The present invention relates to a system of adjuvants for concrete comprising a superplasticizer, such as a polyalkoxylated polycarboxylate comb polymer; an accelerator selected from calcium salts; and a rheology extender, which is a polyalkoxylated polyphosphonate. The system of adjuvants is particularly useful for prefabricated concrete, which is used for producing concrete building elements in series such as slabs, walls, beams, frontage panels. Methods for preparing prefabricated concrete using this system of adjuvants are also provided.