The Effect of Salt-Impregnation on Thermochemical Properties of a Metakaolin Geopolymer Composite for Thermal Energy Storage

Effective and efficient recovery, storage, and reuse of heat, together with renewable energy, play an indispensable role in decarbonising the built environment. Thermochemical energy storage materials possess the highest volumetric energy density compared to latent and sensible heat storage materials under similar conditions. However, conventional thermochemical energy storage materials face several challenges including high cost, low sustainability, and limited heating power. Alkali-activated metakaolin (geopolymer) containing alkali aluminosilicate hydrates (N-A-S-H) has been shown to have a considerable thermochemical heat storage capacity at medium temperatures (below 400 ℃) but is less efficient at low-temperatures (<200 ℃). Here we investigated a salt impregnation method to form geopolymer composites for improving their thermochemical heat storage capacity at low charging temperature. More specifically, the effect of CaCl₂-impregnation on the composition of the composites was examined with X-Ray Diffractometry and Fourier transform infrared (FTIR) spectroscopy. The dehydration enthalpy and volumetric energy density of the composites were assessed with differential scanning calorimetry (DSC), while dynamic water sorption (DVS) was used to study their water cyclic water sorption capacity. It was shown that short-time salt-impregnation can improve the heat storage capacity of the geopolymers, while a prolonged exposure to the salt solution can have adverse effects.