filler coke 

https://doi.org/10.1351/goldbook.F02364
The main constituent of a carbon artifact, introduced as solid component (predominantly in the form of particulate carbon) into the 'carbon mix' from which polygranular carbon and graphite materials are obtained by heat treatment.
Note: Filler coke is not necessarily the only, but it is commonly the most important filler material used in a 'carbon mix' which consists of filler and binder.
Source:
PAC, 1995, 67, 473. (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 489 [Terms] [Paper]