Mitigation of Drying Shrinkage in Cement–CWP Composite Mortar: Effects of CWP Content, W/B and Curing Conditions

Drying shrinkage cracking of hydraulic cementitious materials, induced by moisture loss under varying environmental conditions, significantly compromises structural durability. The utilization of construction waste powder (CWP) in cement composites presents a sustainability opportunity, but its impact on shrinkage behavior remains poorly understood. This study aims to systematically investigate the drying shrinkage characteristics of cement-CWP composite mortar and to identify optimal mix proportions and curing conditions for shrinkage control. A series of experiments were conducted on mortar specimens with varying water-to-binder ratios (W/B = 0.45, 0.50, 0.55) and CWP incorporation rates (0, 5%, 10%, 20%). Three curing regimes were employed: outdoor curing, standard curing (20 °C, 95% RH), and outdoor film curing. Drying shrinkage was monitored over time. Key findings indicate that the optimal CWP content for shrinkage mitigation is 10%. Excessive CWP (>10%) induces a “weak bonding” effect, leading to an increase in shrinkage due to reduced cohesion. Increasing the W/B ratio to 0.55 effectively reduced shrinkage, with the minimum shrinkage value observed at this ratio. Among curing methods, outdoor film demonstrated superior performance in maintaining moisture and suppressing shrinkage. Predictive modeling revealed that the logarithmic model in accurately capturing the nonlinear evolution of shrinkage over time, effectively reflecting the influences of CWP content, W/B ratio, and curing condition. The drying shrinkage of cement-CWP composite mortar can be effectively optimized by incorporating 10% CWP, utilizing a W/B ratio of 0.55, and implementing outdoor film curing. This paper reveals, for the first time, the dual-mechanism regulation of early-age drying shrinkage behavior in cement-based materials by CWP as a supplementary cementitious material and establishes a shrinkage prediction model applicable to various mix proportions and curing conditions, offering practical strategies for enhancing the durability of sustainable construction materials utilizing construction waste powder.