Cool, thanks. I had seen some concrete planters at the Barbican which I wanted to reproduce but thought you might need special concrete for smaller pieces
AIUI for sculpture and for small-scale work some art people add plasticisers and/or strengthening materials (fibres etc.), which allow you to use less water and get finer surface detail, but for planters etc. standard mixes are fine.
Its blue circle ready mixed, but it wasn't an informed decision, just what was in b&q