Mechanical properties of calcium silicate hydrates (C-S-H) with C/S ratios of 0.75, 1, and 1.33 were examined with nanoindentation after gamma-adsorbed doses of 0.145, 0.280, 0.500, and 0.784 MGy, and were compared with control samples. Young' s modulus and stress relaxation tests showed no apparent trend with irradiation dose. Qualitatively, most of the irradiated samples were found to relax more than their respective controls, but not always in a statistically significant manner. Most of the Young' s modulus irradiated–control pairs showed marginally higher stiffness in the irradiated samples, but overall trends with irradiation dose were not obvious. Creep compliance was obtained for the samples irradiated at the highest dose and their respective controls. Two of the three irradiated samples exhibited less creep than their respective controls, but only one of which was statistically significant. The lack of clear changes in mechanical properties for these samples correlates with separate chemical analyses that showed no loss of interlayer water by exposure to irradiation or changes in the mean silicate chain length. Further research evaluating higher doses (25 and 200 MGy) representative of those received by concrete structures in nuclear power plants at prolonged operation is being carried out to complement the present study.