I built my oven 5 years ago and left the surround as plain 1/2" cement board. As you can see below its getting pretty beat up. I want to cut out about 4" surrounding the opening and put in some form of a surround. I'm thinking of either casting a solid piece and putting it in or getting a limestone substitute material and doing four pieces with mitered and mortared corners.

Here are my questions

1) given the expansion and contraction of the heat which would hold up the best: the solid surround or the four separate pieces?
2) if I cast something should it be refractory concrete or is regular concrete OK?
3) How do I adhere it? If I cut back the cement board it will expose the concrete face of the oven opening and the brick hearth face. Is there something that can bond the new surround to those materials?

Thanks in advance.