Preparation of Phase Change Materials from Magnesium Oxysulfide Foamed Cement: Characterization of Paraffin/Foamed Cement Composites and Their Application in Heat Storage Cement

1.1 Mechanical Properties TestingAccording to GB/T 17671—2021 “Test Method for Compressive Strength of Cement Mortar (ISO Method)”, the compressive strength of 100 mm × 100 mm × 100 mm magnesium oxide-sulfur foam standard cubic specimens and 20 mm × 20 mm × 20 mm specimens of the same age was tested using a universal testing machine. Tests were conducted at 7 d, 14 days, and 28 days. The proportional coefficient between the two was calculated and obtained. Subsequently, this proportional coefficient was used to convert the tested strength of the 20 mm × 20 mm × 20 mm specimens after wax adsorption into the equivalent strength of the standard cube specimens. An average value was taken from 3–6 specimens per group as the final result.Mechanical properties of magnesium oxide-sulfate heat-storage cement were tested using a universal testing machine per GB/T 17671—2021 “Cement Mortar Strength Test Method (ISO Method)”, with specimen dimensions of 40 mm × 40 mm × 160 mm.1.2 Thermal Properties Testing of C-PCMsThe melting and crystallization processes of samples under argon atmosphere were studied using a DSC-600C differential scanning calorimeter from Nanjing Huicheng Instrument Co., Ltd. Prior to testing, each sample group was ground into powder and placed in aluminum crucibles. Prior to testing, the temperature range was set to -5 to 100 °C, with a heating rate of 10 °C/min and an argon flow rate of 50 ml/min.1.3 Thermal Conductivity TestingThe thermal conductivity of materials was determined using an HCDR-S thermal conductivity tester. Samples were connected to the thermal conductivity meter for direct measurement of thermal conductivity. Before measuring the next sample, the probe was placed on the cooling plate for 5 minutes. Each sample was measured six times, and the average value was calculated to obtain the thermal conductivity of the sample.1.4 Leakage Performance TestThe effect of encapsulation materials on the leakage rate of C-PCMs was studied through comparative experiments. Three materials—epoxy resin, modified epoxy resin, and polyacrylic acid—were used to encapsulate C-PCMs. The initial mass (M3). After subjecting all samples to 5, 10, 15, 20, 25, 30, and 35 temperature cycles within the range of -25 to 60 °C, their mass (M4) was measured. The leakage rate Z was calculated using Equation (1):1.5 Microstructural CharacterizationScanning electron microscopy (SEM, Model Zeiss Supra-55) was employed to obtain microstructural images of MOSFC and C-PCMs. To facilitate rapid vacuum pumping, samples were divided into small thin sections prior to imaging. All sample surfaces underwent gold sputtering to enhance conductivity.X-ray diffraction (XRD, Model AXS D8 Advance) and Fourier transform infrared spectroscopy (FT-IR, Model PerkinElmer Spectrum 100) were employed to assess the chemical compatibility of C-PCMs. Samples were ground into powder prior to testing.Thermogravimetric analysis (TG-DTG) evaluated the thermal properties, stability, and decomposition characteristics of C-PCMs. Samples were ground into powder prior to testing.1.6 Water Absorption TestingFollowing the specifications of GB17671-2021 “Test Methods for Strength of Cement Mortar (ISO Method)”, standard mortar specimens and three sets of phase-change mortar specimens were prepared. After curing at room temperature for 28 days, the specimens were dried in a 100°C oven for 6 hours and weighed, with the mass recorded as m0. Immerse the specimens in clean water at room temperature for 3 hours. Remove the specimens, wipe off surface moisture, and weigh them, recording the mass as m1. Calculate the water absorption rate using Formula (2): GB/T 17671—2021 “Test Method for Compressive Strength of Cement Mortar (ISO Method)”, the compressive strength of 100 mm × 100 mm × 100 mm magnesium oxide-sulfur foam standard cubic specimens and 20 mm × 20 mm × 20 mm specimens of the same age was tested using a universal testing machine. Tests were conducted at 7 d, 14 days, and 28 days. The proportional coefficient between the two was calculated and obtained. Subsequently, this proportional coefficient was used to convert the tested strength of the 20 mm × 20 mm × 20 mm specimens after wax adsorption into the equivalent strength of the standard cube specimens. An average value was taken from 3–6 specimens per group as the final result.Mechanical properties of magnesium oxide-sulfate heat-storage cement were tested using a universal testing machine per GB/T 17671—2021 “Cement Mortar Strength Test Method (ISO Method)”, with specimen dimensions of 40 mm × 40 mm × 160 mm.