First off, you can buy fireclay at your local hardware store most likely. I know that the Home Depot near me sells it by the bag (probably around 50-80 lbs).
when I googled melting points for the different ingredients I found:
sand - melting point 1600C 2900f
Portland Cement - melting point 1200C 2200f
fire clay - melting point 1500C 2750f
assuming that the mixture is measured by weight I'm pretty sure that you can just average out the temperatures. (3*2900 + 2200 + 2750)/5 = 2730f
