## 1 Introduction  
In recent years, the global issues of excessive energy consumption and high greenhouse gas emissions have threatened the human living environment [1; 2; 3]. The energy consumption of the construction sector (residential and commercial) currently accounts for more than 30% of total global energy consumption, and this figure is up to 40% in developed countries [4]. At the same time, the increasing demand for energy used in the cooling and heating of residential and commercial buildings to maintain human comfort has hugely impacted natural resources and the environment. As such, improving the energy efficiency in buildings is crucial to solving the problems. Solar energy is the most important source of renewable energy available today. Different technologies such as solar photovoltaic systems have been developed to apply solar energy in buildings to ease the current shortage of energy, but their efficiency is not satisfactory. Phase change materials (PCM), having a high latent heat energy storage and isothermal behavior during heat charging and discharging processes, can be used as thermal energy storage materials in buildings to reduce temperature variations, as well as to regulate the indoor temperature to coincide with the human comfort zone (20-28 \({}^{\circ}\)C) [5,6]. Apparently, incorporating PCM into gypsum [7] or the cement-based building elements [8, 9, 10, 11] is a simple and effective way to improve the energy efficiency of buildings. Paraffins are the most studied PCMs for building applications because they have several advantages, such as being non-corrosive and non-supercooling, and having high latent heat, small volume changes in phase change process and good thermal stability [1,12]. They are also available over a wide range of phase change temperatures, ranging from approximately 20 \({}^{\circ}\)C up to 70 \({}^{\circ}\)C. However, they cannot be incorporated directly into building elements because leakage may occur during phase transition process [13]. Therefore, several incorporation techniques, such as shape-stabilized PCM (SSPCM), have been developed to address the leakage issue. Porous materials, such as diatomic [14], expanded graphite [15], porous aggregate [16, 17, 18] and kaolin [19], as well as synthetic porous materials [20, 21, 22], are often chosen as the supporting materials to store PCM. In particular, diatomic is a promising carrier because of its high storage capacity, special pores structure, light weight, thermo-stability property and chemical inertness [23,24]. [PERSON] et al. [25] reported that the adsorptive capacity of diatomic could reach up to 47%, and the thermal properties of the diatomic/magnesium nitrate hexahydrate composite remained stable after 2000 cooling/heating cycles. Besides this, [PERSON] et al. [26] stated that the thermal stability of diatomic/paraffin (DP) remained stable up to 95 \({}^{\circ}\)C, and no paraffin leakage was observed because of the high capillary force of diatomic. So, diatomic/PCM is appropriate to use as a thermal energy storage material in buildings. Nevertheless, it is worth noting that diatomic belongs to the group of pozzolanic materials with high amorphous silica contents [24,27,28], which can react with the portlandite in cement to generate calcium silicate hydrates (C-S-H). If DP is mixed with cement-based materials directly without covering [29,30], the pozzolanic reaction between diatomic and cement would destroy the porous structure of diatomic, and consequently leakage of PCM would happen. However, most studies using DP in cement-based materials did not consider or recognize this problem. To utilize DP successfully in buildings, suitable bulk building materials as alternative matrix materials must be sought. Gypsum is a versatile building material used in partitions, interior walls, soil [31], and decoration [32]. More importantly, gypsum and diatomite are compatible. So, an energy storage building material can be developed by incorporating DP into gypsum.