If i use a surface retarder on the surface of concrete can i then add more concrete on top and still achieve a homogeneous slab?

Yes, using a surface retarder allows for a delay in the setting of the top layer while preventing the topmost layer from bonding with the lower layer. However, to achieve a homogeneous slab, it's essential to properly prepare the surface of the cured concrete by cleaning it and possibly roughening it to ensure good mechanical interlock. Additionally, applying a bonding agent can enhance the bond between the layers and help achieve a more uniform slab.