The firebrick repair cement we use in the factory is "Liquid Kiln Coating & Repair Cement,” available in pint bottles. This cement holds very well.
Several miscellaneous repair pointers:
We have found that the thinner the cement seam, the better. Try to make the two firebrick surfaces match as closely as possible.
Spray water onto both firebrick surfaces before cementing. Tony Rodriguez, a kiln repairman, recommends adding soap to the water. Mix 1 drop of liquid dishwashing soap to one cup of water. The soap reduces surface tension of the water to help it absorb better into the bricks. Use a small spray bottle.
Sometimes kiln users fill in the low spots of their firebrick bottoms with repair cement. The cement breaks out from the firebrick when applied full-strength as a filler. If you need to fill in a gap, use Kaolin Grog mixed 1:1 with cement. Kaolin Grog, available in 5 lb. bags, is powdered firebrick dust. With kaolin grog, the filler more closely matches the coefficient of expansion of the firebrick and stays in place.