Effect of sorbed water on some mechanical properties of hydrated portland cement pastes and compacts

The chief aim of this work was to observe any change in the mechanical properties of cement pastes and compacts when water is sorbed. Thin discs of normal cement paste were cut from 1 1/4-in. diameter cylinders made of 0.3 to 0.7 w/c ratios. Compacts of similar shape were prepared from bottle- hydrated cement at pressures of 15,000, 40,000 and 110,000 psi. All samples were conditioned to different levels of RH from 0 to 98 percent and back to 0; at each level the modulus of elasticity of all samples and the strength of some were determined by means of a miniature testing machine, which is described. Special precautions were taken to avoid any carbonation of the samples by conditioning them over solutions of sodium hydroxide and measuring in gloved conditioned boxes. No change in elastic modulus occurred between 0 and 50 percent RH, but an increase was observed above 50 percent RH. This was maintained on desorption, and appeared to be associated with entry of interlayer water in the tobermorite gel, which supports the mechanism of length change previously proposed by the authors. The maximum fracture-strength for samples of given porosity was attained at 0 percent RH, and the largest reduction of strength was observed from 0 to 15 percent RH. Plots of Young's modulus vs porosity and fracture-strength vs porosity gave common curves for compacts and pastes.