Dual-functional waste-derived geopolymer scaffold for low-supercooling shape-stabilised phase change materials in energy-efficient building envelopes

Porous geopolymers are increasingly being explored for lightweight, fire-resistant building components, thermal insulation, and passive thermal management, as their pore architecture and surface chemistry can be readily tailored to meet specific performance requirements. Motivated by these building-envelope applications, we developed waste-derived shape-stabilised phase change materials (ss-PCMs) for sustainable building energy conservation; however, severe supercooling frequently compromises the reliability and consistency of latent heat release. In this study, a dual-functional open-cell geopolymer skeleton derived from red mud and coal fly ash was engineered to function as both a physical confinement matrix and a chemical nucleating surface for a lauric acid–1-hexadecanol eutectic PCM. The optimised scaffold (RCBG) exhibited a porosity of 75.8 ± 0.5%, a thermal conductivity of 0.132 ± 0.007 W m−1 K−1, and a compressive strength of 4.37 ± 0.13 MPa, thereby enabling the reliable fabrication of the RCBG–LH composite. The hierarchically porous RCBG shifted the phase-change window into the indoor comfort range (20–25 °C), where crystallisation onset increased from 19.61 °C to 22.8–23.4 °C. Melting onset decreased from 30.90 °C to 27.4–28.1 °C, and the degree of supercooling was reduced from 11.29 °C to 4.02–5.32 °C. The composite delivered a latent heat of 114.37 J g−1 and exhibited effective thermal conductivity of 0.284 ± 0.004 W m−1 K−1. EnergyPlus simulations demonstrated a reduction in heating demand while maintaining annual indoor temperature fluctuations within approximately 1 °C.