The fracture behavior of six well-characterized synthetic preparations of C-S-H and C-S-H/CH mixtures was investigated. The C/S ratios of the C-S-H ranged from 0.68 to 1.49. Log crack velocity vs. stress intensity (log V vs. K 1 ) curves were obtained for compacted double-torsion specimens prepared with a wide range of porosity values. The dependence of the critical stress intensity factor K c on porosity, C/S ratio, and polymer/dimer ratio of the C-S-H was determined. The results indicate that an optimum mixture of short and long silicate chains is required for maximum fracture resistance of C-S-H.