Castable Refractory does look like regular concrete mix, but they are definitely different beasts!
Unlike regular 'ol concrete mix, Castable Refractory can withstand temperatures up to 3000 degrees. And that’s about 2000 degrees hotter than you will ever get your oven! Regular concrete would crumble under those extreme temperatures.
On the flip side, Castable is not as forgiving, nor as easy to use as regular concrete mix.
With concrete mix, you can typically eye-ball the consistency of the water-to-mix ratio as the concrete is mixing in a wheelbarrow or mixing machine. If you need to add more water, you just spray it in, but if you add too much, you just add more cement mix to thicken it up a bit.
Castable Refractory on the other hand has a very strict water-to-castable ratio that is set forth by the manufacturer (so be sure to ask for mixing directions when you purchase your Castable).