Evaluating the durability and cyclic thermal performance of lime mortars with microencapsulated PCMs for sustainable energy solutions

Phase change materials (PCMs) have emerged as promising additives for lime renders aimed at moderating indoor temperatures and reducing heating and cooling demand in retrofit applications, including Built Heritage. However, existing studies largely report durability as a macroscopic pass-fail outcome and emphasize initial thermal benefits, with limited evidence on whether thermal functionality is retained after severe environmental ageing. This study addresses this gap by combining durability assessment with an integrated evaluation of post-exposure changes in microstructure, mechanical performance, and, critically, the conservation of thermal behaviour and thermal cyclability. Twelve lime render formulations incorporating microencapsulated paraffin PCMs with melting temperatures of 18 and 24 °C at 5, 10 and 20% by weight of lime, with and without 20% metakaolin, were investigated. Specimens were subjected to natural weathering, freeze-thaw cycling and salt attack, followed by post-durability characterization of microstructure, compressive strength and thermal performance. Functional thermal stability after ageing was assessed through a thermal agreement metric that directly links durability exposure with the preservation of functional thermal behaviour, enabling evaluation of whether thermal performance is merely present initially or effectively retained over time. In parallel, the retention and cyclic stability of latent heat storage and release were evaluated at both material and laboratory envelope scales. The results showed, first, that metakaolin-containing mortars exhibited significantly enhanced durability, successfully withstanding freeze-thaw and salt attack, whereas formulations without metakaolin failed prematurely. Following durability exposure, these optimized formulations displayed a refined pore network and preserved or improved mechanical performance, with compressive strength increases of up to 460%. Regarding the preservation of thermal functionality, PCM-metakaolin formulations generally exceeded 90% thermal agreement and reached 100% in several cases, while maintaining stable heat storage and release under repeated cycling. Overall, the results demonstrate that, when appropriately formulated, PCM-lime renders can combine durability with persistent and cyclable thermal performance, supporting their feasibility as long-lasting solutions for energy-efficient rehabilitation. • PCM-MK mortars maintained matrix integrity after durability exposure. • Post-durability compressive strength increased by up to 460%. • Thermal behaviour was preserved after weathering, freeze-thaw and salt exposure. • Thermal cyclability contributed to sustained energy efficiency over repeated cycles. • Optimised matrix-PCM design delivered integrated durability and thermal performance.