ATH Aluminium Tri-Hydrate filler powder.
Martinal ON921, 300 mesh. Will add fire retardency to any resin, however its worth noting that high levels would be required to achieve a good, low flame rate.
Morover this product lends itself to filling PU Fast Cast quite effectively, reducing its shrinkage to negligible amounts, improving its surface durability and temperature ratings!
White in colour, will pigment resins by itself to some degree.