Cement-based composite materials for thermochemical heat storage

Nowadays, the need of thermal storage is paramount to let the renewable energy sources be competitive and reliable in the energy market. Thermochemical heat storage, especially sorption heat storage, has a strong potential to store huge amount of thermal energy with theoretically no losses. Water absorption of salt hydrates guarantees the highest energy density, up to around 3 GJ/m3 for MgSO4∙7H2O, but lacking in good mass and heat transport properties. On the other hand, pure adsorbents such as zeolites 13X assure a better mass and heat transfer due to their better hydrothermal stability, however having lower energy densities (around 0,6 GJ/m3) and high prices. Composite materials combine the above-mentioned materials properties, achieving acceptable energy densities and low costs. In this work, composite materials made by cement paste (host matrix) and various salt hydrates are investigated. Cement is known to be a porous material, easily available and cheap. At material level, porosity of pure cement paste is studied, performing a screening of cement properties in a wide range of water to cement ratios, by means of geometric density, N2 physisorption analyses (BET and BJH methods) and compressive strength tests. The porosity was proved to increase increasing the w/c ratio. However, beyond a certain value the synthesis undergoes the phenomenon of bleeding. This segregation phenomenon affects the linear dependence of cement properties on w/c ratio, for w/c > 1. Therefore, this value was taken as main benchmark for the composites synthesis. BET and BJH classified cement as a mesoporous material, with pore distribution similar to silica-gel, having specific surface area of w/c = 1 sample around 10 m²/g. The salts investigated are CaCl2 and MgSO4, due to their high energy density and widespread usage. Along with the more common impregnation via aqueous solution, an innovative method called in-situ synthesis was performed to load the matrix with salt. This method takes advantage of pure cement synthesis itself: instead of distilled water, a saline aqueous solution is used. Once the cement hydration process ends, the salt inside the solution should be present as a precipitate inside the matrix pores. Unlike classic impregnation process, this method allows for knowing exactly the amount of salt that should be present inside the matrix, for performing only one process and should have a high degree of repeatability. Nonetheless, reactions occurred between salts and cement powder components. XRD analysis was performed to check the in-situ samples composition. The more promising materials underwent a preliminary energetic analysis, simply pouring water over around 20 g of composite. Interesting ΔT were experienced, up to 30°C. Even dried pure cement showed a low ΔT when hydrated with water. An energy density estimation was also performed by means of DSC analysis, obtaining values around 0,3-0,4 GJ/m³, with desorption temperatures of 120°C for CaCl2 composite, two steps 95-160°C for MgSO4 composite. Equilibrium adsorption isobars were performed on one particular synthesised composite and, thanks to Polanyi-Dubinin theory, a potential thermodynamic cycle of thermal storage was evaluated. In conclusion, those composites show interesting features and, even though their specific surface area and energy density values are not outstanding, their low price may allow them to be further investigated as an attractive alternative to zeolite or silica-gel.