Form-stable PCMs in lime renders: From particle-matrix interactions to thermal buffering and durability

The development of thermally responsive lime renders has been constrained by the lack of materials capable of storing and releasing heat without compromising compatibility, durability or the microstructural integrity required in both modern and heritage applications. Although form-stable phase change materials (PCMs) offer a promising route for latent-heat storage, their behaviour within lime matrices remains insufficiently explored. This study addresses this gap through a comprehensive, multi-scale investigation of lime renders incorporating silica-supported PCMs, linking particle-level interactions to thermal performance and long-term durability. Two PCMs with distinct melting temperatures (5ᵒC and 25ᵒC) were integrated into optimised lime formulations. The results reveal that the silica carriers exhibit excellent compatibility with the lime system, enabling uniform PCM dispersion and preserving a continuous and stable microstructure. Contrary to concerns commonly associated with PCM integration, the modified mortars improved their mechanical performance and displayed no disruption to the pore network essential for durability in lime-based materials. Both PCMs remained fully functional within the hardened matrix, providing reversible latent-heat storage and measurable temperature buffering under dynamic conditions. Enhanced thermal moderation was achieved without compromising durability: PCM-modified mortars outperformed control samples under freeze-thaw cycling and salt crystallisation, with the strongest improvements observed in formulations containing metakaolin. The PCMs themselves demonstrated excellent cycling stability, preserving their thermal response over repeated phase transitions. These findings demonstrate that silica-supported PCMs can be effectively integrated into lime renders to deliver meaningful thermal regulation while maintaining compatibility, structural integrity, and long-term performance, highlighting their potential for energy-responsive and heritage-compatible construction materials.