This　research　investigates　the　electromagnetic　properties　as　well　as　the　microscopic　mechanisms　of　tricalcium　silicate　(C3S)　pastes　maintained　for　a　long　time　at　different　relative　humidities　(RH).　The　electromagnetic　transport　properties　and　electromagnetic　parameters　of　undried　and　dried　specimens　of　C3S　paste　specimens　cured　for　180　d　at　four　RHs　were　tested　for　microscopic　property　changes.　The　results　show　that　the　water　content　significantly　affects　the　electromagnetic　properties　of　the　specimens.　With　the　increase　of　RH,　the　electromagnetic　wave　reflectivity　and　absorption　of　the　specimens　increased　and　the　transmission　decreased.　Based　on　the　changes　in　microphase　properties　it　was　found　that　the　hydrated　calcium　silicate　(C-S-H)　gel　had　the　greatest　effect　on　the　electromagnetic　properties　of　C3S　paste.　In　addition,　the　increase　in　RH　also　resulted　in　a　significant　increase　in　the　volume　percentage　and　area　percentage　of　mesopores　in　the　specimens,　which　in　turn　increased　the　electromagnetic　wave　loss.　This　study　provides　a　microscopic　explanation　for　analyzing　the　effect　of　RH　on　the　electromagnetic　transmission　properties　of　cementitious　materials.　©　2025　Elsevier　Ltd