Copper　slag,　which　contains　Fe-rich　fayalite　(Fe2SiO4),　is　a　valuable　solid　waste　that　warrants　further　research　in　order　to　recover　iron.　Calcium　oxide　(CaO)　can　significantly　enhance　iron　recovery　from　copper　slag;　however,　the　associated　mechanism　has　not　yet　been　explored.　In　this　study,　we　investigated　the　interaction　between　CaO　and　Fe2SiO4　to　obtain　detailed　understanding　of　the　role　of　CaO　in　enhancing　iron　recovery.　The　presence　of　CaO　was　found　to　accelerate　the　decomposition　of　Fe2SiO4　via　an　ion-exchange-like　process.　Specifically,　CaO　dissociated　into　Ca(II)　and　a　Ca-deficient　Ca1-yO　species　at　high　temperatures.　The　Fe(II)　ion　at　the　M2　site　of　Fe2SiO4　was　substituted　by　the　released　Ca(II)　ion,　resulting　in　the　formation　of　[(Fe(2-x)Cax)SiO4]xFe(II).　Subsequently,　the　substituted　Fe(II)　occupied　the　Ca　vacancy　in　Ca1-yO　to　form　(Ca(1-y)Fe(II)y)O.　The　disproportionation　of　Fe(II)　and　the　combination　reaction　between　CaO　and　the　SiO2　separated　from　Fe2SiO4　led　to　the　generation　of　the　final　products,　viz.　Fe2O3,　Fe3O4,　and　CaSiO3.　This　study　explains　the　specific　role　of　CaO　in　decomposing　Fe2SiO4.　It　would　not　only　provide　theoretical　guidance　for　iron　recovery　from　copper　slag　but　also　present　a　new　perspective　on　the　recycling　of　valuable　resources　from　many　other　smelting　slags　(e.g.,　iron　slag,　lead　slag,　and　nickel　slag).　©　2020　Elsevier　B.V.