Search for information on the terms "recrystallized silicon carbide" and "recrystallized silicon carbide ceramics". This material has been known for a long time and you can try to find paper literature about its production. It goes without saying that the companies producing this material today do not publish such information.
many thanks for asking this interesting technical question. In this context please have a look at the following potentially useful links which might help you in your analysis:
Unfortunately the patent is written in Korean, but the Abstract is in English. It already contains the most important details about the raw materials composition for SiC production.