The sludge dewatering performance depends on the chemical and physical characteristics of sludge cakes. To investigate the relation between the sludge cake structural properties, the protein content in loosely bound extracellular polymeric substances (LB-EPS) and the dewatering performance of sludge, the sludge dewatering ability of a combined aluminum-ferrous-starch flocculant (CAFS) was evaluated for capillary suction time (CST), settling volume percentage (SV30), specific resistance to filtration (SRF), time to filter (TTF), dryness and the LB-EPS protein content. The dewatering mechanism was also probed by investigation of the compressibilities, SEM images and structural properties of cakes. The results showed that CAFS could break sludge colloids easily and release more LB-EPS proteins. Cake microscopic structures and compressibility analyses indicated that the CAFS helped to form large incompressible dense cakes with discontinuous surfaces, containing a better adsorption capacity, which were closely related to the synergistic effect of cationic groups and starch mesh chains. (C) 2017 Published by Elsevier Ltd.