Performance Recovery of Phase Change Materials (PCMs)-Modified Limestone Calcined Clay Cement (LC3) Composite Through Air-Void Control Using a Silicone-Based Defoamer

Limestone calcined clay cement (LC3) has emerged as a promising low-carbon alternative to ordinary Portland cement (OPC) due to its reduced clinker content and associated carbon footprint. In parallel, integrating phase change materials (PCMs) into cementitious composites offers a pathway to enhance building thermal regulation through latent heat storage. However, hydrophobic PCMs may adversely affect fresh workability and hardened performance, particularly by promoting entrapped air and increasing porosity. This study investigates the incorporation of capric acid (CA) as an organic PCMs through partial cement replacement (0%, 1%, 5%, and 10% by mass) in OPC and LC3 mortars. In this context, the mortar is a functional organic–inorganic composite, where the organic PCMs phase contributes thermal energy storage functionality. Unlike most PCMs studies focused on OPC systems, this work emphasises LC3 and identifies air-void control as a critical mechanism for performance recovery in LC3–PCMs composites. It also evaluates the effectiveness of a silicone-based defoamer (0.15% by mass of the total mixture) in mitigating air-related performance losses. Fresh flowability, hardened density, ultrasonic pulse velocity, and compressive strength were determined up to 28 days. Results showed that increasing CA content reduced flowability, density, UPV, and compressive strength in both OPC and LC3 systems, indicating that hydrophobic PCMs inclusion adversely affected matrix continuity. However, the incorporation of a silicone-based defoamer enabled performance recovery in both OPC and LC3 composites by improving matrix compactness through air-void control. Overall, the results demonstrate that air-void control is critical for PCMs-modified mortars, and that defoamer addition provides a practical approach to improve the performance of LC3–PCMs systems while maintaining their sustainability benefits.