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Article

Effect on the Thermal Properties of Building Mortars with Microencapsulated Phase Change Materials for Radiant Floors

by
Guo Li
,
Guoqiang Xu
* and
Zhiyi Tao
Green Building Autonomous Region Key Laboratory of Higher Education, School of Architecture, Inner Mongolia University of Technology, Hohhot 010051, China
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(10), 2476; https://doi.org/10.3390/buildings13102476
Submission received: 2 September 2023 / Revised: 25 September 2023 / Accepted: 28 September 2023 / Published: 29 September 2023
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
The use of slag silicate cement mortar as a thermal mass layer for radiant floor heating systems holds significant potential for active thermal energy storage systems in buildings. The main objective of this article is to experimentally test the thermal performance of slag silicate cement mortar thermal storage blocks after the addition of phase change materials. The present study focuses on investigating the thermal performance of thermal storage blocks made of slag silicate cement mortar that incorporates a microencapsulated phase change material (mPCM). The mPCM consists of particles of paraffin-coated resin, which are uniformly distributed in the mortar. The analysis revealed that the introduction of mPCM particles into the mortar decreases the bulk density by approximately 9.4% for every 5% increase in mPCM particles ranging from 0% to 20%. The results obtained utilizing the Hot Disk characterization method demonstrate that the mPCM particles significantly affect the thermal properties of the mortar. Particularly, the thermal conductivity and thermal diffusion coefficient of the SSC30 mortar with a 17.31 wt.% mass of mPCM particles decreased by 59% and 69%, respectively. The results of this study provide a basis for the application of RFHS end-use thermal storage layers.

1. Introduction

Research findings indicate that the building sector accounts for the largest portion of energy consumption and contributes to approximately one-third of global greenhouse gas emissions [1]. Consequently, it is crucial to develop effective and efficient approaches in the construction industry to work towards achieving “carbon neutrality” [2]. In this regard, various heat transfer approaches have been developed. For instance, the radiant floor heating system (RFHS) is an embedded surface heating system [3], which provides a more uniform temperature distribution than conventional heating systems [4,5,6]. This system can improve indoor thermal comfort efficiently while exhibiting great potential for integration with renewable sources of energy [7,8]. The RFHS effectively harnesses the thermal capacity of a building’s structure, offering several advantages, including enhanced thermal comfort, reduced energy consumption, quiet operation, and space efficiency [9,10,11,12]. As a result, the RFHS has been extensively utilized in various types of buildings, including residential, commercial, educational, and airport facilities [13,14,15].
Relevant studies as well as a literature review have shown that the incorporation of phase change materials (PCMs) at the envelope terminals (floors, walls, roofs, etc.) absorbs a large amount of thermal energy [16,17,18,19], which improves the thermal storage capacity [20,21,22] and improves air conditioning by absorbing and releasing heat through phase change [23,24]. Studies show that this approach is an effective scheme to reduce energy consumption and improve indoor air conditioning [25,26].
It is worth noting that the RFHS mainly utilizes electric heating panels or hot water coils embedded in the floor to transfer heat to indoor spaces through radiation [27]. This system is typically classified as a hydronic radiant floor heating system (HRFHS) or an electric radiant floor heating system (ERFHS) according to the type of heat source.
Recently, the incorporation of PCMs in the RFHS has become an emerging research topic attracting scholars worldwide. For instance, Baek et al. [28] combined PCMs in the middle of a mortar layer and simulated the thermal performance and energy-saving potential of the proposed system using EnergyPlus software. Park et al. [29] implemented phase change materials to the floor and experimentally determined the optimal floor thickness and temperature range. Lu et al. [25] developed a model based on phase change materials and experimentally optimized the hot water coil spacing and PCM thickness. Furthermore, Sun et al. [21] developed PCMs with different melting points and designed a double-layer radiant floor system to regulate indoor thermal comfort throughout the year. Faraj et al. [30] designed a simulation chamber and employed an accurate heating-cooling control system to derive the optimal location for PCM bonding. Moreover, Heng et al. [31] incorporated the HRFHS and an air source heat pump (ASHP) to analyze the thermal reaction for complete melting and solidification of the PCM layer. Liu et al. [32] modified the HRFHS for solar heating and compared the results obtained from the numerical model with experimental results. Park et al. [33] analyzed the time lag originating from applying PCMs to the RFHS. Shen et al. [34] prepared a composite phase change material (CPCM), conducted experiments, and demonstrated that as the weight percentage of the PCM in the specimen increases, an increase in the water flow rate and water temperature improves the system efficiency. Cesari et al. [35] developed control strategies considering multiple factors to utilize the benefits of PCMs in the RFHS and validated the developed strategies through simulations and experiments. Jiang et al. [36] introduced a PCM into the RFHC and developed an integrated system involving an air source heat pump (ASHP) and a radiant floor heating condenser (RFHC). Jin et al. [37] utilized a PCM and performed experiments and simulations on the RFHS with different structures. Sun et al. [38] validated the constructed PCM in active and passive coupled systems and optimized the system.
Reviewing the literature indicates that for active thermal energy storage systems (ATES) in the RFHS, the application of a PCM is mainly focused on experimental studies, with an emphasis on the use of PCM materials in adjusting the temperature and encapsulating and embedding the PCMs in certain locations [21,25,29,30,32,34,37], floor construction and thickness optimization [28,29,30,33,36,38], and heating systems such as adjusting the temperature and flowrate of the supply water [21,27,31,39]. Portland cement is mainly used as a thermal storage cementitious material in ATES [27,40,41,42,43].
However, Portland cement is increasingly being replaced by fly ash and slag to prepare more environmentally friendly materials [44,45,46], and its use as a thermal storage block in ATES needs further investigation [34,39,44,47,48,49,50,51]. It is worth noting that the production of 1 kg of ordinary silicate cement generates 0.66–0.82 kg of carbon emissions [52], and global cement production accounts for 5–7% of anthropogenic CO2 emissions [53], necessitating consideration of the sustainability of mortar and concrete buildings [54]. Additionally, the integration of PCMs into mortar and concrete has been found to significantly improve the thermal storage capabilities of buildings [44,51,55,56,57] and reduce the carbon footprint and greenhouse gas (GHG) emissions of the construction industry [58]. Ground granulated blast furnace slag (GGBFS) has been utilized as a substitute for a portion of clinker or cement in cement or concrete production. This innovative process can effectively dispose of the waste slag produced in steel mills and reduce the production cost of cement or concrete. The use of slag silicate cement (SSC) mortar as a thermal layer for radiant floor heating systems has great potential for active thermal energy storage systems in buildings [59].
In this study, a specific process was used to introduce mPCM particles into slag silicate cement mortar with the main objective of analyzing the thermal properties of slag silicate cement mortar thermal storage blocks with the addition of a mPCM, contributing to the existing knowledge on the use of a mPCM in mortar and its further application to radiant floor heating systems (Figure 1). The first part of the study introduces the materials, the preparation process of the thermal storage blocks, and the thermal performance test methods and procedures. The second part focuses on the experimental results, with an analysis of the evolution of the thermal characteristics of the slag silicate cement mortar with respect to the PCM particle content. Finally, the main achievements are summarized in the Section 4.

2. Materials and Methods

2.1. Raw Materials

Considering the exceptional properties of solid-liquid PCMs, such as negligible volume change during phase transition and a high latent heat capacity, these materials have become an emerging substance with wide engineering applications for heat storage [60]. Organic PCMs are the most common materials used in construction applications. It should be indicated that these materials are safe, non-reactive, and chemically stable [61]. Microencapsulation is the most common method of incorporating PCMs into mortar [44]. In this study, a Thermalcare 32 microencapsulated phase change material (mPCM, Shanghai Xinya New Material Technology, Shanghai, China) with a melting temperature of 32.0 ± 2.5 °C was used in the experiments. The product was wrapped with paraffin wax using a polymer resin and had a white powder appearance (D90 ≤ 20.0 µm) with a moisture content of ≤2.5%.
The mortar was prepared using PSA32.5 slag silicate cement with a mass fraction of GGBS ranging from 20% to 50% and ISO medium-grade standard sand. Table 1 presents the chemical composition of the reference mortar with a water–cement ratio of 0.5 (W/C).

2.2. Preparation of the PCM Mortar Samples

There are two approaches to incorporating mortar mixtures and a PCM: (1) additive approach using the PCM as an additional component in the mortar [62] and (2) alternative method using the PCM to partially replace the sand in the mixture [63]. A PCM can be incorporated into mortar as a replacement for fine aggregates, with a replacement percentage ranging from 5% to 30% [44,45,58]. To prevent damage to the microencapsulated PCM powder, it is typically added to the cementitious material in the final stage of mixing under low rotational speed conditions [42]. Moreover, it can be added in the early stage of mixing to achieve a mortar microstructure with uniform distribution and global integrity [41].
The mass ratio of mortar prepared in this study was 1 part cement, 3 parts standard sand, and 0.5 parts water, i.e., 1 unit weight (1 g) of cement corresponds to a 3 unit weight (3 g) of standard sand and a 0.5 unit weight (0.5 g) of water, and the PCM was introduced at the early stage of mixing. Quantitative cement, sand, and phase change microencapsulated materials were weighed according to the proportioning scheme, and dry materials were added to the mixing bucket to achieve a homogeneous mixture. Different specimens were made, including a reference cement mortar (SSC0) and mortar samples with various contents of mPCM particles. Table 2 shows the chemical composition of the prepared specimens. The mass of the slag silicate cement was kept constant in various mixtures, and the specimens were labeled as SSCx, where x is the mass percentage of mPCM particles incorporated into the mortar mixture, which ranged from 0% to 30%.
The mixing process of SSC0 was as follows (Figure 2): (1) The cement and sand were weighed using an electronic balance and stirred evenly in a measuring cup. (2) A quantitative amount of tap water was weighed and poured into the mixture. (3) The mortar was manually mixed at a low speed for 60 s. The same procedure was used for the PCM mortar, the only difference being that the mPCM particles were added during the mixing and weighing of the dry mix. To achieve a homogeneous mixture, water was added to the dry mix in three stages.
The W/C ratio in Table 2 is defined as the water requirement of the mPCM particles in the mortar proportion. It was observed that as the content of the mPCM particles in the mixture increased, the water requirement also increased. This finding aligns with that of previous reports [64].
After mixing, the mixtures were molded to prepare test blocks with dimensions of 70 × 70 × 20 mm. The samples were placed in a cool environment to cure for 24 h. After demolding, the samples were continuously sprinkled with water for 7 days and left to naturally cure at room temperature for 28 days.

2.3. Characterization Techniques

2.3.1. Calorimetry

The thermal characteristics of the mPCM particles and PCM mortar were analyzed using a differential scanning calorimeter (DSC Q10, TA Instruments, New Castle, DE, USA) at various scan rates ranging from 5 to 20 °C min−1. Moreover, the melting properties and crystallization peaks of the particles during the heating and cooling phases were analyzed. Each sample was analyzed in the temperature range of −20 °C to 80 °C. First, the samples were cooled from room temperature to −20 °C and maintained in isothermal equilibrium for 5 min. Then, the samples were heated from −20 °C to 80 °C and maintained in isothermal equilibrium at 80 °C for 5 min. Second, the samples were cooled from 80 °C to −20 °C and maintained in isothermal equilibrium for 5 min. Finally, the samples were returned to room temperature.

2.3.2. Bulk Density of mPCM Mortars

The bulk density is defined as the mass per unit volume of the material in its natural state, which can be mathematically expressed as follows:
ρ 0 = m V 0
The PCM mortar was kept in the laboratory until the mass of the test block reached a constant value after complete curing. After weighing the test blocks using an electronic balance and calculating the bulk density of the different blocks, variations in the PCM mortar bulk density with an increasing mass of the mPCM were analyzed.

2.3.3. Microstructural Analysis

To explore the microstructure of specimens under low vacuum conditions, a scanning electron microscope (SEM Sigma 500, ZEISS, Jena, Germany) with an operating voltage of 3 kV and 5 kV was employed. The morphology of the mPCM particles in the natural state and the fracture surfaces of the PCM mortar samples after 28 days of curing were analyzed. To examine the fractured surfaces, the samples were initially cut into 10 × 10 × 50 mm blocks using an angle grinder. Subsequently, the blocks were broken with a hammer to ensure that the resulting mortar samples were smaller than 10 × 10 × 10 mm in size.

2.3.4. Thermogravimetric Analysis

Samples of the mPCM particles in their natural state and fines from different PCM mortars were analyzed using an analyzer (Discovery SDT 650, TA Instruments, New Castle, DE, USA). The degradation point of the organic/mineral components was determined by monitoring the mass loss. The procedure involved heating the samples from room temperature to 600 °C in a nitrogen atmosphere, with a heating rate of 10 °C min−1. To minimize experimental errors, each sample was tested three times.

2.3.5. Thermophysical Characteristics

In this article, the thermophysical characteristics of mPCM particles in their natural state and fines from different PCM mortars were explored utilizing the Hot Disk (HD) technique. To this end, a testing device (TPS 3500 HotDisk, Hot Disk AB, Gothenburg, Sweden) equipped with a 6.4 mm radius sensor (5501 Kapton, Hot Disk AB, Gothenburg, Sweden) was employed. The sensor transmitted data to a thermal analyzer.
The HD method is based on the principle of instantaneous planar sources as defined by the ISO 22007-2 standard. In this method, a double helix-shaped sensor is positioned between two samples of the same material. Figure 3 shows the configuration of the test setup. The device is capable of rapidly, accurately, and non-destructively determining the thermal conductivity (λ), thermal diffusivity (α), and volumetric heat capacity (CV) with an accuracy of 3%, 5%, and 7%, respectively.
The melting point of mPCM particles is about 32 °C so the HD measurements were carried out at room temperature (24 °C). To minimize measurement errors, each test was repeated three times and the average value was recorded.

3. Results and Discussion

3.1. Characterization of the mPCM Particles

To facilitate an analysis of the size distribution and thermal properties, the mPCM particles were preliminarily characterized.

3.1.1. Particle Size Distribution

Figure 4 shows SEM images of the mPCM particles at 500× and 1000× magnifications, indicating that mPCM particles exhibit a spherical shape. Furthermore, the SEM images reveal the presence of broken resin shells at a magnification of 1000×. These broken resin shells were detected alongside the mPCM particles, which appear irregular in shape, as observed at a magnification of 500×. It is possible that these irregularly shaped particles were formed during the fabrication process of the mPCM particles.

3.1.2. DSC Characterization

Figure 5 illustrates the DSC heat fluxes at various scan rates during the heating process. The results reveal the correlation between the DSC signal and the heating rate. It is observed that higher heating rates result in more significant heat flux. Additionally, it is found that as the heating rate increases, the peak curve shifts towards higher temperatures.
Figure 6 shows the heat flow diagram during the heating/cooling process. The DSC histograms show that the onset of melting and crystallization in mPCM particles occurs at 29.6 °C and 29.3 °C, respectively. Furthermore, it is found that the latent heat of melting and crystallization is 10.64 and 123.06 J g−1, respectively. The results demonstrate that the melting and crystallization transition of mPCM particles occurs over a broad temperature range, spanning from 14 °C to 44 °C. This phenomenon is related to the phenomenon of subcooling, which tends to zero when the heating/cooling rate is sufficiently low to allow the establishment of thermodynamic equilibrium within the sample, and, as illustrated in Figure 5, the DSC results using lower heating rates are closer to the properties of the material itself. In practical applications, it is important to consider the end water supply system for radiant floor heating to prevent hysteresis due to too rapid heating. Moreover, two peaks can be distinguished in both the melting and crystallization transition curves: the smaller peak reflects the mPCM polymer resin shell transition [42], and the other peak corresponds to the phase transition of the paraffin.

3.1.3. Thermogravimetric Analysis

To analyze the degradation of mPCM particles at high temperatures, TG tests were performed. In this regard, Figure 7 illustrates the variations in the DTG curves and mass of the specimens against temperature. It is observed that the initial minor weight loss commences around 50 °C and extends up to 135 °C. This stage may be due to the reduction in water content within the mPCM pellets. The second stage commences at 135 °C, which may be attributed to the decomposition of the polymer resin shell. It should be indicated that this stage is consistent with the shell softening temperature of 150 °C as provided by the manufacturer. Significant weight loss occurs within the temperature range of 135 to 480 °C, which may originate from the decomposition of the resin shell surrounding the mPCM particles containing a paraffin core. Figure 8 also demonstrates that as the temperature increases from 480 to 600 °C, the mass of the particles reduces slightly. The observed weight loss may originate from the degradation of the additives employed during the microencapsulation process, including surfactants [65], as well as highly cross-linked phases within the polymer shell [66].

3.1.4. Thermophysical Characteristics

This research employs the HD method to investigate the thermophysical characteristics of the mPCM powder at various temperatures, and the results are presented in Table 3.
Table 3 indicates that as the temperature of the mPCM increases from room temperature to 40 °C, the thermal conductivity decreases while the thermal diffusion coefficient increases, where λ24 denotes the thermal conductivity at 24 °C and α24 denotes the thermal diffusion coefficient at 24 °C, and λ40 denotes the thermal conductivity at 40 °C and α40 denotes the thermal diffusion coefficient at 40 °C. The decrease in thermal conductivity is due to the change in the core density resulting from the conversion of paraffin from the solid phase to the liquid phase. It is observed that the thermal conductivity (λ) and the thermal diffusivity (α) of the mPCM particles after melting decreased by 1.9–13.0% and increased by 64.6–69.8%, respectively.

3.2. Experimental Investigations of the PCM Mortars

This section focuses on the experiments on the PCM mortars. The bulk density of each PCM mortar was calculated to analyze the influence of the mass of the mPCM on the microstructure of the hardened mortar and analyze the variation in the bulk density based on microstructural aspects. Finally, the HD method was employed to investigate the thermal characteristics of the specimens.

3.2.1. Bulk Density

Bulk density calculations were performed using different PCM mortar samples after 28 days of curing. Figure 8 illustrates that as the mass of the mPCM replacing the fine aggregates increases, the bulk density of the mortar reduces sharply. It is observed that when the mass of the mPCM ranges from 0% to 20%, the reduction curve is linear. Within this linear region, a 5 wt% increase in the mass of the mPCM reduces the bulk density of the mortar by 9.4%. At a higher mass of the mPCM, however, the decrease rate is smaller. It is important to note that at the mixing stage, when the mass of the mPCM exceeds 25%, the volume of the mPCM has exceeded the volume of the cemented sand. This variation in density can be attributed to a variety of factors [67,68]:
(1) The density of the mPCM particles is lower than the other components in the mortar. Consequently, substituting these components with the mPCM leads to a reduction in the mortar density; (2) The presence of the mPCM particles also affects the particle distribution within the mortar, thereby affecting the density; (3) The incorporation of the mPCM particles introduces air into the mortar during the mixing process. Moreover, as the mass of the mPCM increases, excess water is required, further contributing to the decrease in the density.

3.2.2. SEM Observations

Figure 9 shows SEM images of mortars with different masses of the mPCM at different magnifications (500×, 1000×, and 3000×). The results reveal a compact microstructure with limited surface porosity for the reference mortar SSC0.
Figure 9a shows an SEM image of SSC0 in the absence of the mPCM granular slurry at a magnification of 500×. It is observed that the shape of the mechanical fracture surface is regular after 28 days of curing, and there are many mineral aggregates in the form of flakes after hydration on the fracture surface. The results demonstrate that the mortar crystals, which appear filamentary after hydration, are interspersed among the massive and flaky crystals. On a macroscopic scale, this arrangement leads to a dense surface appearance of the mortar. In contrast, as the mass of the mPCM increases in the mortar, the shape of the mechanical fracture surface gradually becomes upland and staggered, exhibiting irregular morphology. Filamentous mortar crystals are not observed at the microscopic scale, and the aggregated crystals form agglomerates. These changes are reflected on the macroscopic scale as a coarse and sparse mortar surface.
After adding the mPCM particles to the mortar, the microstructure became more porous. Moreover, the SEM images revealed that as the mass of the mPCM increased, small voids were more uniformly distributed throughout the material. This increase in porosity may be attributed to the addition of excess water to adjust the workability of the fresh mortar mixture and the entrainment of air during the addition of the mPCM particles. Moreover, Figure 9f’ shows that the mPCM particles in the mortar samples maintained a regular spherical shape, indicating that damage to the particles during the mixing and pouring processes is negligible, thus preserving the integrity of the resin shell.

3.2.3. Thermogravimetric Analysis

Figure 10 exhibits the TG curves under a heating rate of 10 °C min−1. The curve for the SSC0 reference mortar reveals three distinct events: The first minor weight loss phase occurs in the temperature range from room temperature to 95 °C, resulting from the evaporation loss of free and bound water. The second phase of weight loss is observed between 95 °C and 190 °C, which is caused by the loss of water bound to calcium silicate hydrate (CSH) [41]. The third and smallest mass loss occurs in the range of 400 °C to 600 °C, demonstrating the good heat resistance of the slag silicate cement.
The TGA curves reveal that a substantial weight loss beyond 200 °C is significantly high in the PCM mortars, and this difference exhibits a positive correlation with the mass of the mPCM. This correlation may originate from the degradation of the mPCM particles and the excess water added during the preparation process, which forms a weak bond with the cement hydrate.
Figure 11 shows DSC heat flow diagrams obtained for different PCM mortars during the heating step at a heating rate of 10 °C min−1. It is observed that the SSC0 reference mortar reaches a stable heat flow of 1 mW. Under this condition, the mPCM particle content increases, more heating is required, the peak value of the curve increases, and the thermal characteristics of the PCM mortar change significantly. In real-world engineering applications, adding a larger mass of mPCM particles means that the floor absorbs and stores more heat during the heating process and this stored heat continues to be released for a longer period of time after the heating stops. This facilitates the utilization of peak and valley tariffs and is in line with the reality of the imbalance between the supply and demand of energy sources such as solar energy.

3.2.4. Thermophysical Properties

In this article, the Hot Disk approach was employed to analyze the thermophysical properties of PCM mortars. To minimize measurement errors, each experiment was repeated three times, and the average value was recorded.
Figure 12 shows the distribution of the thermal conductivity (λ) and thermal diffusivity (α) in various PCM mortars, indicating that the thermal conductivity ranges from 0.4 to 2.4 W m−1 K−1 and the thermal diffusivity varies from 0.2 to 1.2 mm2 s−1 in various types of PCM mortars. It also shows a significant decrease in thermal conductivity and thermal diffusivity after adding mPCM particles to the mortar. It is found that the use of mPCM particles to replace fine aggregates at 0–15 wt.% significantly reduces the thermal conductivity and thermal diffusivity of the material. When the content of the mPCM particles exceeds 20 wt.%, the reduction rate decreases. More specifically, the thermal conductivity and thermal diffusivity of the SSC30 mortar with a mass of mPCM particles of 17.31 wt.% decreases by 59% and 69%, respectively. The results confirm that the presence of mPCM particles in the SSC0 reference mortar significantly affects its thermophysical characteristics, including its thermal conductivity and thermal diffusivity.
The thermal conductivity measured at 40 °C in the liquid state exhibits slightly lower values compared to those in the solid state. This observation has remained a topic of debate in the literature, as some authors obtained slightly higher thermal conductivity for PCM mortars in the solid state [67], while others reported higher values in the liquid state [68,69].
The decrease in the thermal conductivity and thermal diffusivity of the PCM mortars may originate from two factors: Firstly, the lower conductivity of mPCM itself compared to the mineral composition it replaces. Secondly, the increase in porosity resulting from the inclusion of mPCM particles leads to the entrainment of air, further reducing the thermal conductivity and thermal diffusivity [67,70].

4. Conclusions

This article proposes a procedure for the preparation of slag silicate cement mortar samples with a high mass of mPCM particles. According to the proposed procedure, mPCM particles are added in the early stages of the mixing, and the water content of the mortar is maintained at a certain level.
An analysis of different hardened PCM mortars through SEM images revealed that the mPCM particles distribute uniformly within the material. As the mass of the mPCM increased and fine aggregates were replaced, the bulk density of the mortar decreased by approximately 9.4% for every 5% increase in mPCM particles. However, when the mass of the mPCM exceeded 20 wt.%, the decrease in the bulk density slowed down, potentially due to the excess water used to enhance the workability and the entrainment of air during mixing. TGA analysis indicated changes in the composition of the cement hydrate in the PCM mortar.
The addition of mPCM particles has a significant impact on the thermal characteristics of PCM mortars. The presence of mPCM particles leads to a notable decrease in the thermal conductivity and thermal diffusion in PCM mortars. Particularly, when the mass of the mPCM ranges from 0% to 15%, there is a rapid reduction in the thermal conductivity and thermal diffusion. However, when the replacement of the fine aggregates with the mPCM particles exceeded 20%, the rate of reduction slowed down. In the case of the SSC30 mortar (with 17.31 wt.% of mPCM), the thermal conductivity and thermal diffusion decreased by 59% and 69%, respectively. This effect may be due to both the intrinsic thermal properties of the mPCM and the higher porosity of the PCM mortar compared to the reference mortar. Furthermore, it is worth noting that the slag silicate cement exhibited good thermal performance in this context.
The use of slag silicate cement mortar with mPCM particles as a thermal mass layer for radiant floor heating systems offers great potential for active thermal energy storage systems in buildings. It has a good prospect for the actual situation where the electricity consumption policy of peak and valley tariffs is implemented, the imbalance between the supply and demand of energy sources such as solar energy is utilized, and ordinary cement is produced with high energy consumption, etc. It can be further used for the construction of solar energy building-integrated houses, etc., and can effectively alleviate the pressure of the construction field in the actual operation of the HVAC system as well as the energy industry that utilizes non-renewable resources, such as fossil energy sources. This study primarily focuses on investigating the thermal properties of the samples at the micro- and mesoscopic scales through experimental analysis. It should be noted that these findings may have limitations in terms of representing the properties of the materials at the macroscopic scale. For practical engineering applications, the construction cost is an issue that requires special attention. According to the results of DSC, adding more mPCM particles may obtain a better thermal storage effect, but this will also lead to a high construction cost. Of course, with the continuous progress of mPCM manufacturing technology, it is possible to reduce the cost; for the current stage, this needs to be followed up with further research. In terms of thermophysical properties, the addition of a higher mass of mPCM particles also leads to a reduction in the thermal conductivity and thermal diffusion coefficient of the mortar, thereby increasing the thermal inertia, which can be time-consuming as well as energy-costly during the heating of the floor, which needs to be followed up by determining the optimum mass of mPCM particles to be used in the slag silicate cement floor. Moreover, the addition of a PCM to mortars can affect the processability, microstructure, compressive strength, flexural strength, and adhesion. To obtain data that is more representative of actual building applications, further research should be carried out. In this article, the PCM alternative method was employed to incorporate mPCM particles into concrete. A specific direction for future research is to investigate the relationship between the mechanical strength and thermal properties of slag silicate cement mortars with added mPCM particles for better application in real-world projects. In addition, it is crucial to investigate PCM mortars on a whole-floor scale to gather construction cost data and assess the full life-cycle environmental benefits.

Author Contributions

Conceptualization, G.L. and G.X.; Formal analysis, G.L. and G.X.; Funding acquisition, G.X.; Investigation, Z.T.; Methodology, G.L.; Supervision, G.X.; Visualization, Z.T.; Writing—original draft, G.L.; Writing—review and editing, G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant no. 52168006).

Data Availability Statement

The data supporting the findings of this study are available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Radiant floor heating system using slag silicate cement with PCM.
Figure 1. Radiant floor heating system using slag silicate cement with PCM.
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Figure 2. The preparation of mortar: (a) weighing materials, (b) mixing of dry materials, (c) manual mixing, (d) pouring test blocks, and (e) watering maintenance.
Figure 2. The preparation of mortar: (a) weighing materials, (b) mixing of dry materials, (c) manual mixing, (d) pouring test blocks, and (e) watering maintenance.
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Figure 3. Configuration of the Hot Disk setup.
Figure 3. Configuration of the Hot Disk setup.
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Figure 4. SEM images of mPCM particles at various magnifications.
Figure 4. SEM images of mPCM particles at various magnifications.
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Figure 5. DSC measurements of mPCM particles with different scanning rates.
Figure 5. DSC measurements of mPCM particles with different scanning rates.
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Figure 6. Typical DSC thermograms of mPCM particles during the heating/cooling process at a rate of 10 °C min−1.
Figure 6. Typical DSC thermograms of mPCM particles during the heating/cooling process at a rate of 10 °C min−1.
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Figure 7. TGA and DTG curves of mPCM particles at a heating rate of 10 °C min−1 under nitrogen atmosphere.
Figure 7. TGA and DTG curves of mPCM particles at a heating rate of 10 °C min−1 under nitrogen atmosphere.
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Figure 8. Density of mortar against the mass of mPCM.
Figure 8. Density of mortar against the mass of mPCM.
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Figure 9. SEM images of the microstructure of different PCM mortars at various magnifications: (a–a″) SSC0; (b–b″) SSC5; (c–c″) SSC10; (d–d″) SSC15; (e–e″) SSC20; (f–f″) SSC25; and (g–g″) SSC30.
Figure 9. SEM images of the microstructure of different PCM mortars at various magnifications: (a–a″) SSC0; (b–b″) SSC5; (c–c″) SSC10; (d–d″) SSC15; (e–e″) SSC20; (f–f″) SSC25; and (g–g″) SSC30.
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Figure 10. TGA curves for different PCM mortars under a heating rate of 10 °C min−1.
Figure 10. TGA curves for different PCM mortars under a heating rate of 10 °C min−1.
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Figure 11. Distribution of heat flow against temperature for various PCM mortars under a heating rate of 10 °C min−1.
Figure 11. Distribution of heat flow against temperature for various PCM mortars under a heating rate of 10 °C min−1.
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Figure 12. The distribution of λ and α in different mortars.
Figure 12. The distribution of λ and α in different mortars.
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Table 1. Main components of the reference cement mortar (SSC0).
Table 1. Main components of the reference cement mortar (SSC0).
ComponentSSCSandWaterW/C Ratio
Dosage (g)50150250.5
Table 2. Main compositions of different PCM mortars.
Table 2. Main compositions of different PCM mortars.
DesignationSSC (g)Sand (g)mPCM (g)Water (g)W/C RatiomPCM Ratio (wt.%)
SSC0501500250.50
SSC550142.57.5300.63.26
SSC105013515350.76.38
SSC1550127.522.5400.89.38
SSC205012030450.912.24
SSC2550112.537.5501.015
SSC305010545551.117.31
Table 3. Thermophysical characteristics of the mPCM powder at 24 °C and 40 °C.
Table 3. Thermophysical characteristics of the mPCM powder at 24 °C and 40 °C.
λ24 (W m−1 K−1)α24 (m2 s−1)λ40 (W m−1 K−1)α40 (m2 s−1)
Test 10.0920.030.0920.05
Test 20.0920.030.0810.10
Test 30.0920.030.0820.11
Average0.0920.030.0850.09
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Li, G.; Xu, G.; Tao, Z. Effect on the Thermal Properties of Building Mortars with Microencapsulated Phase Change Materials for Radiant Floors. Buildings 2023, 13, 2476. https://doi.org/10.3390/buildings13102476

AMA Style

Li G, Xu G, Tao Z. Effect on the Thermal Properties of Building Mortars with Microencapsulated Phase Change Materials for Radiant Floors. Buildings. 2023; 13(10):2476. https://doi.org/10.3390/buildings13102476

Chicago/Turabian Style

Li, Guo, Guoqiang Xu, and Zhiyi Tao. 2023. "Effect on the Thermal Properties of Building Mortars with Microencapsulated Phase Change Materials for Radiant Floors" Buildings 13, no. 10: 2476. https://doi.org/10.3390/buildings13102476

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