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.