People who make concrete counter tops use a lot of fibreglass fillers to get them fairly thin but if you wanted it truly light weight you’d probably need to make it out of a dense foam and coat it with something that looks like concrete.
There are a range of rendering mixes that look pretty much like concrete, or at least mortar, that would do the job. In fact just rendering it entirely (base coat, plastic reinforcing mesh, top coat(s)) should give a concrete-ish look while keeping things fairly light.
Concrete counter top mixes usually use either much smaller, or no aggregate and use more sand. The mixes resemble mortar more than concrete and they are typically a little harder and less forgiving to work with.
