The synthesis of CaO/Fe3O4 composites has been carried out using the coprecipitation method with a weight ratio of 1:1. CaO used from blood clam shells were calcined at 1000 °C for 6 hours. Analysis of CaO/ Fe3O4 composite results using BET, XRD, FTIR and VSM, then applied to the adsorption of congo red dye with variables of contact time, concentration of congo red dye and temperature. The res…