[ophic]

You want something stronger than concrete????
High strength concrete then, perhaps with fibre reinforcement mixed in. You can add a bit of PVA glue to allow it to adhere better to surroundings.

I wouldn't expect anyone to be able to get past a hard concrete mix, such as 3:1, with reinforcing fibres in it. Is the design poor? Is it such a small piece of concrete you could just lift out the whole thing? If so the solution is simple... more concrete!