I assume you mean concrete, of which portland cement is a component. It seems to be a widespread belief that concrete would make a well damped speaker enclosure. What could be more solid that what sidewalks are cast from, right? The fact is concrete rings pretty significantly when cast in a thin, monolithic structure as shown. Not to mention the need for reinforcement. To get it to a place of non-consequential resonance it would need to be heavily damped, which mitigates the original notion of suitability for intended purpose.
My suggestion would be to use something else as your baffle. If you want to cast it and are willing to make a mold, epoxy or some other cast-able resin might be a better choice. One could even add damping material directly into pour, but I imagine some testing would be prudent. Easier might be a stacked lamination of MDF or Baltic Birch, orienting the laminations top to bottom.
Off the top of my head, the most difficult part in either method might be getting the driver holes positioned and aligned correctly. Here's an illustration of lamination I was thinking of.
