Cut a Styrofoam block the same diameter and shape as the sink to make a barrier/dam to preserve the proper opening for the sink. Ensure that the vertical sides of the foam are slightly greater than the intended thickness of the concrete countertop to prevent the wet mortar from burying the Styrofoam block. Taping the edges with electrician’s tape will help prevent the concrete from sticking to the edge of the Styrofoam and give it a little support.
