Volume Stability and Mechanical Property of Hardened Cement Paste under the First Drying: Effects of Water-to-Cement Ratio and Cement Type

We investigated eight hardened cement paste (HCP) mixtures with different water-to-cement ratios (W/C) and cement types (C3S/C2S ratio and Blaine fineness). We systematically measured pore structure evolution, C–S–H surface properties, mass and length changes, bending strength, and Young's modulus during the first drying. The strength and Young’s modulus exhibited a two-stage response, decreasing over 95%–40% relative humidity (RH) and increasing again over 40%–11% RH. Based on the Griffith–Irwin energy balance, the strength decrease in the 95%–40% RH range was accurately reproduced using only the changes in surface free energy and porosity. In contrast, in the 40%–11% RH range, C3S-rich, high–W/C systems showed pronounced hardening of the HCP skeleton, accompanied by a corresponding increase in strength. For drying shrinkage, we quantitatively demonstrated that different mechanisms dominate in the 100%–40% RH and 40%–11% RH ranges using a model that incorporates the low-density/high-density C–S–H volume fractions and the water-vapor BET specific surface area per unit HCP volume. These results provide a unified microstructural basis for understanding the coupled evolution of volume stability and mechanical properties of HCP during the first drying process.