carbon since linestone is calcium carbonate, but don’t worry about this burning. However, plaster is not resistant to very high temperatures: it can melt or otherwise decompose unless it is designed to be heat-resistant.
And, of course! I should have realized this. Powder bed and inkjet head 3D printing processes use a gypsum plaster powder and apply water very much in a plaster-making process. So, if you 3D print your electronics cases with this material, you need not worry one bit about flammability and fire retardants.
20190309/https://en.wikipedia.org/wiki/Plaster
Vermiculite is a mineral rock that is popular for many aggregate or concrete uses and does not contain carbon, enough said.
20190309/https://en.wikipedia.org/wiki/Vermiculite
Mineral wool and asbestos are also fire resistant, but these ought to be avoided due to their risk to human health.
20190309/https://en.wikipedia.org/wiki/Mineral_wool
20190309/https://en.wikipedia.org/wiki/Asbestos