How do I seal off my brick fireplace so I can install unvented gas logs?

How would I go about sealing my fireplace so that I can install unvented gas logs. I have a brick chimney that is a wood fireplace. The damper broke off a few years ago and it is stuck in the open position. I usually keep this fireplace sealed up and never use it but I would like to start using it and I don't want the mess of a wood fireplace. Should I use concrete or something else?