The　main　hydration　product　of　cement　is　C-S-H.　C-S-H　has　a　great　influence　on　the　electromagnetic　properties,　but　there　are　few　studies　on　the　independent　study　of　the　electromagnetic　properties　of　C-S-H.　Therefore,　nanocrystalline　C-S-H　was　synthesized　by　CaO　and　nano-SiO2,　multi-scale　analysis　method　was　adopted,　and　the　electromagnetic　properties　of　its　hydration　products　were　analyzed　from　the　mechanism.　C-S-H　powders　with　Ca　to　Si　molar　ratio　(C/S)　of　0.8,　1.2,　1.6,　2.0　and　2.5　were　synthesized　and　tested　by　the　coaxial　method.　Four　types　of　materials　were　defined　in　this　paper:　HPB　(composed　of　nanocrystalline　C-S-H,　Ca(OH)2,　CaCO3　and　pore　phase);　CSH　(composed　of　nanocrystalline　C-S-H　and　pore　phase　with　diameter　less　than　5　nm);　P-CSH　(composed　of　pore　phase　and　CSH　phase);　C-CSH　(composed　of　Ca(OH)2,　CaCO3　and　CSH　phase).　The　complex　permittivity　and　complex　permeability　of　HPB　with　different　C/S　at　2-18　GHz　were　tested.　The　influence　of　molecular　structure,　interlayer　physically　bound　water,　phase　types　and　content　of　HPB　on　the　electromagnetic　properties　were　investigated　by　XRD,　MIP,　N2　adsorption,　TGA　and　29Si　NMR　spectroscopy.　The　results　showed　that　with　the　increase　of　C/S,　the　permittivity　of　CSH,　P-CSH,　C-CSH　and　HPB　were　decreased.　Both　Ca(OH)2　phase　and　pore　phase　can　reduce　the　permittivity　and　loss　tangent　of　HPB.　It　was　worth　noting　that　the　reduction　degree　of　pores　phase　on　the　permittivity　of　the　HPB　were　about　4　times　than　that　of　Ca(OH)2　phase　(or　CaCO3　phase).　Meanwhile,　the　results　confirmed　that　the　carbonization　of　Ca(OH)2　phases　had　no　effect　on　the　elec-tromagnetic　properties　of　HPB.