How to make custom concrete brick?
If purely decorative, plaster-of-Paris (aka gypsum paster) sets quickly, takes detail from molds well, and paints well.

The brick link you provided is for "sun-dried mud brick" or "adobe" (more or less) - I suspect plaster would be at least as strong as that. Normal clay brick is kiln fired (your home oven won't do it, but odds are there's a potter or 3 in your town with kilns.) Any clay brick requires some drying time (or it will crack if dried too swiftly) unless it is pressed (at very high pressure) from dry powdered clay. 

Brickmaking link