What type of cement do i use to make my forge?

It depends a lot on what temperatures you will be getting from the forge. And that depends on the type of fuel and whether or not you will be using an air blast to get higher temperatures. I am a hobbyist blacksmith, I use coal with a bottom air blast and can get around 3000 deg. F (1650-1700 deg. C). My forge has a plywood bottom. I have regular cement, like for laying blocks or bricks, around the sides of the forge, but not on the bottom on the plywood. The wood doesn't burn because of the air blowing the heat up, the cement has a few small cracks in it. I set this forge up like this over 20 years ago and never had any problems with it.

You need to use kaowool then coat with satinite or other suitable refractory cement. Using refractory cement or fire bricks alone will cost you a lot of heat as they aren't true insulators. Since you didn't say what you intend, just blacksmithing you can get away with doing it that way. If you intend to forge weld you'll need all the heat that your forge can build.

Concrete never looses the water, when it hardens the water is trapped. When heated it will "pop" like pop corn. That can hurt or put an eye out pretty easy.