Waste-derived calcium chloride hexahydrate phase-change hydrogel composite for energy-efficient building thermal regulation

Decarbonising building energy use requires thermal energy storage materials that can reduce heating and cooling demand while remaining economically viable and resource efficient. Herein, a waste-derived calcium chloride hexahydrate phase-change hydrogel was developed for building envelope applications using recovered calcium chloride as the primary raw material. Bentonite, strontium chloride hexahydrate, and a three-dimensional polyacrylamide network were incorporated to improve nucleation, structural stability, and leakage resistance. The phase-change hydrogel was also characterised for its thermophysical properties and cycling stability, and its building-envelope performance was evaluated through thermal tests and energy assessment. The developed material exhibited a very low supercooling degree of 0.3 °C and a high latent heat of 173.2 J/g while showing good cycling stability by retaining over 86% of its latent heat with no observable phase separation after 600 cycles. When incorporated into building envelopes, a 20 mm layer reduced the average peak indoor temperature by 5.6 °C. Upon testing on 175 representative locations in China, the annual heating and cooling electricity saving ratio reached 3.0–17.5% in hot-summer–warm-winter zones. Furthermore, the developed material also exhibited a substantial cost advantage, with an estimated unit cost 38.6–90.3% lower than that of commercial alternatives. These results demonstrate that the waste-derived hydrogel offers a practical and economically viable approach to building thermal energy storage, with particularly strong potential in cooling-dominated regions.