The plan is to build a one dm deep shelf (approximately) that goes from the floor and up half a meter. It will be built against the existing light concrete wall (perhaps partially other material), and at first, we thought to just continue with light concrete. Then we thought we might want to make the wall a bit easier to undo/adjust in the future. What can be used then? It should hold tiles and some shampoo bottles. You might notice that there's a small gap between the bathtub template and the wall? That's where the wall will be.
If you build up a "loose" low course of bricks in front of the wall with lecablock or similar, it is fairly easy to knock it down with a hammer and chisel if you change your mind.
