Preparation and performance evaluation of expanded vermiculite-based phase change materials for building thermal insulation and energy saving

Abstract

To meet the demand for building envelope materials with efficient heat storage and release capabilities in the harsh climate of Northwest China, this study reports the development of a novel ternary composite phase change material (CEV@SiC). The material was prepared by vacuum impregnation, encapsulating calcium chloride hexahydrate into highly porous expanded vermiculite and further incorporating silicon carbide nanoparticles as thermal conductivity enhancers. Thermal analysis showed that the sample with 5% SiC doping exhibited the highest enthalpy of fusion (90.3 J g⁻¹) and retained 95% of its enthalpy after 100 thermal cycles, demonstrating excellent cycling stability. A microscale heat transfer model of phase change mortar was established through simulation, and test modules incorporating CEV@SiC as a partial replacement for aggregate in cement mortar were constructed. Compared with ordinary modules, the CEV@SiC-modified modules reduced the indoor peak temperature by 2.04 °C and delayed the occurrence of the peak temperature by 1 h, confirming their significant energy-saving and temperature-regulation potential. This study provides crucial insights into component design and process optimization for developing high-performance phase change materials tailored for building envelopes in Northwest China. The developed material effectively fulfils the dual requirements of high thermal storage density and rapid thermal response under the region's unique climatic conditions.

To meet the demand for building envelope materials with efficient heat storage and release capabilities in the harsh climate of Northwest China, this study reports the development of a novel ternary composite phase change material (CEV@SiC).

1. Introduction

The continental climate¹ of Northwest China—characterized by harsh winters, scorching summers, sharp diurnal temperature swings, low humidity, and intense solar radiation—poses significant challenges for building insulation² and dynamic thermal management.³ These conditions exacerbate building energy consumption, making efficient thermal regulation a critical concern for sustainable development in such regions.⁴ Conventional building materials⁵ often struggle under these extremes. Their limited thermal mass and heat storage capacity lead to pronounced indoor temperature fluctuations, increasing the reliance on active heating and cooling systems and, consequently, overall energy use.⁶

To simultaneously achieve energy efficiency⁷ and indoor comfort, integrating advanced functional materials into building envelopes has become a key research focus.⁸ In this context, improving building energy performance through optimized design and material innovation is essential. Studies have explored various mechanisms, such as optimizing temperature setpoints under different envelopes for cooling energy savings,⁹ and assessing the impact of facade-integrated photovoltaic retrofits on carbon emissions in residential buildings.¹⁰ Phase change materials (PCMs),¹¹ particularly hydrated salts¹² such as calcium chloride hexahydrate (CaCl₂·6H₂O), are regarded as a highly promising technology for enhancing thermal energy storage capacity. They offer suitable phase transition temperatures, high latent heat,¹³ low cost, and abundant availability.¹⁴ However, their widespread application is hindered by inherent drawbacks like liquid leakage, supercooling, and phase separation,^15–17 which compromise long-term stability and reliability.¹⁸

A common strategy to overcome these issues is to encapsulate hydrated salts within porous matrices, such as expanded perlite or expanded vermiculite (EV).^19–21 These shape-stabilized composites prevent leakage but introduce a new challenge: their thermal conductivity²² is often significantly lower than that of conventional foam insulation materials.²³ For instance, while polystyrene foam exhibits a conductivity of about 0.03–0.04 W m⁻¹ K⁻¹, typical salt/porous matrix composites report values around 0.178 W m⁻¹ K⁻¹. This low conductivity, while providing some insulation, severely impedes the rates of heat storage and release,²⁴ which is critical for responsive thermal management. Research into enhancing the functional properties of composite materials, such as improving the thermal stability and enthalpy of wood-based composites via in situ polymerization of graphene oxide-modified PCM microcapsules,²⁵ highlights the ongoing efforts to refine PCM performance. Similarly, studies on optimizing other material systems, like the design of cylindrical thermal classifiers based on transformation thermodynamics²⁶ or the multi-objective optimization of thermoelectric modules,²⁷ underscore the importance of tailored thermal transport and energy conversion in advanced material design.

Under the sharp diurnal temperature fluctuations typical of Northwest China, the low thermal conductivity of standard PCM composites restricts their ability to rapidly absorb excess heat during the day and release it at night, thereby diminishing their effectiveness in dynamic thermal regulation.²⁸ Therefore, enhancing the thermal conductivity of shape-stabilized PCMs without compromising their storage density, shape stability, or cost-effectiveness is a crucial research direction. Innovations in other cementitious and ceramic composites, such as developing novel green lithium slag expansion agents for mortar,²⁹ utilizing carrier-embedded microbial calcium carbonate in cement mortars,³⁰ or understanding the mechanisms behind microstructure regulation in foamed ceramics,³¹ demonstrate the broad potential of composite material engineering for achieving enhanced and multifunctional properties.

To address this specific conductivity challenge, this study innovatively introduces high-thermal-conductivity silicon carbide (SiC) into a calcium chloride hexahydrate/expanded vermiculite composite system as a thermal-conductivity-enhancing phase. This approach aligns with the broader need for intelligent energy management in buildings, which can be informed by strategies considering factors like price fluctuations in HVAC system operation.³² By precisely controlling the SiC content, we aim to significantly improve the thermal transport rate of the composite while maintaining effective PCM encapsulation, high thermal storage density, and adequate insulation properties. This design targets a substantial enhancement in the dynamic heat storage and release performance of the composite under frequent and large temperature swings. The development of such a multifunctional composite, which synergistically combines high energy density, rapid thermal response, and good stability, holds significant scientific value and promising practical prospects for energy-efficient buildings in extreme climates.

2. Materials and methods

2.1. Reagents and instruments

Anhydrous calcium chloride (CaCl₂) and strontium chloride hexahydrate (SrCl₂·6H₂O, AR grade) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Vermiculite was supplied by Xinlong Vermiculite Co., Ltd. (Yuli, Xinjiang, China). Silicon carbide (SiC, AR grade) was obtained from Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Cement mortar was provided by Zhejiang Construction New Building Materials Group Co., Ltd.

2.2. Sample preparation

Firstly, 10.14 g of anhydrous CaCl₂ was dissolved in 9.86 g of deionized water, followed by the addition of 0.40 g of SrCl₂·6H₂O. The mixture was stirred in a water bath at 50 °C for 1 h and subsequently cooled to crystallize CaCl₂·6H₂O. Vermiculite particles (2 mm in diameter) were dried and expanded using a 1200 W microwave treatment to obtain expanded vermiculite (EV). A vacuum impregnation method was then employed to incorporate molten CaCl₂·6H₂O into EV at mass ratios of 40%, 50%, and 60% under conditions of 50 °C and 85 kPa for 30 min, followed by drying for 5 h to yield CaCl₂·6H₂O/EV (CEV) composite phase change materials (PCMs). Furthermore, nano-SiC particles were incorporated into the molten CaCl₂·6H₂O at 1, 5, 9, and 13 wt% loadings, and the above impregnation process was repeated to prepare CaCl₂·6H₂O/EV@SiC (CEV@SiC) ternary composites. All prepared samples were finally stored under refrigeration. The preparation procedure is schematically illustrated in Fig. 1.

Fig. 1. Schematic diagram of the preparation process for CaCl₂·6H₂O/expanded vermiculite@SiC (CEV@SiC) composite phase change material.

2.3. Characterization

The morphology and elemental composition of the samples were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, ZEISS Gemini 300/Smartedx). The chemical functional groups and component compatibility of the composite were investigated by Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet 6700) over a wavenumber range of 4000–400 cm⁻¹. The crystal structure was analyzed by X-ray diffraction (XRD, Bruker D8-ADVANCE) with 2θ scanned from 10° to 90° at a rate of 5° min⁻¹. The phase transition temperatures and enthalpies were determined using differential scanning calorimetry (DSC, Mettler DSC1) under a nitrogen atmosphere, with a heating rate of 5 °C min⁻¹ between 10 and 60 °C. Thermal conductivity was measured via the steady-state heat flow method using a DRE-III instrument. Thermal stability was evaluated by thermogravimetric analysis (TGA, Mettler TGA/DSC3+) from 10 to 600 °C at a heating rate of 5 °C min⁻¹. Leakage behavior was assessed by heating the samples at 60 °C for 1 h, with regular photographic and infrared monitoring. A mesoscopic heat transfer model was established using COMSOL Multiphysics, and the practical temperature regulation performance was evaluated by incorporating the composite into a cement plaster model room (300 mm × 300 mm × 300 mm) for outdoor testing.

3. Results and discussion

3.1. Maximum absorption capacity of composite phase change materials

To determine the maximum uptake (loading) capacity of expanded vermiculite (EV) for CaCl₂·6H₂O, CaCl₂·6H₂O/EV (CEV) composites with three different mass fractions were placed on separate filter papers. The papers were kept in an oven at 50 °C for 1 h to ensure complete melting of the PCM. After removal, the samples were lifted off the papers, and the filter papers were carefully inspected for any wetting indicative of leakage. Representative results are shown in Fig. 2.

Fig. 2. Leakage behavior and maximum absorption capacity of CEV composite phase change materials with different CaCl₂·6H₂O mass fractions ((a) 40 wt%; (b) 50 wt%; (c) 60 wt%).

The experimental results indicate that when the mass fraction of CaCl₂·6H₂O in the CEV composite is 40% or 50% (Fig. 2a and b), the material's adsorption capacity does not reach saturation. No liquid residues were detected on the filter paper after heating, demonstrating that the solute is fully retained within the vermiculite pores. However, increasing the CaCl₂·6H₂O content to 60% led to liquid seepage on the filter surface (Fig. 2c), suggesting that the adsorption sites of EV had reached saturation and the excess solute exceeded the structural carrying capacity, causing leakage. Overall, the maximum adsorption capacity of the CEV composite for CaCl₂·6H₂O is 50%, at which the material exhibits optimal adsorption stability and solute encapsulation efficiency during the thermal phase change process.

3.2. Morphology and chemical compatibility of CEV and CEV@5% SiC composite PCM

Fig. 3a displays the typical layered porous structure of expanded vermiculite (EV), which features uniform interlayer spacing and continuous pore distribution, providing ideal adsorption sites and thermal energy storage space for phase change materials. Fig. 3b illustrates the microstructure of the CEV composite after impregnation with 50 wt% calcium chloride hexahydrate (CaCl₂·6H₂O). The molten salt, driven by capillary forces, uniformly fills the pores of vermiculite, forming a dense composite interface without significant phase separation or salt leakage, indicating that the vacuum impregnation method effectively achieves efficient encapsulation of the phase change material Fig. 3c corresponds to the CEV@5% SiC ternary composite, in which SiC nanoparticles (bright black dots) are uniformly dispersed on the surface and within the pores of vermiculite without agglomeration, while calcium chloride hexahydrate remains fully loaded. The introduction of silicon carbide does not disrupt the porous network of expanded vermiculite but instead enhances interfacial heat conduction pathways, improving thermal conductivity. This is consistent with the differential scanning calorimetry (DSC) results discussed later—the melting enthalpy of this composite material increases (ΔH_m = 90.3 J g⁻¹).

Fig. 3. (a) SEM images of (a) EV, (b) CEV, and (c) CEV@5% SiC. (d–f) Elemental mapping images of (d) Cl, (e) Si, and (f) Ca corresponding to the area in (c). (g) EDS survey spectrum and (h) the corresponding elemental atomic and weight percentages derived from the area shown in (c).

Energy-dispersive X-ray spectroscopy analysis of the scanning electron microscopy image Fig. 3(c) (Fig. 3d–f) reveals the presence of characteristic elements in the CEV@5% SiC composite: calcium (Ca, 28.18 wt%), chlorine (Cl, 59.92 wt%), and silicon (Si, 11.90 wt%). Detailed data are summarized in Fig. 3(g) and (h). The atomic ratio of calcium to chlorine is approximately 1 : 2.4 (the theoretical atomic ratio for calcium chloride is 1 : 2), confirming the successful loading of calcium chloride hexahydrate into the vermiculite matrix. The atomic percentage of silicon (15.04 wt%) is significantly higher than its mass percentage (11.90 wt%), which is attributed to the contribution of low-density silicon carbide nanoparticles (density ρ = 3.21 g cm⁻³). After deducting the silicon inherently present in vermiculite (approximately 8.4 wt%), the additional silicon content (approximately 3.5 wt%) matches the designed 5% silicon carbide doping level exactly, further confirming the successful construction of the CEV@5% SiC ternary composite structure.

Fig. 4a shows the FT-IR spectrum of the CEV@5% SiC composite. The broad peak at 3435.44 cm⁻¹ corresponds to O–H stretching vibrations from crystalline or adsorbed water.¹⁸ The band at 1638.79 cm⁻¹ is attributed to C O stretching or aromatic C C vibrations, and the sharp peak at 1030.34 cm⁻¹ is associated with asymmetric stretching of Si–O–Si bonds. The characteristic Si–C vibration of pure SiC at 848.67 cm⁻¹ remains clearly visible in the composite, indicating that the addition of SiC nanoparticles did not significantly alter the chemical structure of the matrix. Furthermore, the preservation of C–H stretching vibrations (2845–2952 cm⁻¹) from CEV and metal–oxygen vibrations (400–600 cm⁻¹) from CaCl₂·6H₂O suggests that the components are combined through physical interactions without chemical recombination.³³

Fig. 4. (a) FTIR and (b) XRD spectra of CaCl₂·6H₂O, SiC, CEV, and CEV@5% SiC.

Fig. 4b shows the X-ray diffraction (XRD) patterns of the individual components and the CEV@5% SiC composite. For pure CaCl₂·6H₂O, the characteristic diffraction peaks at 2θ = 13.01°, 22.61°, 26.03°, 32.03°, 39.62°, and 41.81° can be indexed to the (100), (110), (101), (111), (300), and (211) crystal planes, respectively, matching well with the standard pattern of CaCl₂·6H₂O (JCPDS no. 26-1053). The expanded vermiculite (EV) matrix displays a broad, low-intensity peak around 2θ = 8–9° and another around 26–28°, which are characteristic of its layered silicate structure, indicating a predominantly amorphous or poorly crystalline nature. Pure SiC nanoparticles show sharp peaks at 2θ = 35.64° and 60.01°, corresponding to the (111) and (220) planes of its cubic β-phase (JCPDS no. 29-1129).

The XRD pattern of the final CEV@5% SiC composite retains all the characteristic peaks of CaCl₂·6H₂O and β-SiC, while also showing the broad feature of the EV matrix. No new diffraction peaks are observed, and there is no detectable shift in the peak positions of the constituent phases. These results confirm that: (1) the crystalline integrity of CaCl₂·6H₂O and SiC is preserved during the composite preparation process, and (2) the components are combined through physical interaction without any chemical reaction, successfully forming a ternary composite phase change material.

3.3. Thermal properties characterization of CEV and CEV@5% SiC composite PCM

Fig. 5a and b present the DSC thermograms of CEV composite phase change materials with varying SiC doping ratios, along with that of pure CaCl₂·6H₂O. Analysis of the melting and crystallization behaviors indicates that all samples exhibit characteristic phase change peaks, with the melting enthalpy (ΔH_m) of pure CaCl₂·6H₂O serving as the reference. As the SiC content increases from 0% to 13%, the melting enthalpy of the composites initially rises and then decreases, as summarized in Table 1. The ΔH_m reaches a maximum for CEV@5% SiC, suggesting that an appropriate amount of SiC enhances thermal conductivity and interfacial adhesion, thereby improving crystallization efficiency. However, when the SiC content exceeds 5% (e.g., 9% and 13%), ΔH_m decreases significantly, which is attributed to phase separation and pore blockage caused by excessive SiC, limiting the loading capacity of CaCl₂·6H₂O.

Fig. 5. DSC curves of the composites. (a) Melting and (b) crystallization curves of CaCl₂·6H₂O, CEV, and CEV@SiC composites with varying SiC content (0–13 wt%).

Table 1. Phase change enthalpy of composites with different SiC contents.

Sample	ΔHc (J g−1)	ΔHc (J g−1)
CaCl2·6H2O	178.5	111.03
CEV	92.23	90.3
CEV@1% SiC	87.41	84.23
CEV@5% SiC	90.3	85.47
CEV@9% SiC	86.63	79.18
CEV@13% SiC	83.29	79.03

At a 5% SiC doping ratio, the composite exhibits high melting enthalpy, low supercooling, and excellent cycling stability. This performance enhancement arises from the synergistic interaction between SiC and the CEV matrix, which both reinforces the thermal conduction pathways and preserves the integrity of the porous structure. These findings provide critical experimental guidance for the compositional design of highly stable composite phase change materials.

Fig. 6 shows the thermal conductivities of expanded vermiculite (EV), calcium chloride hexahydrate (CaCl₂·6H₂O), CEV (CaCl₂·6H₂O/EV), and the CEV@5% SiC composite PCM. The thermal conductivity of EV is as low as 0.12 W (m⁻¹ K⁻¹), which is attributed to its inherently poor heat conduction and the limited ability of EV alone to improve thermal transfer in composites. In contrast, the thermal conductivity of CEV@5% SiC is higher than that of EV, mainly because SiC possesses high thermal conductivity, which helps to reduce internal temperature gradients within the PCM, thereby minimizing heat loss and improving overall thermal management efficiency.

Fig. 6. Thermal conductivity of EV CaCl₂·6H₂O, CEV, and CEV@5% SiC.

During the experiments, the temperature evolution of CEV and CEV@5% SiC was investigated. As shown in Fig. 7, both samples were placed on a heating plate maintained at 60 °C. CEV@5% SiC reached its phase transition temperature (29 °C) at 5 min and the peak temperature (60 °C) at 18 min, whereas CEV reached 29 °C at 8 min and 60 °C at 27 min. In both cases, the rate of temperature rise slowed significantly during the phase transition. However, CEV@5% SiC exhibited a faster heating rate than pure CEV due to the presence of SiC. This acceleration in temperature rise can be attributed to the improved thermal conductivity of the composite, which facilitated uniform heat distribution throughout the sample and prevented localized overheating that could cause material degradation.

Fig. 7. Temperature variation curves of CEV and CEV@5% SiC.

Viewed together, the thermal conductivity and heating behavior demonstrate a balanced design of the CEV@5% SiC composite. The incorporation of SiC significantly enhances the thermal conductivity (compared to pure EV), establishing efficient heat transfer pathways to overcome the slow thermal response inherent to traditional shape-stable PCMs. Crucially, the resulting conductivity (∼0.51 W (m⁻¹ K⁻¹)) remains considerably lower than that of metals and is comparable to that of the cement mortar matrix. This indicates that while acting as an efficient “thermal storage unit,” the composite retains the necessary thermal insulation capacity required for building envelopes against ambient temperature swings. This combination of moderate thermal insulation and rapid thermal responsiveness is key to its function in dynamic thermal management.

3.4. Characterization of the phase change performances

To further investigate thermal stability, as shown in Fig. 8a, the temperature was increased from 20 °C to 60 °C over 15 minutes. By comparing the thermal stability results of CaCl₂·6H₂O and CEV@5% SiC, the following conclusions can be drawn: CaCl₂·6H₂O begins melting at its melting point (29 °C) after 3 minutes and is completely liquefied by 15 minutes at 60 °C.·In contrast, the CEV@5% SiC composite phase change material remains stable under the same conditions, showing no melting within 15 minutes, demonstrating a significant thermal stability advantage. These results indicate that the CEV@5% SiC composite prepared in this study exhibits higher thermal stability and enhanced anti-leakage performance compared to pure CaCl₂·6H₂O.

Fig. 8. (a) Photographs showing the leakage behavior of CaCl₂·6H₂O and CEV@5% SiC. (b) Infrared thermal images of CaCl₂·6H₂O and CEV@5% SiC.

To investigate the heat storage capacity and buffering performance of CaCl₂·6H₂O and CEV@5% SiC, an infrared thermography experiment was conducted. Leakage of the phase change material from CaCl₂·6H₂O during heating was directly observed, as shown in Fig. 8b. During the gradual temperature increase, no significant difference in buffering capacity or heat storage ability was observed between the two samples below 30 °C. As the temperature continued to rise, CaCl₂·6H₂O melted, resulting in a substantial decrease in both its buffering capacity and heat storage ability. In contrast, the buffering capacity and heat storage ability of CEV@5% SiC continued to increase significantly. Comparison reveals that CEV@5% SiC demonstrates a notable improvement in both buffering capacity and heat storage performance, with only minimal leakage observed.

Fig. 9a presents the thermogravimetric (TGA) curves of EV, CaCl₂·6H₂O, and the CEV@5% SiC composite PCM. EV shows negligible decomposition from room temperature to 600 °C.³⁴ In contrast, CaCl₂·6H₂O begins decomposing at around 29 °C, followed by three distinct stages of water loss at 137.7 °C, 163.7 °C, and 183.8 °C, with mass losses of 26.32%, 7.92%, and 11.89%, respectively. The total mass loss corresponds to 6.031 water molecules per formula unit, which is consistent with the theoretical value of 6. The CEV@5% SiC composite exhibits a similar dehydration process. The total water loss of the composite is 23.56%, which aligns well with the theoretical crystal water content (26.63%). After 100 thermal cycles (Fig. 9b), the melting enthalpy (ΔH_m) of the composite is 85.15 J g⁻¹, a decrease of only 3.3%, indicating minimal leakage of CaCl₂·6H₂O. FTIR and XRD analyses (Fig. 9c and d) demonstrate the high structural stability of the composite after cycling. The FTIR spectra before and after cycling show high overlap between 3500 and 500 cm⁻¹, with no noticeable shift or reduction in characteristic peaks (e.g., C–H, C O, Si–O), confirming the integrity of chemical bonds and functional groups. The XRD patterns remain unchanged within the 20°–90° range, with no additional peaks, indicating excellent retention of the crystal structure, including phase composition and interplanar spacing. Overall, the DSC, FTIR, and XRD results confirm the exceptional cycling durability of the CEV@5% SiC composite, with no significant changes in chemical composition or crystalline structure after 100 cycles. The excellent retention rate (95%) over 100 thermal cycles demonstrates promising short-to-medium-term stability for the CEV@SiC composite. This 100-cycle test protocol is a well-established and widely adopted method for evaluating the medium-term stability of shape-stabilized phase change materials, as commonly practiced in related studies.^35,36 For building applications that require material performance over decades, long-term stability under thousands of cycles is indeed the ultimate benchmark. While accelerated testing over several hundred to thousands of cycles is recommended for comprehensive lifetime prediction, the minimal degradation observed in our 100-cycle test, coupled with the absence of structural or chemical changes (as confirmed by FTIR and XRD), provides a strong positive indicator of the material's inherent durability. This forms a solid basis for future long-term cyclic aging studies.

Fig. 9. Thermal stability and structural durability of the composite. (a) Thermogravimetric analysis curves of EV, CaCl₂·6H₂O, and the CEV@5% SiC composite. (b–d) Characterization of the CEV@5% SiC composite before and after 100 thermal cycles: (b) DSC curves, (c) FTIR spectra, and (d) XRD patterns.

3.5. Thermal insulation test of the simulated unit house

The porous and lightweight structure of expanded vermiculite, combined with the surface reinforcement, water absorption control, and interfacial enhancement provided by the SiC coating, makes CEV@5% SiC a well-balanced material capable of effectively replacing conventional sand and gravel aggregates, and an excellent candidate for producing functional cement mortars with properties such as low density, thermal insulation, fire resistance, and sound absorption.

Based on the assumption that the cement mortar matrix is a homogeneous phase, a mesoscale model of phase change energy-storage mortar was constructed using a two-phase random composite approach, as shown in Fig. 10a. This model can be applied to calculate the effective thermal conductivity of the mortar and to analyze the internal heat transfer process. As illustrated in Fig. 10b, the incorporation of shape-stabilized PCM caused slight deflections in the isothermal lines within the mortar, indicating that the addition of PCM increases the conduction pathways of heat flow inside the energy-storage mortar.³⁷

Fig. 10. (a) Mesoscopic and (b) heat transfer models of phase-change thermal storage mortar.

The CEV@5% SiC prepared in Section 2.2 was used to replace an equal mass of aggregate in the cement mortar and was mixed into the mortar, producing CEV@5% SiC composite cement mortar, as summarized in Table 2. A model house was then constructed using PVC panels, and its surfaces were plastered with either ordinary cement mortar or CEV@5% SiC composite mortar, resulting in a reference house and a phase change house.

Table 2. Composite cement mortar mix proportions.

Mortar mix proportion
	PCM	Fine sand	Fly ash	Cement	Admixture	Water-cement ratio
Ordinary room
Mass (kg)	0	799.7	75	125	0.3	0.5
Percentage (%)	0	79.97	7.5	12.5	0.03	0.5
Phase-change room
Mass (kg)	100	699.7	75	125	0.3	0.5
Percentage (%)	10	69.97	7.5	12.5	0.03	0.5

Fig. 11 presents the monitoring results of indoor air temperature. It can be observed that the outdoor temperature during the day ranged from a minimum of 18.01 °C to a maximum of 35 °C. Regarding indoor conditions, the peak temperature in the phase change unit house was 2.04 °C lower than that in the reference house, and the occurrence of the indoor maximum temperature was delayed by 1 h. This effect can be explained by the fact that, during the daytime when the temperature exceeds the phase transition point, the PCM within the storage mortar melts and absorbs heat, thereby reducing heat transfer into the room and lowering the indoor peak temperature. At night, when the temperature drops below the phase transition point, the PCM solidifies and releases the heat stored during the day, which helps to maintain a higher indoor temperature.

Fig. 11. 24 hour indoor temperature variations in the ordinary room and phase-change room.

The magnitude of this peak temperature reduction (2.04 °C) represents a balanced outcome achieved under multiple practical constraints. This performance level can be attributed to three primary factors: (1) the PCM content within the mortar was limited to 10 wt% (as an aggregate replacement) to preserve the essential mechanical strength and workability of the construction material, highlighting the inherent trade-off between thermal storage capacity and structural integrity. (2) A thermal interface resistance exists between the cement matrix and the PCM composite, as indicated by the deflection of isothermal lines in the simulation (Fig. 10b), which implies that not all incident heat is uniformly absorbed by the PCM, allowing partial heat to bypass the storage mechanism. (3) The extreme climatic conditions of Northwest China, characterized by intense solar radiation and rapid diurnal temperature rise, posed a significant challenge by exceeding the instantaneous heat storage rate of the passive mortar system. These factors collectively define the current performance envelope and, more importantly, translate the experimental findings into clear motivations for future optimization—such as developing PCMs with better-matched phase-change temperatures, incorporating interfacial thermal enhancers, and integrating the phase-change mortar with other building envelope systems for synergistic effects.

This process effectively increases the thermal inertia of the wall assembly by adding a “capacitive thermal buffer,” thereby delaying the arrival of the outdoor heat wave peak indoors. This phenomenon provides applied-performance evidence for the composite's thermal insulation capability. It demonstrates a synergistic effect where the material's dynamic heat storage complements the static insulation provided by the building envelope, collectively enhancing overall energy efficiency and thermal comfort.

3.6. Comprehensive evaluation of CEV@5% SiC

To objectively evaluate the application potential of the as-prepared CEV@5% SiC composite in thermal insulation and energy saving, a systematic performance comparison with representative composite PCMs reported in recent literature was conducted. The comparison focuses on three critical parameters for practical applications: the phase change temperature (T_m), the latent heat of fusion (ΔH_c), and the thermal conductivity (k). Key composite systems employing similar porous matrices (e.g., expanded vermiculite, EV; expanded perlite, EP) for shape stabilization were selected as benchmarks. The comparative data are summarized in Table 3, which provides a clear overview of the performance landscape.

Table 3. Comparison of thermal properties of CEV@5% SiC and reported PCMs for building applications.

Material system	T m (°C)	ΔHc (J g−1)	Thermal conductivity (W (m−1 K−1))	References
Na2SO4·10H2O–Na2CO3·10H2O/EV	23.98	110.3	0.192	38
CaCl2·6H2O/EV	28.18	94.46	—	35
CA-PA/EV	23.61	67.5	0.25	39
CaCl2·6H2O/EP	27.38	87.44	0.11	40
CEV@5% SiC	29	90.3	0.51	This work

Based on the comparative analysis above, the CEV@5% SiC composite demonstrates well-balanced and competitive overall performance, making it suitable for thermal insulation and energy-saving applications. Although its latent heat of phase change is slightly lower than some reported systems, its phase transition temperature aligns more closely with the building thermal comfort range, and its thermal conductivity is significantly higher than that of similar porous matrix-based composite phase change materials. This combination of considerable heat storage capacity, suitable phase change temperature, and excellent heat transfer performance highlights its strong application potential in scenarios requiring efficient thermal buffering and rapid thermal response.

3.7. Outlook for energy-saving performance assessment

To translate the demonstrated thermoregulatory performance (Section 3.5) into directly quantifiable energy-saving indicators, we will construct structurally identical full-scale prototype rooms. Under real climatic conditions, by maintaining the same indoor temperature-control settings and performing long-term monitoring of air-conditioning power consumption, direct energy-use comparisons will be conducted. This work aims to quantitatively evaluate the annual energy-saving efficiency of the phase-change mortar and to carry out a preliminary economic analysis, thereby providing a critical decision-making basis for its practical engineering applications.

4. Conclusions

A ternary composite phase change material (CEV@SiC) was successfully developed via vacuum impregnation for building thermal regulation. The optimized composite, with 50 wt% CaCl₂·6H₂O loaded into expanded vermiculite and enhanced by 5% SiC nanoparticles, exhibits a high latent heat of 90.3 J g⁻¹ and excellent cyclic stability (95% retention after 100 cycles). Simulation and model-building tests demonstrated its effective thermal buffering capacity, reducing the indoor peak temperature by 2.04 °C and delaying it by one hour. This work provides a practical strategy for developing high-performance, shape-stable PCMs to improve energy efficiency in buildings under large diurnal temperature variations. Future work will focus on direct energy-consumption monitoring to bridge laboratory validation with practical application.

Author contributions

Chaoqun Fan: writing, methodology; Hanning Zhang: conceptualization; Jiaxing Gao: data curation; Yang Haiming: formal analysis; Xifeng Lv: supervision, funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data generated or analyzed during this study are included in this published article.