The　magnetic　susceptibility　and　turbidity　of　calcium　silicate　hydrate　(C-S-H)　gel　hydrothermally　synthesized　under　various　conditions　were　studied.　The　experimental　results　indicate　that　C-S-H　gel　with　diatomite　and　lime　has　an　optimum　time　for　hydrothermal　synthesis,　in　terms　of　susceptibility　(chi(m))　and　turbidity　(tau).　C-S-H　gel　hydrothermally　synthesized　for　the　optimum　time　has　higher　susceptibility　and　lower　turbidity,　i.e.,　the　gel　exhibits　high　dispersibility　(fineness)　and　numerous　structural　defects,　properties　that　provide　a　better　strengthening　effect　for　cements.　Susceptibility　and　turbidity　have　obvious　characteristics　and　can　be　used　as　indexes　of　quality　control　for　the　synthesis　of　C-S-H　gel.