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My bathroom is a couple mm of microcement over Schluter Kerdi-Board foam, it's fairly strong. I think it can hold a laptop no problem.

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.

> if you wanted it truly light weight

Aircrete with a good sealant because obviously it would be even more porous than regular concrete https://www.youtube.com/watch?v=z4_GxPHwqkA