# A review of microencapsulated and composite phase change materials: Alteration of strength and thermal properties of cement-based materials

Sarra Drissi, Tung Chai Ling, Kim Hung Mo, Anissa Eddhahak

► To cite this version:

Sarra Drissi, Tung Chai Ling, Kim Hung Mo, Anissa Eddhahak. A review of microencapsulated and composite phase change materials: Alteration of strength and thermal properties of cement-based materials. Renewable and Sustainable Energy Reviews, 2019, 110, pp.467-484. 10.1016/j.rser.2019.04.072. hal-02164863v2

HAL Id: hal-02164863

https://hal.science/hal-02164863v2

Submitted on 28 Jun 2019

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
<figure><img src="image_2.png" /></figure>

# A review of microencapsulated and composite phase change materials: Alteration of strength and thermal properties of cement-based materials

<figure><img src="image_3.png" /></figure>

Check for
updates

Sarra Drissiª, Tung-Chai Lingª,*, Kim Hung Moᵇ, Anissa Eddhahakᶜ

## ARTICLE INFO

### Keywords:

Cement-based material

Phase change material

Microencapsulated

Composite

Compressive strength

Thermal energy storage

## ABSTRACT

Due to the population growth and the increased reliance on cooling and heating systems, buildings have become the largest energy consumer worldwide. The use of phase change material (PCM) has shown great potential to reduce the annual cooling and heating load by up to 50%. Nowadays, the direct incorporation of PCM in cement-based materials (CBM) is creating a considerable debate in the research community with regards to the proper selection and the beneficial utilization of PCM (microencapsulated or composite) in CBM. Therefore, this paper reviews the pros and cons of using microencapsulated and composite PCM in CBM by highlighting the mechanisms involved in the mechanical strength loss and thermal properties enhancement. Generally, a high thermal energy storage CBM was obtained. However, PCM exhibited a negative effect on the compressive strength of CBM. In view of the literature review, the compressive strength reduction varies considerably with no clear trend which is understandable in view of the differences in mix designs as well as the variety of materials used in each study. Finally, an up-to-date PCM case studies, gaps and future directions are also presented to provide a reliable basis and helpful reference for the future development of eco-friendly and energy-efficient building materials containing PCM.

## 1. Introduction

The building sector has a huge impact on the environment. This is mainly due to buildings' life cycle (design, construction, operation, demolition and waste treatment) which contributes to a large share of CO₂ emission [1]. In fact, most of the CO₂ emission is coming mainly from the production of building materials as well as electricity to provide modern comfort for buildings [2]. Luckily, many techniques, such as carbon dioxide capture and storage (CCS), have been proposed to reduce the CO₂ concentration in the atmosphere [3,4]. The development of low cost means to capture CO₂ can promote the use of this technology at an industrial scale. The enrichment of indigenous microorganism present for example in activated sludge seems to be an economical and promising way to capture CO₂ [5,6].

The energy consumption by buildings is projected to increase by

about 48% from 2012 to 2040 [7] which is mostly caused from the rapid growth of some of the non-OECD (Organization for Economic Co-operation and Development) countries [8]. According to the international energy outlook 2016 reference case (IEO2016), the residential energy use in China (non-OECD country), is expected to grow by an average of 2.4% from 2012 to 2040 compared to only 0.1% in the USA (OECD country) [9]. This increased demand will result in a shortage of fossil fuels and consequently a sharp increase in the energy cost. Therefore, there has been a growing interest in applying energy storage systems in building to overcome the mentioned problem.

For the past few decades, the feasibility of using PCM as a thermal energy storage system in buildings was extensively investigated [10–12]. PCM can obviously reduce energy consumption due to its phase change process by maintaining a small fluctuation between indoor and outdoor temperatures. This phase change process occurs only

**Abbreviations:** PCM, Phase change materials; CO₂, Carbon dioxide; CCS, carbon dioxide capture and storage; OECD, Organization for economic co-operation and development; IEO2016, International energy outlook 2016; CBM, Cement-based materials; mPCM, Microencapsulated phase change materials; cPCM, Composite phase change materials; SiO₂, Silica gel; CaCO₃, Calcium carbonate; TiO₂, Titanium dioxide; ECA, Expanded clay aggregates; LWA, Lightweight aggregate; LDPE, Low density polyethylene; EVA, Ethylvinylacetate; CA, Capric acid; LA, Lauric acid; MA, Myristic acid; PEG, Polyethylene glycol; DP, Diatomite; EP, Expanded perlite; EG, Expanded graphite; PCMOW, PCM-panel mounted on the outer surface of walls; PCMIW, PCM-panel mounted on the inside surface of walls

* Corresponding author.

E-mail addresses: tcling@hnu.edu.cn, tcling611@yahoo.com (T.-C. Ling).

<footnote>ªKey Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, 410082, Hunan, China</footnote>

<footnote>ᵇDepartment of Civil Engineering, Faculty of Engineering, University of Malaya, Malaysia</footnote>

<footnote>ᶜPIMM Laboratory, UMR 8006: Arts et Métiers ParisTech, CNRS, CNAM, Arts et Métiers ParisTech, France</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Observed and projected climate classification (2001–2025) [17]. Copyright (2019), with permission from authors.</figcaption></figure>

due to the outdoor/indoor temperature variation and does not require any electrical energy (electrical power or fuel). Therefore, the careful selection of PCM's phase change temperature specifically according to the outdoor climatic conditions is crucial to achieve better thermal performances [13].

As known, there are mainly five climatic zones (Fig. 1) as defined by Köppen-Geiger climate classification [14]. For a very cold or hot climate, characterized by little temperature variation and fluctuations, PCM showed a very limited impact on the energy consumption of buildings whereas PCM was more efficient when the climate has large temperature swings covering the PCM's phase change temperature. For example, the total energy demand of a typical Mediterranean climate could be reduced by about 23.5% (heating) and 37.4% (cooling) when using PCM [15] while the heating demand can be reduced only by about 15% for cold climate [16].

Moreover, it is well known that each season is distinguished by a special climate condition and weather patterns. Therefore, the effect of PCM on the building will vary also during seasons and based on weather conditions [18]. As can be seen in Fig. 2, the energy saving that could be achieved in the same city during summer differs from that achieved in winter. A more detailed survey on the PCM's applications in different climate zones can be found in Refs. [19,20].

However, the relatively high cost of PCM technologies is limiting their widespread application. Therefore, extensive studies are carried out in both OECD and non-OECD countries, in particular during the last 10 years (Fig. 3 and Fig. 4) to investigate the feasibility of incorporating PCM directly in cement, mortar and concrete (hereafter referred to as cement-based materials or CBM) in order to save the cost associated with installation of PCM in buildings as a separate component [21]. The main efforts devoted in this specific scientific research topic is to explore and provide sustainable technologies to face the growing trend in energy consumption. According to the literature survey, China and the USA are the most active in this particular research topic. Furthermore, the majority of countries conducting research on PCM in buildings are located in the Mediterranean climate zone. It should be noted also that only few published researches are available on evaluating the effect of

<figure><img src="image_5.png" /><figcaption>Fig. 2. Heating and cooling energy saving rate in different Chinese cities (modified from Ref. [18]). Copyright (2019), with permission from Elsevier.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Number of published research in the area of PCM in construction building technology since 1992 according to the web of science database.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Distribution of published research worldwide: (a) Developed and, (b) Developing countries (the countries were selected based on the corresponding author address).</figcaption></figure>

<figure><img src="image_8.png" /></figure>

PCM in buildings (case studies) under real outdoor conditions and some of them are reviewed in this paper.

Various strategies have been practiced in the literature to efficiently utilize PCM in CBM. One of these methods consisted of incorporating bulk PCM (e.g. paraffin) as a latent heat material in CBM [22]. Despite the economic benefits of this method, the direct contact of paraffin wax with highly alkaline cementitious environment often results in the poor stability of PCM which can affect its thermal properties [23–25]. To overcome this issue, researchers are focusing on developing shape stabilizing PCM by using encapsulation (micro and macro) techniques [26,27]. Besides, PCM suffers also from a very low thermal conductivity which limits its overall performances and results in a low heat transfer [28,29]. For this, recent studies suggested to use both PCM and insulation materials in the building elements (such as walls) [15]. The total energy saving that can be achieved was about 66.2% which is 2.3 times and 1.09 times higher than that achieved by PCM or insulation materials, respectively. This results in a significant reduction of the long payback period. On the other hand, other researchers suggested adding high thermal conductivity nanofillers during the encapsulation process of PCM [30–33].

Obviously, the application of PCM in buildings is not limited only to thermal energy storage systems but also to micro electro mechanical systems, data storage, smart textiles, etc. [34–36]. Most of the previous reviews regarding the potential application of PCM in CBM were focused mainly on the synthesizing process of PCM and the potential benefits of adopting PCM-CBM systems in terms of energy saving and indoor comfort improvement. Cabeza et al. [37], Paloma et al. [38] and Soares et al. [39] presented a detailed summary of the different type of bulk PCM and the main criteria that should be considered when selecting PCM for building applications as well as the different methods to investigate its thermal properties. The encapsulation techniques (micro and macro) were extensively reviewed in Refs. [38,40–46] and compared in Ref. [47]. Furthermore, the different methods employed in the literature to enhance the thermal conductivity of PCM were comprehensively reviewed in Refs. [31,33,48–50]. The effect of PCM in particular microencapsulated PCM (mPCM) on the fresh and hardened properties of CBM was briefly discussed in Ref. [51]. Rao et al. [52] gathered information regarding the effect of PCM on different type of mortars. They highlighted that geopolymer mortar is more suitable to be a medium for PCM than lime and cement based mortar. Several other papers reviewed the numerical simulation methods used to analyse the passive cooling potential of PCM when integrated in buildings [20,39,53,54]. Finally, Gracia et al. [55], Tyagi et al. [45], Zhu et al. [56] and Song et al. [57], provided an insight into the application of PCM in the building envelope (wall, roof, floors) and building systems (heat pump, ventilation systems, etc.).

The previous reviews provide generally repeating information

related to PCM itself, the encapsulation techniques and heat transfer, the different methods to evaluate its thermophysical properties, the PCM's thermal conductivity enhancement as well as its application in CBM. However, none of the published reviews highlighted the pros and cons or compared the use of mPCM and cPCM in CBM. Understanding the difference between the effect of mPCM and cPCM on the fresh and hardened properties of CBM is crucial for real concrete building applications. Therefore, this paper summarizes the main studies related to PCM in CBM and highlights the pros and cons of using mPCM and cPCM in CBM. On the other hand, most of the available reviews have neglected the effect of PCM on the compressive strength of CBM. To bridge this gap, this paper will give a clear overview on how mPCM and cPCM affect the strength of CBM by pointing out the underlying mechanisms involved in this strength loss. Finally, the recent case studies across different countries and climate regions, as well as research gaps are also provided to possibly establish future directions in basic research and real application of PCM in CBM.

## 2. Phase change material for building application

PCM can be incorporated into buildings as a passive (wall, ceiling, façade element) or active (separate heat and cold storage devices) system to increase their thermal energy storage and efficiency [23,58]. It presents a promising solution to increase the thermal mass of buildings by providing inertia against temperature fluctuations. Different organic materials (paraffin, fatty acids), inorganic materials (hydrated salts, metals) and eutectics with various phase change temperatures and thermal energy storage densities have been studied by several researchers for building applications. A database was published based on the type and working temperature of PCM to facilitate its selection for building applications [59]. Recently, several studies have focused on the incorporation of solid-liquid phase change PCM in CBM. Different methods were considered for incorporating PCM into CBM, namely direct incorporation, immersion and encapsulation [11].

Nowadays, the use of microencapsulated PCM (mPCM) and composite PCM (cPCM) have attracted great attention since they can prevent liquid leakage during the solid-liquid phase transition of PCM. In general, organic PCM, typically paraffin with a melting point that is close to the average temperature range for human comfort (18 °C–28 °C) and high storage densities up to 240 kJ/kg [60], is chosen as the core material in mPCM and cPCM. Other advantages of using paraffin as PCM are its chemical stability, effective cost and low supercooling phenomenon and phase segregation [61]. As can be seen in Fig. 5, mPCM is composed of a core material encapsulated in a shell, while cPCM is composed of a porous material that can retain PCM by capillarity [62–64].
<figure><img src="image_9.png" /><figcaption>Fig. 5. Schematic representation of (a) mPCM and (b) cPCM (modified from Ref. [65]). Copyright (2019), with permission from Elsevier.</figcaption></figure>

## 2.1. Microencapsulated PCM (mPCM)

The mPCM consists of a core (active) material microencapsulated/ coated in a thin spherical shell or capsule (usually polymers). The shell can be formed using various techniques including physical and chemical methods which are detailed in Refs. [40,46,66,67]. The most often used physical methods include pan coating, spray drying, solvent evaporation, centrifugal and fluidized bed processes while the chemical methods include in situ polymerization, interfacial polymerization, sample or complex coacervation, phase separation, suspension-like polymerization and other methods [68]. Generally, microcapsules smaller than 100 µm cannot be synthesized by physical methods, therefore, chemical methods, in particular in situ polymerization, are widely used [40].

After the manufacturing process, mPCM particles (up to 5 µm in diameter) agglomerate together and form spherical capsules ranging between 100 and 500 µm as illustrated in Fig. 6 [25,61,69]. Organic polymer shells (e.g. poly methyl methacrylate, melamine-formaldehyde resin, urea formaldehyde resin, etc.) were usually employed to protect the PCM core and improve its structural, impermeability and thermal stability properties [40].

As known, PCM has to maintain good physical and thermal stability upon thermal cycling [24]. According to the previous experimental study [71], mPCM had excellent thermo-physical stability. In fact, the geometrical shape and energy storage capacity of mPCM remained stable after 3000 repetitive dynamic cycles of fusion and solidification [72,73]. However, the undesirable properties of mPCM such as the low strength and stiffness of the shell usually caused it to be susceptible to damage (Fig. 6 (b)) [62]. Another typical drawback of mPCM was its relatively low thermal conductivity; for example, the thermal conductivity of paraffin waxes and poly methyl methacrylate shell is only about 0.2–0.4 W/m·°C and 0.17–0.25 W/m·°C, respectively [67]. To

overcome this problem, graphite powder with a thermal conductivity of 390 W/m·°C (nearly 1000 times higher than paraffin) was added to PCM during the microencapsulation process [74,75]. Inorganic shells were also used to improve the thermal conductivity of mPCM such as SiO₂ [76], CaCO₃ [77] and TiO₂ [78], and the resulting microcapsules (Fig. 7) had also a spherical shape with an average size of 80–800 nm. Although these inorganic shells could provide higher thermal conductivity and stability as well as better phase change performance to mPCM, microcapsules formed by the silica shell, for example, could still be easily damaged (Fig. 7 (c)) [77–79].

## 2.2. Composite PCM (cPCM)

Recently, cPCM was also developed by several researchers. The process consisted of absorbing liquid PCM into highly porous inorganic materials such as expanded graphite [80], expanded clay [81], diatomite [82] and expanded perlite [83]. Before the impregnation process, porous materials were usually treated in order to improve their absorbability and purity as well as to expand their interlayer spacing [44]. Several techniques were adopted in the literature to prepare cPCM. One of these methods is the fusion adsorption process [84,85]. This method consists of placing the porous materials in melted paraffin then drying the composite using either compressed air or combination of compressed air and ventilated oven to remove any superficial residue of PCM. Compared to the previous method, a higher encapsulation ratio can be achieved when using the vacuum absorption technique [86–89]. Generally, it is recommended to coat the developed cPCM with a mixture of epoxy, graphite powder and silica fume to enhance the thermal conductivity and prevent the leakage of PCM [81].

Compared to mPCM, it is noteworthy that the use of a porous material in cPCM had mainly no effect on the encapsulation efficiency (ratio) since the percentage of the absorbed PCM in porous materials

<figure><img src="image_10.png" /><figcaption>Fig. 6. SEM observation of mPCM capsules: (a) Agglomeration of small mPCM capsules [21], and (b) Damaged capsules [70]. Copyright (2019), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Example of mPCM encapsulated in inorganic shells: (a) CaCO₃ shell, (b) TiO₂ shell, and (c) SiO₂ shell [77–79]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Mass ratio of organic active PCM in different supporting materials (data taken from Refs. [44,61,74–76,80–82,86,87,91,92]).</figcaption></figure>

could be up to 70% (Fig. 8). Therefore, the total energy storage capacity of cPCM was usually comparable to that of mPCM [90]. On the other hand, as shown in Fig. 8 the type of medium had an obvious influence on the encapsulation efficiency. For example, the encapsulation ratio of paraffin in the Micronal DS 5038 X, Microtek MPCM24D, SiO₂, diatomite and expanded graphite capsules was about 48%, 85%, 69%, 47% and 85%, respectively.

Studies also indicated that cPCM had good chemical stability and thermal behavior over time [86,93,94]. Moreover, improved compatibility with cementitious materials was noticed in comparison to the performance of mPCM in CBM [63,64,74,80,82]. Studies reported that

the use of hydrophobic supporting materials and hydrophobic surface protective coating (hydrophobic fumed silica, precipitated silica, epoxy) or mixing the cPCM with dry powder (e.g. silica fume) before being added to the CBM could improve the bonding with cement matrix and prevent the leakage of PCM. This is particularly obvious for hydrophilic supporting materials [63,81,87,91,95,96].

### 3. Properties of CBM mixed with PCM

In the past decade, most studies have concentrated on examining the feasibility of incorporating mPCM and cPCM into CBM. The
reported literature has excessively emphasized their effect on compressive strength and thermal properties. Lately, other properties including workability, hydration behavior, etc. of CBM incorporating PCM have also been studied and reported.

## 3.1. Fresh properties

### 3.1.1. Workability

In order to optimize the mix design, it is important to understand the impact of adding PCM on the fresh properties to minimize any consequential drawbacks on the CBM's mixing process as well as the resulting hardened properties of CBM. One of the main factors influencing the workability of CBM is the leakage of paraffin from the composite (micro or macro) [96].

Generally, the incorporation of mPCM in CBM significantly reduced its workability. In fact, the presence of fin spherical mPCM particles increases the specific surface area and thus decreases the amount of water available to lubricate the CBM system [97]. Moreover, the water absorption characteristics of the polymeric shell of the microcapsules as well as the adsorption of water by the hydrogen bonds of the hydroxyl and imino groups of the microcapsule could contribute to the increase in the water demand of the CBM mix [22,98]. As can be seen in Fig. 9, the incorporation of mPCM significantly reduced the amount of free water and resulted in an increase in the mix water demand (up to 22%) in order to compensate the workability loss [11,29,62,99,100].

With regards to the effect of cPCM, there is relatively limited information reported on the workability in the literature. In general, the high-water affinity of cPCM [63] could contribute to the increase of water demand of the CBM mix. In addition, the use of superplasticizer to maintain constant workability for all mixes (with and without PCM) indicates that cPCM affected the workability [82,101].

Based on the above-mentioned findings, it is suggested that both mPCM and cPCM had similar impact on the workability of CBM. The use of extra water or an appropriate amount of superplasticizer were the common methods adopted to compensate for the loss of workability.

<figure><img src="image_13.png" /><figcaption>Fig. 9. Amount of water added to the mix before and after adding 5% of mPCM (data taken from Refs. [62,69]).</figcaption></figure>

### 3.1.2. Hydration reaction

As known, PCM as a latent heat storage material is likely to absorb the heat released by the cement's hydration process. Hence, PCM has been suggested to be used to control temperature rise during the early age of cement hydration and mitigate the thermal stress for mass concrete structures. Generally, the same conventional isothermal and non-isothermal calorimetry were used to evaluate the hydration reaction of CBM incorporating PCM. As known, the isothermal calorimetry measures directly the heat rate (thermal power) produced by cement at a constant temperature condition while the non-isothermal (adiabatic

and semi-adiabatic) calorimetry measures the temperature released during hydration and then the produced heat will be calculated [102]. Kim et al. [103] studied the adiabatic temperature rise of concrete using different types of binder. It was found that the maximum hydration temperature could be decreased by approximately 3 °C when mPCM was used in a ternary mix containing ordinary Portland cement, fly ash and slag.

Other than decreasing the hydration temperature [69], the addition of mPCM could delay the time required to reach the maximum hydration temperature, and could exhibit a disruptive effect for the development of the hydration reaction. This finding was highlighted in the work of Eddhahak et al. [61] who used the semi-adiabatic calorimetry test to study the hydration reaction of mortar incorporating 0%, 1%, 3% and 5% mPCM. Similarly, Li et al. [104] noticed that mPCM decreased the hydration heat peak by 5 °C and delayed it by 60 min. Furthermore, based on a numerical model, Savija and Schlangen [105] found that the low-temperature rise and the delay of maximum hydration temperature could help in minimizing the maximum tensile stress caused by thermal effects that occurs in the structure, consequently leading to less thermal cracking of CBM.

Similar to mPCM, the use of cPCM affected also the heat evolution of cement hydration. Sharifi et al. [106,107] used the semi-adiabatic calorimetry and isothermal calorimetry to study the hydration reaction of mortar with different contents of cPCM. For the semi-adiabatic test, the mortar was filled in an insulated cylinder while the temperature in the centre of the cylinder as well as the ambient air were recorded for three days using a k-type thermocouple. A 10% decrease in the released heat and a delay of approximately 120 min was noticed [107]. On the other hand, mortar was filled into two sealed capped ampoules in a machine maintained at 25 °C in order to measure the heat of hydration using isothermal calorimetry. A similar result to the semi-adiabatic test was observed [106]. Bentz and Turpin [108] developed a home-built semi-adiabatic calorimeter to evaluate the hydration reaction of PCM-mortars. Specimens were produced by adding expanded shale/PCM particles and paraffin wax particles as sand replacement in the mix. The presence of the cPCM successfully delayed the peak temperature by 60 min and lowered the temperature by 8 °C. In another study [109], the peak hydration development of mortar incorporating 2% cPCM was delayed by several hours and the maximum temperature was decreased by nearly 40%.

Based on these results, both mPCM and cPCM could be potentially used to control and mitigate the cracks of mass concrete structure caused by cement hydration, primarily because of the PCM's ability to absorb heat released by the fresh mixtures.

## 3.2. Compressive strength

The influence of PCM on the mechanical strength of CBM tends to be inconsistent in the literature, which is understandable in view of the lack of proper mix designs as well as the variety of PCM types and incorporation methods (e.g. replacement or addition to cement or for sand) used in the CBM production. Studies have highlighted that early strength development (up to 28 days) was strongly influenced by the presence of PCM. The mechanical strengths results of CBM modified with mPCM and cPCM are summarized in Table 1 and Table 2, respectively. In general, a significant decrease in the mechanical strength was noticed with the incorporation of PCM. The mPCM had a slightly higher loss in compressive strength as compared to mixes with cPCM. Moreover, it was clear that the mechanical strength loss was higher when the PCM dosages exceeded a certain optimum value, depending on the types of PCM used as shown in Tables 1 and 2. Additionally, the PCM's incorporation methods did not influence the mechanical strength loss when considering the same amount of PCM. For example, when approximately 5% of mPCM was incorporated into the mix either by total mass of concrete [69] or by weight replacement of fine aggregates in concrete [28], the relatively same mechanical strength reductions of
<table><caption>**Table 1**
Effect of mPCM on the mechanical strength of CBM.</caption><thead><tr><th colspan="7">(a) Replacement</th></tr><tr><th>Ref</th><th>MPCM type</th><th>MPCM (%)</th><th>Mix/technique</th><th>Increase/ decrease* (%)</th><th>Reasons given by authors for the decrease or increase of strength</th><th></th></tr></thead><tbody><tr><td rowspan="4">[21]</td><td rowspan="4">PCM-E</td><td>5</td><td rowspan="4">Mortar/Replacement (wt% fine aggregate)</td><td>23</td><td rowspan="4">• MPCM particles were uniformly dispersed<br>• MPCM particles were not damaged and coated with the hydration product<br>• The interface between mPCM particles and mortar was denser</td><td></td></tr><tr><td>10</td><td>34</td></tr><tr><td>15</td><td>23</td></tr><tr><td>20</td><td>8</td></tr><tr><td rowspan="4">PCM-M</td><td rowspan="4"></td><td>5</td><td rowspan="4"></td><td>-14</td><td rowspan="4">• MPCM particles had low mechanical strength<br>• MPCM particles were damaged and the leaked organic PCM interfered with the hydration reaction<br>• Agglomerated mPCM particles behaved like voids in the mortar</td><td></td></tr><tr><td>10</td><td>-20</td></tr><tr><td>15</td><td>-37</td></tr><tr><td>20</td><td>-39</td></tr><tr><td rowspan="5">[110]</td><td rowspan="5">MPCM: paraffin encapsulated in melamine-formaldehyde shell</td><td>20<sup>(**)</sup></td><td rowspan="5">Aerial lime-mortar/Replacement (wt% fine aggregate)</td><td>-50</td><td rowspan="5">• The water to binder ratio increased<br>• The porosity increased</td><td></td></tr><tr><td>40</td><td>-75</td></tr><tr><td>60</td><td>-90</td></tr><tr><td>20<sup>(**)</sup></td><td>-38</td></tr><tr><td>40</td><td>-57</td></tr><tr><td rowspan="6">[28]</td><td rowspan="6">Micronal DS 5040X, BASF</td><td>1</td><td rowspan="6">CEM II-mortar/Replacement (wt% fine aggregate)</td><td>-65</td><td rowspan="6">• MPCM particles had lower strength compared to sand<br>• The porosity increased<br>• High volume of mPCM made the mix more linear</td><td></td></tr><tr><td>3</td><td>-33</td></tr><tr><td>5</td><td>-63</td></tr><tr><td>5</td><td>-69</td></tr><tr><td>10</td><td>-38</td></tr><tr><td>20<sup>(**)</sup></td><td>-14</td></tr><tr><td rowspan="4">[111]</td><td rowspan="4">Wet cake</td><td>5</td><td rowspan="4">Concrete/Replacement (vol% fine aggregate)</td><td>-27</td><td rowspan="4">• MPCM had low mechanical strength<br>• The volume fraction of cement decreased with an increasing amount of mPCM</td><td></td></tr><tr><td>10</td><td>-43</td></tr><tr><td>15</td><td>-17</td></tr><tr><td>20</td><td>-50</td></tr><tr><td rowspan="4">[100]</td><td rowspan="4">m-LDPE-EVA-Rubitherm RT27</td><td>5</td><td rowspan="4">Concrete/Replacement (vol% fine aggregate)</td><td>-9</td><td rowspan="4">• MPCM had low mechanical strength<br>• The porosity increased<br>• The interface between mPCM particles and concrete was poor</td><td></td></tr><tr><td>10</td><td>-15</td></tr><tr><td>15</td><td>-26</td></tr><tr><td>20</td><td>-42</td></tr><tr><td rowspan="2">[111]</td><td rowspan="2"></td><td>3.2</td><td rowspan="2">Concrete/Replacement (vol% fine aggregate)</td><td>-51</td><td rowspan="2">• MPCM had low mechanical strength<br>• The porosity increased<br>• The interface between mPCM particles and concrete was poor</td><td></td></tr><tr><td>2.7</td><td></td><td></td></tr></tbody></table>

<table><thead><tr><th colspan="7">(b) Addition</th></tr><tr><th>Ref</th><th>MPCM type</th><th>MPCM (%)</th><th>Mix/technique</th><th>Increase/decrease* (%)</th><th>Reasons given by authors for the decrease or increase of strength</th><th></th></tr></thead><tbody><tr><td rowspan="4">[112]</td><td rowspan="4">Micronal DS 5008 X</td><td>10</td><td rowspan="4">Hydrated lime-mortar/Addition (wt% total solids)</td><td>-20</td><td rowspan="4">1/Mechanical strength increased:<br>• MPCM contributed to a better pore size distribution (The pore size (macroporosity) of lime mortar decreased)</td><td></td></tr><tr><td>20</td><td>7</td></tr><tr><td>30</td><td>53</td></tr><tr><td>10</td><td>CEM II-mortar/Addition (wt% total solids)</td><td>-13</td></tr><tr><td rowspan="4">[75]</td><td rowspan="4">MPCM/Flaky graphite-doped</td><td>10</td><td rowspan="4">Cement paste/Addition (wt% cement)</td><td>-24</td><td rowspan="4">2/Mechanical strength decreased:<br>• The total porosity increased<br>• MPCM had low mechanical strength<br>• MPCM increased pores of samples</td><td></td></tr><tr><td>20</td><td>-36</td></tr><tr><td>25</td><td>-41</td></tr><tr><td>30<sup>(**)</sup></td><td>-48</td></tr><tr><td rowspan="6">[29]</td><td rowspan="6">MPCM28</td><td>14.8</td><td rowspan="6">Mortar/Addition (vol% mortar)</td><td>-50</td><td rowspan="6">• MPCM particles had higher absorption of water than other aggregates<br>• MPCM acted as a lubricant and a fluid after the damage of capsules<br>• MPCM particles behaved like voids in the mix in terms of mechanical strength</td><td></td></tr><tr><td>22.1</td><td>-62</td></tr><tr><td>29.3</td><td>-73</td></tr><tr><td>8.6</td><td>-44</td></tr><tr><td>13</td><td>-61</td></tr><tr><td>25</td><td>-85</td></tr><tr><td rowspan="3">[61]</td><td rowspan="3">Micronal DS 5038 X</td><td>1</td><td rowspan="3">Concrete/Addition (vol% concrete)</td><td>-14</td><td rowspan="3">• The leaked paraffin affected the hydration reaction<br>• MPCM-mortar released lower hydration heat than conventional mortar<br>• Cement hydration was delayed<br>• The interfacial bond between MPCM particles and cement grains was weak</td><td></td></tr><tr><td>3<sup>(**)</sup></td><td>-39</td></tr><tr><td>5</td><td>-47</td></tr><tr><td rowspan="4">[113]</td><td rowspan="4">CA-LA (inorganic PCM)</td><td>1</td><td rowspan="4">Concrete/Addition (vol% concrete)</td><td>-20</td><td rowspan="4">• Solid mPCM particles were difficult to distribute in the mix<br>• Agglomerated mPCM particles created irregular void space in the concrete<br>• MPCM had low mechanical strength<br>• MPCM increased pores in the cement mortar<br>• The interface between mPCM particles and concrete was poor</td><td></td></tr><tr><td>2</td><td>-30</td></tr><tr><td>1</td><td>-13</td></tr><tr><td>2</td><td>-28</td></tr><tr><td rowspan="4">[74]</td><td rowspan="4">MPCM-graphite</td><td>5</td><td rowspan="4">Concrete/Addition (wt% cement)</td><td>-26</td><td rowspan="4">• The leaked paraffin affected the hydration reaction by inhibiting the water transport<br>• The leaked paraffin could develop a phase interface</td><td></td></tr><tr><td>10</td><td>-34</td></tr><tr><td>15</td><td>-43</td></tr><tr><td>20</td><td>-49</td></tr><tr><td rowspan="3">[69]</td><td rowspan="3">Micronal DS 5008 X</td><td>1</td><td rowspan="3">Concrete/Addition (wt% concrete)</td><td>-30</td><td rowspan="3">• The leaked paraffin affected the hydration reaction by inhibiting the water transport<br>• The leaked paraffin could develop a phase interface</td><td></td></tr><tr><td>3<sup>(**)</sup></td><td>-53</td></tr><tr><td>5</td><td>-71</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued)**</caption><caption>(b) Addition</caption><thead><tr><th>Ref</th><th>MPCM type</th><th>MPCM (%)</th><th>Mix/technique</th><th>Increase/decrease* (%)</th><th>Reasons given by authors for the decrease or increase of strength</th></tr></thead><tbody><tr><td rowspan="4">[111]</td><td rowspan="4">Wet cake</td><td>5</td><td rowspan="4">Concrete/Addition (wt% concrete)</td><td>-24</td><td rowspan="4">• MPCM particles had no mechanical strength<br>• The volume fraction of cement decreased with an increasing amount of mPCM</td></tr><tr><td>10</td><td>-40</td></tr><tr><td>15</td><td>-38</td></tr><tr><td>20</td><td>-41</td></tr><tr><td>[11]</td><td>Micronal DS 5001X, BASF</td><td>3.2</td><td>Concrete/Addition (wt% concrete)</td><td>-68</td><td>• The incorporation method of mPCM<br>• MPCM increased the water content in the mix</td></tr><tr><td rowspan="8">[114]</td><td rowspan="8">PEG/SiO2 (inorganic shell)</td><td>1.5</td><td rowspan="8">Concrete/Addition (wt% concrete)</td><td>-37</td><td rowspan="8">• MPCM had low density and compressive strength</td></tr><tr><td>3</td><td>-50</td></tr><tr><td>4.5</td><td>-68</td></tr><tr><td>6</td><td>-71</td></tr><tr><td>7.5(**)</td><td>-81</td></tr><tr><td>9</td><td>-84</td></tr><tr><td>12</td><td>-90</td></tr><tr><td>15</td><td>-93</td></tr><tr><td>18</td><td>-95</td><td></td><td></td><td></td></tr></tbody></table>

(*) the sign (−) and (+) means that the mechanical strength of cement-based materials decreased and increased respectively with the use of PCM.

(**) optimum amount of PCM defined by the authors.

<caption>**Table 2**</caption>

Effect of cPCM on the mechanical strength of CBM.

<table><caption>(b) Addition</caption><caption>(a) Replacement</caption><thead><tr><th>Ref</th><th>CPCM type</th><th>CPCM (%)</th><th>Technique</th><th>Increase/decrease* (%)</th><th>Reasons given by authors for the decrease or increase of strength</th></tr></thead><tbody><tr><td rowspan="3">[82]</td><td rowspan="3">Paraffin/DP</td><td>10</td><td rowspan="3">Mortar/Replacement (wt% cement)</td><td>-27</td><td rowspan="3">• Not mentioned</td></tr><tr><td>15</td><td>-29</td></tr><tr><td>20</td><td>-44</td></tr><tr><td rowspan="2">[87]</td><td rowspan="2">Paraffin/Vermiculite</td><td>50</td><td rowspan="2">Mortar/Replacement (vol% fine aggregate)</td><td>-47</td><td rowspan="2">• CPCM had lower mechanical strength compared to sand</td></tr><tr><td>100</td><td>-56</td></tr><tr><td rowspan="4">[101]</td><td rowspan="4">Paraffin/EP</td><td>20</td><td rowspan="4">Mortar/Replacement (vol% fine aggregate)</td><td>12</td><td rowspan="4">• CPCM had a low stiffness</td></tr><tr><td>40</td><td>33</td></tr><tr><td>60</td><td>53</td></tr><tr><td>80</td><td>70</td></tr><tr><td>[115]</td><td>Paraffin/EP<br>CaCl<sub>2</sub>-Paraffin/EP</td><td>30</td><td>Geopolymer mortar with EP/Replacement (vol% fine aggregate)</td><td>1</td><td>• CPCM improved the pore structure and decreased the porosity</td></tr><tr><td>[81]</td><td>Paraffin/LWA</td><td>50</td><td>Concrete with LWA/Replacement (wt% coarse aggregate)</td><td>4</td><td>• The coating material of cPCM had a high strength</td></tr><tr><td></td><td></td><td>100</td><td></td><td>7</td><td>• Inclusion of silica fume as a layer for the coating material</td></tr></tbody></table>

<table><thead><tr><th>Ref</th><th>CPCM type</th><th>CPCM (%)</th><th>Technique</th><th>Increase/decrease* (%)</th><th>Reasons given by authors for the decrease or increase of strength</th></tr></thead><tbody><tr><td rowspan="4">[91]</td><td rowspan="4">Paraffin/EP</td><td>5</td><td rowspan="4">Mortar/Addition (wt% cement)</td><td>-23</td><td rowspan="4">• CPCM had lower mechanical strength compared to sand and cement past</td></tr><tr><td>10</td><td>-25</td></tr><tr><td>15</td><td>-41</td></tr><tr><td>20(**)</td><td>-43</td></tr><tr><td rowspan="5">[80]</td><td rowspan="5">n-Octadecane/EG</td><td>30</td><td rowspan="5">Mortar/Addition (wt% mortar)</td><td>-61</td><td rowspan="5">• Not mentioned</td></tr><tr><td>0.5</td><td>-32</td></tr><tr><td>1.2</td><td>-41</td></tr><tr><td>1.7</td><td>-49</td></tr><tr><td>2.5</td><td>-56</td></tr><tr><td rowspan="2">[107]</td><td rowspan="2">PCM 28/LWA</td><td>15.7</td><td rowspan="2">Mortar/Addition (vol% mortar)</td><td>-25</td><td rowspan="2">• LWA was more porous and had less strength than sand</td></tr><tr><td>22.6</td><td>-31</td></tr><tr><td rowspan="5">[85]</td><td rowspan="5">Process A: surface dried with compressed air</td><td>13.1</td><td rowspan="5">Mortar/Addition (vol% mortar)</td><td>-29</td><td rowspan="5">• The interface between cPCM and cement paste was poor</td></tr><tr><td>21.5</td><td>-41</td></tr><tr><td>33.6</td><td>-53</td></tr><tr><td>13.1</td><td>-26</td></tr><tr><td>21.5</td><td>-53</td></tr><tr><td>33.6</td><td>-56</td><td></td><td></td><td></td></tr></tbody></table>

(*) the sign (−) and (+) means that the mechanical strength of cement-based materials decreased and increased respectively with the use of PCM.

(**) optimum amount of PCM defined by the authors.
about 71% and 69% were noticed, respectively. Also, in the case of cPCM, the mechanical strength loss of the mix was almost identical at a given cPCM content. In fact, when 20% of cPCM was used to replace cement (by mass) in the mix [82] or added as total mass of cement [91], about 43% of mechanical strength loss was reported.

Some explanations have been reported in the literature for this
mechanical strength loss; however, the actual mechanism of the
strength loss is still unclear. According to the summary in Tables 1 and
2, the increase/decrease of compressive strength of CBM incorporating
PCM was mainly dependent on the type and content of PCM. Some
researchers attributed the mechanical strength loss to the low intrinsic
strength and density of PCM [28] whereby the inherent weaker nature
of PCM became a weak point when subjected to loading. Furthermore,
according to the “Bolomey” equation, PCM could be assimilated to
voids [29]. The presence of voids in concrete or mortar was known to
be the major cause of CBM’s strength reduction. In addition, the ag-
glomeration of PCM in the mix could also result in irregular void space
in concrete [113]. On the other hand, the poor workability associated
with the inclusion of PCM, the presence of increased entrapped air
voids and the loss of free water could negatively affect the hydration
process and consequently the mechanical strength of the matrix
[29,62]. As reported earlier, additional water used to compensate for
the workability loss could effectively increase the water/cement ratio in
the CBM mix, hence decreasing the compressive strength [110].

The phase change process of PCM was known also to alter the hy-
dration process of CBM. In addition, the increase of PCM volume re-
sulted in the decrease of cement content which impacts also the hy-
dration mechanism [61]. Moreover, the incompatibility between PCM
and cement matrix could be another possible factor affecting the
strength development of the CBM mixes. The poor bonding between the
mortar matrix and PCM, the damage of PCM and the subsequent
leakage of paraffin resulted in lower compressive strength [62,74]. In
fact, the leaked PCM coated on the cement particles could prevent the
cement hydration reaction and formation of the strength-bearing hy-
dration products [109].

On the contrary, an opposite trend of mechanical strength results
was also reported by few studies [21,81,112,115]. In fact, according to
some researches, PCM capsules which were uniformly dispersed
without damage in the mix could act as filler, contributing to a better
pore structure and interfacial bond with cement, thereby increasing the
compressive strength.

3.3. Thermal properties

The main objective of using PCM is to store and release heat,
therefore, PCM should have a high thermal conductivity and latent heat
in order to improve efficiently the storage capacity and heat transfer of
CBM. A large number of experimental and theoretical studies have been
conducted and some of their results are highlighted in the following
section.

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** (a) Thermal cycle, and (b) Thermal behavior of PCM-mortar and PCM-concrete specimens [29]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

3.3.1. *Thermal conductivity*

The low thermal conductivity of PCM is considered as one of the
major drawbacks for building applications. Because of this low thermal
conductivity, PCM would lose their main functionality as a thermal
energy storage material. Furthermore, this will impact the overall
thermal conductivity of CBM. It was noticed that even at small amounts
(up to 5%), mPCM (encapsulated in organic or inorganic shells) de-
creased the thermal conductivity of concrete by up to 37% [28]. Apart
from the low thermal conductivity of mPCM, the reduction could also
be related to both the low density and the increased entrapped air in
CBM caused by PCM [100,114].

A similar trend was also found for the use of cPCM in CBM, where
the thermal conductivity of mortar was found to decrease by 13.8%,
21.0%, 26.2% and 33.6% with the incorporation of 10%, 15% 20% and
30% of cPCM, respectively [82]. A similar range of reduction was ob-
served in Ref. [101], where the thermal conductivity of mortar de-
creased by 15.8%, 31.6%, 52.6% and 65.8% with an increase of cPCM
level at 20%, 40%, 60% and 80%, respectively. Likewise, the thermal
conductivity of mortar decreased by 15.5% with an increase in the mass
percentage of cPCM up to 2.5% [80]. Like mPCM, the reduction of
thermal conductivity of CBM was also attributed to the increase of
porosity and the reduction of CBM apparent density [80,101].

3.3.2. *Thermal energy storage capacity*

To the best of the author's knowledge, there is no standard or spe-
cifications related to the measurement of the thermal properties of
PCM-CBM system [116]. Therefore, some self-made equipment, ex-
tensively reviewed in Refs. [117,118], were developed to determine the
effect of PCM on the thermal energy storage capacity of CBM.

The performances of mortar and concrete samples exposed to
thermal cycles are presented in Fig. 10 [29]. The experiments were
performed on 2-cm-thick mortar and 5-cm-thick concrete samples. The
contribution of mPCM could be clearly identified for the concrete
sample. Compared with conventional mixes, the increase/decrease in
temperature of mPCM mix was at a much slower rate before stabiliza-
tion (Fig. 10 (b)). This reflects that mPCM could effectively improve the
apparent thermal inertia of mortar and concrete which is in line with
the results reported by Hunger et al. [69]. In fact, the thermal mass of
PCM-concrete during the phase change process of PCM was clearly
higher than that of the control mix [69]. A similar result was reported
by other studies [62,70]. On the other hand, a shorter time was needed
in the case of mortar to stabilize the temperature of the sample. This
was attributed to the small size of the mortar samples compared to
concrete samples [29].

Furthermore, mPCM can lower the temperature of cement modified
with mPCM by up to 13.1 °C which highlight the positive effect of PCM
in increasing the thermal mass of buildings [75].

Similar to mPCM, the use of cPCM exhibited a significant impact on
the thermal energy storage capacity of CBM. Xu and Li [82] showed
that the use of 30% of cPCM resulted in an increase in the thermal

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 11. Temperature evolution inside the room model (modified from Ref. [119]). Copyright (2019), with permission from Elsevier.</figcaption></figure>

energy storage capacity up to 5.438 kJ/kg. Ramakrishnan et al. [95] demonstrated that a cPCM panel had higher thermal inertia compared to the control panel without PCM. The thermal inertia was determined by calculating the temperature difference between the top and bottom surfaces of the panels. The maximum temperature differences were about 2.4 °C and 5.2 °C for the control and PCM panels, respectively. In addition, it was noticed that both panels had the same initial heat storage rate during the heating and cooling process but subsequently the storage rate of the cPCM panel increased at a much higher rate. On the other hand, a reduction of about 37% and an attenuation of approximately 30 min were observed in the internal heat flux peak of the cPCM panel during the heating process. A significant reduction of the internal heat flux measurement during the cooling process was also recorded. These results underlined the improvement of indoor thermal comfort when using cPCM panels. Moreover, the cPCM panels exhibited a similar thermal performance after 100 thermal cycles. A smaller temperature fluctuation during heating and cooling of a small wooden room model was noticed when the top panel wall was modified with cPCM [119]. As shown in Fig. 11, the room modified with cPCM reduced the temperature by about 1.9 °C when the room was heated up to 50 °C. During the cooling process (to 25 °C), the temperatures of the control room and PCM room were reduced by 5.8 °C and 2.3 °C, respectively. This further indicates that the use of PCM is beneficial in reducing indoor temperature fluctuations.

<figure><img src="image_17.png" /><figcaption>Fig. 12. An overview of the wall (a) With cPCM, and (b) Without cPCM [120]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

# 4. PCM in CBM: case studies, current gaps and future directions

As discussed earlier, PCM is a very attractive technology in terms of energy saving and sustainable development, however, this technology has not yet been widely adopted in the construction industry. Only few case studies concerning space heating and cooling strategies using PCM were reported in the literature and these are summarized in detail in the following section.

## 4.1. Case studies

To promote actual field implementation of PCM in buildings, case studies focusing on the application of PCM (mPCM/cPCM) in house-like cubicles and real buildings have been carried out. In situ measurements of the thermal behavior, in particular thermal conductivity and temperature change within the building envelopes are the main parameters to assess the PCM efficiency in buildings. Wang et al. [120] evaluated the influence of temperature on the effective thermal conductivity of a wall prepared with cPCM (Fig. 12). The developed cPCM consisted of 70 wt% paraffin impregnated in 15 wt% high-density polyethylene and 15 wt% of expanded graphite. Control and PCM walls of size 1.5 m × 1.5 m × 0.24 m and 1.5 m × 1.5 m × 0.12 m, respectively were tested. The control wall was made with Portland cement vitrified bricks (225mmx115mmx45mm) while the cPCM bricks (240mmx120mmx90mm) mixture consisted of 37.5% Portland cement, 22.5% sand, 40% cPCM and water (the water to cement ratio was 0.2). Less than 5 mm thick mortar was used to bind the bricks. A good linear relationship has been found between the effective thermal conductivity and the temperature of the control wall. The same behavior was observed for the PCM-wall when the temperature was below 15 °C and above 30 °C; however, when the air temperature increased from 15 °C to 30 °C, the thermal conductivity decreased. This result suggested that cPCM was experiencing phase change under these temperature conditions since the thermal conductivity of PCM is mainly affected by the latent heat of fusion and solidification [120].

Kong et al. [121] prepared a PCM wallboard (0.5 m × 0.5 m × 0.02 m) by incorporating cPCM (paraffin/expanded perlite) in styrene acrylic emulsion glass fiber and environmental protection coating. As shown in Fig. 13, the wallboard was installed on the internal surface of the walls and ceiling of a test room (1.7 m × 1.7 m × 2.1 m) located in Tianjin, China. Three different cases were tested and are summarized in Table 3. As indicated in all three cases, cPCM could passively regulate the indoor air temperature by smoothing the temperature fluctuations. In fact, the cPCM absorbed the inside heat and released it by natural convection to the outside environment when the temperature was lower than the phase change temperature of PCM. However, it was noticed that cases 1 and 2 were
<figure><img src="image_18.png" /><figcaption>Fig. 13. cPCM wallboards installed on the ceiling and walls of the test cell [121]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

<table><caption>**Table 3**

Studied cases and temperature peak variation compared to conventional test cell. Data taken from Ref. [121].</caption><thead><tr><th>Case no.</th><th>ΔT (°C) <sub>\with regards to conventional cell</sub></th><th>Temperature peak delay (min)</th></tr></thead><tbody><tr><td>Case 1: Doors and windows opened in the night</td><td>4.42</td><td>150</td></tr><tr><td>Case 2: Doors and windows kept closed until the outdoor temperature was lower than the phase change point</td><td>7.05</td><td>160</td></tr><tr><td>Case 3: Doors and windows closed</td><td>2.8</td><td>80</td></tr></tbody></table>

more effective in reducing and delaying the change in temperature of the room as compared to case 3 [121]. This indicated the importance of using night ventilation in order to enhance significantly the natural convection.

The thermal performances of paraffin/polypropylene/elastomer cPCM sheets in lightweight huts constructed in Chiba in Japan (Fig. 14) were evaluated [122]. Test results confirmed that cPCM was effective in reducing the peak indoor air temperature by 3.1 °C during daytime and increasing it by 2.5 °C during night, under natural condition. Additionally, cPCM could reduce the heating load by about 18.4%. However, it was reported that higher thermal benefits could be obtained when PCM received direct solar radiations.

In Portland, 130 Kg of PCM mats were installed in the second story of the west unit of a newly constructed duplex (Fig. 15). This region was characterized by a marine climate and a large diurnal temperature swing in the summer [123]. During summer, the indoor temperature of the PCM-unit was reduced by about 1.1 °C. Thus, the use of PCM reduced the annual overheated hours (the time outside thermal comfort) from 400 to 200.

A PCM panel composed of two aluminium sheets filled with PCM was designed by Kong et al. [124]. As indicated in Fig. 16, the PCM panels were installed in two different perforated brick test room (2 m × 2 m × 2.4 m). The panels were integrated on the outer surface walls and roof of the first test room (PCMOW) and on the inside surface walls and roof of the second room (PCMIW). Three different cases were evaluated namely case (1) free cooling experiments, case (2) open

<figure><img src="image_19.png" /><figcaption>Fig. 15. The passive house duplex home [123]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

window and door at night and case (3) forced ventilation at night. According to the test results presented in Table 4, installing PCM panels inside the wall (PCMIW) was more effective in reducing and delaying the indoor peak temperature [124]. Additionally, opening windows and door along with ventilation at night-time had improved the discharging process of PCM and hence more heat can be stored during the daytime.

In order to investigate light-building envelope, a PCM test cell

<figure><img src="image_20.png" /><figcaption>Fig. 14. Test huts and PCM sheets installed on the wall of the hut [122]. Copyright (2019), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Fig. 16. (a) Test room, sections of the wall structure of the (b) Conventional test cell, (c) PCMOW, and (d) PCMIW [124]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

<table><caption>**Table 4**
Results of different studied cases. Data taken from Ref. [124].</caption><thead><tr><th rowspan="2">Case no.</th><th rowspan="2">ΔT (°C) \with regards to conventional cell</th><th colspan="2">Temperature peak delay (h)</th></tr><tr><th>PCMOW</th><th>PCMIW</th></tr></thead><tbody><tr><td>Case 1</td><td>1</td><td>2.3</td><td>2.1</td><td>3</td></tr><tr><td>Case 2</td><td>0.41</td><td>2.46</td><td>Not mentioned</td><td>Not mentioned</td></tr><tr><td>Case3</td><td>Not mentioned</td><td>1.2</td><td>Not mentioned</td><td>Not mentioned</td></tr></tbody></table>

(3 m × 3 m × 3 m) was constructed at the University of Technology of Saint-Pierre in Reunion Island (tropical and humid climate) [125]. The PCM panels were integrated into the roof of the cell as shown in Fig. 17. For the purpose of verifying the efficiency of the suggested configuration (Fig. 17 (b)), a heat flux meter was installed on each surface of the roof layers. Test results showed (Fig. 17 (c)) that the PCM was effective

for the given configuration. In addition, it was found that the temperature of the inside plasterboard surface was 5 °C higher than that of the outside PCM surface. Furthermore, the use of PCM lowered the indoor temperature by approximately 2 °C compared to the conventional test cell [125].

Most of the studies focused on testing the efficiency of PCM during summer. However, as known, PCM is designed to be used for both summer and winter applications. Therefore, recent studies [126,127] were conducted to investigate the efficiency of PCM in buildings during winter. A mPCM plaster was developed and applied on the walls and ceiling of a laboratory test cell (2.08 m × 4.55 m × 2.40 m) in Hokkaido, Japan [126]. The composition of the mPCM plaster and the thermal properties of the mPCM are presented in Fig. 18. The test cell was divided into two halves by an insulated partition wall, and one of the halves was finished with mPCM plaster. A 35% reduction in the heating load was reported when mPCM plaster was used. In addition,

<figure><img src="image_22.png" /><figcaption>Fig. 17. (a) Test cell, (b) Schematic view of the roof, and (c) Temperature evolution of the roof [125]. Copyright (2019), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. 18.** (a) Thermal properties of PCM and composition of mPCM plaster, (b) Application of PCM plaster to the wall, and (c) Overview of the room after applying the mPCM plaster [126]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

PCM decreased significantly the temperature fluctuation. Later, PCM plaster (100 kg) was also applied in a full-scale residential building, offering a comfortable and stable indoor temperature around 20 °C [126].

A PCM-tile (660 mm × 660 mm × 52 mm) was installed into the floor of a full-scale bioclimatic house located at the University of Madrid in order to test the efficiency of PCM in controlling the indoor temperature during winter [127]. The tile consisted of 4 pieces as shown in Fig. 19: (1) pure clay stoneware (20 mm thick), (2) top metal sheet (3 mm), (3) metal container (32 mm thick) containing 4.8 l of paraffinic mixture; and (4) a layer of thermal insulation (22 mm thick). The surface temperature of the floor was measured in the morning, afternoon and night for 60 days. In comparison to the conventional floor tile without PCM (see Table 5), it was found that PCM tiles could release the stored energy to the room when the outside temperature was low and store the energy when the outside temperature was high. Furthermore, it was noticed that the amount of energy stored by the PCM-tile could be reduced by the presence of any obstacles which could cause any significant shadows on the tile and this might affect the discharge process of the PCM. Thus, in order to evaluate the effect of shadows on the performances of the PCM-tile, 3 different cases (Table 6) were tested [127]. According to Table 6, the temperature of PCM-tile that didn't receive direct sunlight was 0.5 °C higher than the

<caption>**Table 5**</caption>

Temperature stored and released by PCM tile exposed to direct sunlight. Data taken from Ref. [127].

<table><thead><tr><td>Outdoor temperature</td><td>ΔT (°C) \ with regards to conventional tile</td><td>Phase change process</td></tr></thead><tbody><tr><td>2–10 °C (low)</td><td>(–) 1.5–2 °C</td><td>Discharge</td></tr><tr><td>17–26 °C (high)</td><td>(+) 4–10 °C</td><td>Charge</td></tr><tr><td>Less than 26 °C</td><td>(–) Not mentioned</td><td>Discharge</td></tr></tbody></table>

(–) temperature released.

(+) temperature stored.

conventional tiles while it could reach 2 °C when the PCM tiles received direct solar radiation. Thus, to maximize energy efficiency, it is necessary that PCM receives enough solar radiation during the thermal storage period [127].

The effect of shadows provided by an awning on the performances of a full-scale outdoor test house (3 m × 3 m) with mPCM was also evaluated by Arce et al. [128]. The small house-sized cubicle was located at Lleida in Spain (Fig. 20) and constructed using a novel pre-fabricated concrete panel containing 5 wt% of mPCM. The awning (4.4 m × 4 m) was installed 12 cm above the roof of the cubicle. Unlike [127], it was concluded that the presence of shadows facilitated the

<figure><img src="image_24.png" /><figcaption>**Fig. 19.** (a) Cross section of the tile, and (b) Image of the tile installed onto the floor of the room [127]. Copyright (2019), with permission from Elsevier.</figcaption></figure>
<table><caption>**Table 6**
Experiment set and energy stored and released by a tile with PCM not being exposed to direct sunlight. Data taken from Ref. [127].</caption><thead><tr><th>Case no.</th><th>Blinds</th><th>Tiles position</th><th>Temperature difference evolution</th></tr></thead><tbody><tr><td>Case 1</td><td>Closed</td><td>Near the window</td><td rowspan="3">Temperature difference<br>0.00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:01:001.002.003.004.005.0016.0017.0018.0019.0020.0021.0022.0023.00<br>Time (h)</td></tr><tr><td>Case 2</td><td>Opened</td><td>In the interior of the corridor</td></tr><tr><td>Case 3</td><td>Closed</td><td>In the interior of the corridor</td></tr></tbody></table>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>**Fig. 20.** A house-like cubicle with awning [128]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

melting and solidification of mPCM, when incorporated in the roof and the walls of the test cell, which allowed higher comfort hours inside the cubicle during summer. In fact, compared to cubicle without awning and for a free cooling condition (windows remained open at night) the peak temperature reduction and the time delay of the peak increased by about 6% and 36%, respectively [128].

In order to optimize the cost of the experimental investigation, the

real thermal behavior of the PCM-test cubicle was also numerically studied by changing the system variables. Mainly the numerical model considered the real environment conditions (i.e. temperature, wind speed) and took into account the phase change of PCM. The temperature changes inside a wall with and without PCM are presented in Fig. 21 [129]. Compared to the wall without PCM, the maximum and minimum temperature were reduced and increased by about 3 °C which contributed to 8% of energy saving. When the PCM wall received heat, PCM started to melt by absorbing the heat which resulted in lowering the indoor temperature. The heat absorption process ended when the PCM melted completely (PCM liquid fraction = 1). However, the absorbed heat couldn't fully discharge at night (PCM liquid fraction not equal to 0) as shown in Fig. 22. Therefore, selecting another PCM with a lower phase change temperature will be more efficient to release the stored heat and hence results in better management of the indoor temperature [129].

Different test cells (2 m × 2 m × 0.12 m) were built in Adana, Turkey [128]. The south and west walls of the mPCM-test cell were cast using a 0.06 m thick PCM-concrete layer (towards the inside of the room) and 0.06 m thick concrete layer (Fig. 23 (b)). The temperature variation of the walls was recorded (for 2 consecutive years) using an IR camera at noon during autumn (Figure 23) and winter (Fig. 24). As can be seen in Fig. 23 (a), the highest external temperature of the

<figure><img src="image_27.png" /><figcaption>**Fig. 21.** Evolution of the temperature inside the wall (a) without PCM and (b) with PCM [129]. Copyright (2019), with permission from authors.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Fig. 22.** Evolution of the PCM liquid fraction (a) during several hours (b) after 144 h [129]. Copyright (2019), with permission from authors.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>Fig. 23. Temperature distribution in a panel without PCM and with PCM recorded in November [130]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

reference test cell (0% mPCM) wall was about 35 °C. After being dif-
fused in the wall, the temperature decreased and was about 27 °C.
Compared to the reference panel, a similar temperature was observed in
the first half layer (no mPCM) of the wall. However, the temperature on
the right side of the wall (with mPCM) was 5 °C lower than that of the
reference test cell [130].

On the other hand, during winter, the temperature of the PCM wall was 5 °C higher than that of the reference wall (Fig. 24) [130]. This result highlights that the use of this novel double layer panels can result in annual energy savings of up to 13%. The same results were recorded during the whole test duration (2 years) which demonstrated the good thermal stability of PCM inside the wall [130].

4.2. Current gaps and future directions for the application of PCM in CBM

So far, extensive research works on CBM incorporating PCM (mPCM/cPCM) have been undertaken worldwide to promote its use as an energy efficient building material. In overall, the examined literature findings showed that PCM could be a promising material to improve the thermal performances of CBM. However, several critical issues were identified including the selection of a suitable PCM which could meet all the desired properties. Based on the findings, PCM in the melted state could leak from its containment medium. In addition, PCM tended to have a very low thermal conductivity which affected the ef- ficiency of energy storage and release. Therefore, innovative ways to develop a leak-free PCM with enhanced thermal conductivity have been recently studied, by using additives and coating materials to provide extended surfaces to the PCM. However, such solutions might in turn increase the overall cost and sometimes weaken the PCM performances. Therefore, there is still a need to explore alternative and economical mediums for PCM which will be able to contain and prevent the PCM leakage and provide it with sufficiently high thermal conductivity and good intrinsic strength. For example, one of the novel ideas consists of preparing a PCM-foamed cement and casting it into hardened hollow concrete block (Fig. 25). Another innovative approach consists of

<figure><img src="image_30.png" /><figcaption>Fig. 25. Schematic overview of the filling process of PCM-foamed cement onto the concrete block.</figcaption></figure>

incorporating PCM in artificial aggregate during the granulation pro-
cess. The proposed benefits of this concept are the reduced cost when
compared to cPCM as well as the prevention of both the damage of PCM
particles and leakage problem due to the low friction induced during
the pelletization process and the dense shell layer of the produced ag-
gregate.

Another major research gap in utilizing PCM is to optimize both
thermal and mechanical properties when using PCM for the develop-
ment of a sustainable CBM for building application. As discussed in
section 3.2, the use of PCM in CBM deteriorates its mechanical strength
considerably. Thus, conducting a micro-mechanical studies will be
beneficial to identify the underlying mechanism influencing this

<figure><img src="image_31.png" /><figcaption>Fig. 24. General view of the test buildings in January [130]. Copyright (2019), with permission from Elsevier.</figcaption></figure>
deterioration and to develop consistent optimal mixture composition for using PCM in CBM. As shown in Tables 1 and 2, the increase of CBM porosity was one of the factors causing the strength alteration. Therefore, it is important to identify and confirm whether the observed increase of CBM porosity is entirely related to the presence of PCM particles or because of the chemical degradation (dissolution and leaching) of cement hydration products following the leakage of PCM [131]. Furthermore, a comprehensive micro-mechanical study can shed light on the possible presence of micro-cracks in CBM containing PCM. The micro-cracks could occur due to the presence of extra water absorbed by PCM which could wet the cement particles and lead to the formation of additional hydration products, thus causing internal stress [132,133].

Finally, PCM-CBM is produced and tested currently using the conventional concrete/mortar standards. Therefore, adopting the conventional standards with appropriate modifications to consider the unique thermophysical properties of PCM is suggested to encourage and expedite wider acceptance of the product by the relevant industries.

## 5. Conclusions

Phase change materials (PCM) can be an effective solution to improve the energy efficiency of buildings. In the past decades, several studies have been conducted to incorporate mPCM and cPCM into cement-based materials (CBM). In general, mPCM and cPCM demonstrated similar negative and positive impacts on the overall performance of CBM with regard to workability, hydration reaction, compressive strength and thermal properties. Therefore, the authors couldn't select one type of PCM over the other. It was also highlighted that in some situations in particular when related to the compressive strength mPCM is more appropriate to be used than the other cPCM and vice versa. Therefore, through this comparison, it's apparent that further work is needed to develop a new PCM having a negligible effect on the compressive strength of CBM. Field studies also demonstrated that the use of mPCM and cPCM is feasible and can improve the indoor thermal comfort of buildings during summer and winter. Nevertheless, continuous research and development are still required for the use of PCM in CBM commercially in a larger scale.

## Compliance with ethical standards

### Funding

The research funding from Young Talent Program of Hunan Provincial Science and Technology Program (801201048) is gratefully acknowledged.

## Conflicts of interest

The authors declare that they have no conflicts of interest.

## References

[1] Meggers F, Leibundgut H, Kennedy S, Qin M, Schlaich M, Sobek W, et al. Reduce CO2 from buildings with technology to zero emissions. Sustainable Cities and Society 2012;2:29–36.

[2] United Nations Environment Program (UNEP). Buildings and climate change, summary for decision-makers. 2009. p. 62.

[3] Leung DYC, Caramanna G, Maroto-Valer MM. An overview of current status of carbon dioxide capture and storage technologies. Renew Sustain Energy Rev 2014;39:426–43.

[4] Kaliyavaradhan SK, Ling T-C. Potential of CO2 sequestration through construction and demolition (C&D) waste—an overview. J CO2 Utilization 2017;20:234–42.

[5] Segehri A, Sarrafzadeh M-H. Effect of nitrifiers community on fouling mitigation and nitration efficiency in a membrane bioreactor. Chemical Engineering and Processing - Process Intensification 2018;128:10–8.

[6] Mohd Yasin NH, Maeda T, Hu A, Yu C-P, Wood TK. CO2 sequestration by methanogens in activated sludge for methane production. Appl Energy 2015;142:426–34.

[7] Energy Information Administration (eia). International energy outlook. 2016 https://www.eia.gov/outlooks/ieo/pdf/0484(2016).pdf, Accessed date: 9 November 2017.

[8] Energy Information Administration (eia). International energy outlook. 2017 https://www.eia.gov/outlooks/ieo/pdf/0484(2017).pdf, Accessed date: 9 November 2017.

[9] Energy Information Administration (eia). Buildings sector energy consumption. https://www.eia.gov/outlooks/ieo/pdf/buildings.pdf, Accessed date: 9 November 2017.

[10] Ling T-C, Poon C-S. Use of phase change materials for thermal energy storage in concrete: an overview. Constr Build Mater 2013;46:55–62.

[11] Figueiredo A, Lapa J, Vicente R, Cardoso C. Mechanical and thermal characterization of concrete with incorporation of microencapsulated PCM for applications in thermally activated slabs. Constr Build Mater 2016;112:639–47.

[12] Laura Pisello A, Fabiani C, Cotana F. New experimental technique to investigate the thermal behavior of PCM/doped concrete for enhancing thermal/energy storage capability of building envelope. Energ Procedia 2017;126:139–46.

[13] Khan MMA, Ibrahim NI, Mahbubul IM, Muhammad Ali H, Saidur R, Al-Sulaiman FA. Evaluation of solar collector designs with integrated latent heat thermal energy storage: a review. Sol Energy 2018;166:334–50.

[14] Souayfane F, Fardoun F, Biwole P-H. Phase change materials (PCM) for cooling applications in buildings: a review. Energy Build 2016;129:396–431.

[15] Panayiotou GP, Kalogirou SA, Tassou SA. Evaluation of the application of Phase Change Materials (PCM) on the envelope of a typical dwelling in the Mediterranean region. Renew Energy 2016;97:24–32.

[16] Athienitis AK, Liu C, Hawes D, Banu D, Feldman D. Investigation of the thermal performance of a passive solar test-room with wall latent heat storage. Build Environ 1997;32:405–10.

[17] Rubel F, Kottek M. Observed and projected climate shifts 1901-2100 depicted by world maps of the K \ o open-Geiger climate classification. Meteorol Z 2010;19:135–41.

[18] Liu J, Liu Y, Yang L, Hou L, Wang M, Liu J. Annual energy saving potential for integrated application of phase change envelopes and HVAC in Western China. Procedia Eng 2017;205:2470–7.

[19] Cui Y, Xie J, Liu J, Wang J, Chen S. A review on phase change material application in building. Adv Mech Eng 2017;9: 1687814017700828.

[20] Saffari M, de Gracia A, Usak S, Cabeza LF. Passive cooling of buildings with phase change materials using whole-building energy simulation tools: a review. Renew Sustain Energy Rev 2017;80:1239–55.

[21] Aguayo M, Das S, Maroli A, Kabay N, Mertens JCE, Rajan SD, et al. The influence of microencapsulated phase change material (PCM) characteristics on the micro-structure and strength of cementitious composites: experiments and finite element simulations. Cement Concr Compos 2016;73:29–41.

[22] Cunha S, Lima M, Aguiar JB. Influence of adding phase change materials on the physical and mechanical properties of cement mortars. Constr Build Mater 2016;127:1–10.

[23] Ahmad M. Nouveaux composants actifs pour la gestion énergétique de l'enveloppe légère des bâtiments. Couplage matériaux à changement de phase, super-isolation, apports solaires [PhD Thesis] Université Joseph Fourier; 2004.

[24] Wei Z, Falzone G, Wang B, Thiele A, Puerta-Falla G, Pilon L, et al. The durability of cementitious composites containing microencapsulated phase change materials. Cement Concr Compos 2017;81:66–76.

[25] Drissi S, Eddhahak A, Caré S, Neji J. Thermal analysis by DSC of Phase Change Materials, study of the damage effect. J Build Eng 2015;1:13–9.

[26] Cabeza LF, Castellón C, Nogués M, Medrano M, Leppers R, Zubillaga O. Use of microencapsulated PCM in concrete walls for energy savings. Energy Build 2007;39:113–9.

[27] Pereira da Cunha J, Eames P. Thermal energy storage for low and medium temperature applications using phase change materials – a review. Appl Energy 2016;177:227–38.

[28] Jayalath A, San Nicolas R, Sofi M, Shanks R, Ngo T, Aye L, et al. Properties of cementitious mortar and concrete containing micro-encapsulated phase change materials. Constr Build Mater 2016;120:408–17.

[29] Lecompte T, Le Bideau P, Glouannec P, Nortershauser D, Le Masson S. Mechanical and thermo-physical behaviour of concretes and mortars containing phase change material. Energy Build 2015;94:52–60.

[30] Amaral C, Vicente R, Marques PAAP, Barros-Timmons A. Phase change materials and carbon nanostructures for thermal energy storage: a literature review. Renew Sustain Energy Rev 2017;79:1212–28.

[31] Lin Y, Jia Y, Alva G, Fang G. Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage. Renew Sustain Energy Rev 2017;82:2730–41.

[32] Liu L, Su D, Tang Y, Fang G. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2016;62:305–17.

[33] Tauseef ur R, Ali HM, Janjua MM, Sajjad U, Yan W-M. A critical review on heat transfer augmentation of phase change materials embedded with porous materials/foams. Int J Heat Mass Transf 2019;135:649–73.

[34] Wilhelm E, Richter C, Rapp BE. Phase change materials in microactuators: basics, applications and perspectives. Sensor Actuator Phys 2018;271:303–47.

[35] Wuttig M, Yamada N. Phase-change materials for rewriteable data storage. Nat Mater 2007;6:824.

[36] Mondal S. Phase change materials for smart textiles – an overview. Appl Therm Eng 2008;28:1536–50.

[37] Cabeza LF, Castell A, Barreneche C, de Gracia A, Fernández AI. Materials used as PCM in thermal energy storage in buildings: a review. Renew Sustain Energy Rev
2011;15:1675-95.

[38] Giro-Paloma J, Martínez M, Cabeza LF, Fernández AI. Types, methods, techniques, and applications for microencapsulated phase change materials (MPCM): a review. *Renew Sustain Energy Rev* 2016;53:1059-75.

[39] Soares N, Costa JJ, Gaspar AR, Santos P. Review of passive PCM latent heat thermal energy storage systems towards buildings' energy efficiency. *Energy Build* 2013;59:82-103.

[40] Zhao CY, Zhang GH. Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications. *Renew Sustain Energy Rev* 2011;15:3813-32.

[41] Alva G, Lin Y, Liu L, Fang G. Synthesis, characterization and applications of microencapsulated phase change materials in thermal energy storage: a review. *Energy Build* 2017;144:276-94.

[42] Umair MM, Zhang Y, Iqbal K, Zhang S, Tang B. Novel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energy storage-A review. *Appl Energy* 2019;235:846-73.

[43] Zhang P, Xiao X, Ma ZW. A review of the composite phase change materials: fabrication, characterization, mathematical modeling and application to performance enhancement. *Appl Energy* 2016;165:472-510.

[44] Lv P, Liu C, Rao Z. Review on clay mineral-based form-stable phase change materials: preparation, characterization and applications. *Renew Sustain Energy Rev* 2017;68:707-26.

[45] Tyagi VV, Kaushik SC, Tyagi SK, Akiyama T. Development of phase change materials based microencapsulated technology for buildings: a review. *Renew Sustain Energy Rev* 2011;15:1373-91.

[46] Jamekhorshid A, Sadrameli SM, Farid M. A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium. *Renew Sustain Energy Rev* 2014;31:531-42.

[47] Liu Z, Yu Z, Yang T, Qin D, Li S, Zhang G, et al. A review on macro-encapsulated phase change material for building envelope applications. *Build Environ* 2018;144:281-94.

[48] Qureshi ZA, Ali HM, Khuskood S. Recent advances on thermal conductivity enhancement of phase change materials for energy storage system: a review. *Int J Heat Mass Transf* 2018;127:838-56.

[49] Drissi S, Ling T-C, Mo KH. Thermal efficiency and durability performances of paraffinic phase change materials with enhanced thermal conductivity – a review. *Thermochim Acta* 2019;673:198-210.

[50] Nematpour Keshlhti A, Sheikholeslami M. Nanoparticle enhanced PCM applications for intensification of thermal performance in building: a review. *J Mol Liq* 2019;274:516-33.

[51] Navarro L, de Gracia A, Niall D, Castell A, Browne M, McCormack SJ, et al. Thermal energy storage in building integrated thermal systems: a review. Part 2. Integration as passive system. *Renew Energy* 2016;85:1334-56.

[52] Rao VV, Parameshwaran R, Ram VV. PCM-mortar based construction materials for energy efficient buildings: a review on research trends. *Energy Build* 2018;158:95-122.

[53] Verma P, Varun, Singal SK. Review of mathematical modeling on latent heat thermal energy storage systems using phase-change material. *Renew Sustain Energy Rev* 2008;12:999-1031.

[54] Asgharian H, Baniasadi E. A review on modeling and simulation of solar energy storage systems based on phase change materials. *J Energy Storage* 2019;21:186-201.

[55] de Gracia A, Cabeza LF. Phase change materials and thermal energy storage for buildings. *Energy Build* 2015;103:414-9.

[56] Zhu N, Li S, Hu S, Wei S, Deng R, Lei F. A review on applications of shape-stabilized phase change materials embedded in building enclosure in recent ten years. *Sustainable Cities and Society* 2018;43:251-64.

[57] Song M, Niu F, Mao N, Hu Y, Deng S. Review on building energy performance improvement using phase change materials. *Energy Build* 2018;158:776-93.

[58] Cellat K, Beyhan B, Kazanci B, Konuklu Y, Paksoy H. Direct incorporation of butyl stearate as phase change material into concrete for energy saving in buildings. *J Clean Energy Technol* 2017;5:64-8.

[59] Barreneche C, Navarro ME, Cabeza LF, Fernández AI. New database to select phase change materials: chemical nature, properties, and applications. *J Energy Storage* 2015;3:18-24.

[60] Baldassarri C, Sala S, Caverzan A, Lamperti Tornaghi M. Environmental and spatial assessment for the ecodesign of a cladding system with embedded Phase Change Materials. *Energy Build* 2017;156:374-89.

[61] Eddhahak A, Drissi S, Colin J, Caré S, Neji J. Effect of phase change materials on the hydration reaction and kinetic of PCM-mortars. *J Therm Anal Calorim* 2014;117:537-45.

[62] Drissi S. Développement de nouveaux bétons "accumulateurs d'énergie" : investigations expérimentale, probabiliste et numérique du comportement thermique. *Université Paris-Est*; 2015.

[63] Ramakrishnan S, Wang X, Sanjayan J, Wilson J. Assessing the feasibility of integrating form-stable phase change material composites with cementitious composites and prevention of PCM leakage. *Mater Lett* 2017;192:88-91.

[64] Memon AS, Liao W, Yang S, Cui H, Shah FS. Development of composite PCMs by incorporation of paraffin into various building materials. *Mater* 2015;8.

[65] Shaid A, Wang L, Islam S, Cai JY, Padhye R. Preparation of aerogel-eicosane microparticles for thermoregulatory coating on textile. *Appl Therm Eng* 2016;107:602-11.

[66] Shchukina EM, Graham M, Zheng Z, Shchukin DG. Nanoencapsulation of phase change materials for advanced thermal energy storage systems. *Chem Soc Rev* 2018;47:4156-75.

[67] Hassan A, Shakeel Laghari M, Rashid Y. Micro-encapsulated phase change materials: a review of encapsulation, safety and thermal characteristics. *Sustainability-Basel* 2016;8.

[68] Cui Y, Xie J, Liu J, Pan S. Review of phase change materials integrated in building walls for energy saving. *Procedia Eng* 2015;121:763-70.

[69] Hunger M, Entrop AG, Mandilaras I, Brouwers HJH, Founti M. The behavior of self-compacting concrete containing micro-encapsulated Phase Change Materials. *Cement Concr Compos* 2009;31:731-43.

[70] Eddhahak-Ouni A, Drissi S, Colin J, Neji J, Care S. Experimental and multi-scale analysis of the thermal properties of Portland cement concretes embedded with microencapsulated Phase Change Materials (PCMs). *Appl Therm Eng* 2014;64:32-9.

[71] Hawes DW, Banu D, Feldman D. The stability of phase change materials in concrete. *Sol Energy Mater Sol Cell* 1992;27:103-18.

[72] Silakhori M, Naghavi SM, Metselaar SH, Mahlia MT, Fauzi H, Mehrali M. Accelerated thermal cycling test of microencapsulated paraffin wax/polyaniline made by simple preparation method for solar thermal energy storage. *Mater* 2013;6.

[73] Wang Y, Shi H, Xia TD, Zhang T, Feng HX. Fabrication and performances of microencapsulated paraffin composites with polymethylmethacrylate shell based on ultraviolet irradiation-initiated. *Mater Chem Phys* 2012;135:181-7.

[74] Cui H, Liao W, Mi X, Lo TY, Chen D. Study on functional and mechanical properties of cement mortar with graphite-modified microencapsulated phase-change materials. *Energy Build* 2015;105:273-84.

[75] Zhang H, Xing F, Cui H-Z, Chen D-Z, Ouyang X, Xu S-Z, et al. A novel phase-change cement composite for thermal energy storage: fabrication, thermal and mechanical properties. *Appl Energy* 2016;170:130-9.

[76] Şahan N, Paksoy H. Determining influences of SiO2 encapsulation on thermal energy storage properties of different phase change materials. *Sol Energy Mater Sol Cell* 2017;159:1-7.

[77] Yu S, Wang X, Wu D. Microencapsulation of n-octadecane phase change material with calcium carbonate shell for enhancement of thermal conductivity and serving durability: synthesis, microstructure, and performance evaluation. *Appl Energy* 2014;114:632-43.

[78] Cao L, Tang F, Fang G. Preparation and characteristics of microencapsulated palmitic acid with TiO2 shell as shape-stabilized thermal energy storage materials. *Sol Energy Mater Sol Cell* 2014;123:183-8.

[79] Zhang H, Sun S, Wang X, Wu D. Fabrication of microencapsulated phase change materials based on n-octadecane core and silica shell through interfacial polycondensation. *Colloids Surf, A* 2011;389:104-17.

[80] Zhang Z, Shi G, Wang S, Fang X, Liu X. Thermal energy storage cement mortar containing n-octadecane/expanded graphite composite phase change material. *Renew Energy* 2013;50:670-5.

[81] Memon SA, Cui HZ, Zhang H, Xing F. Utilization of macro encapsulated phase change materials for the development of thermal energy storage and structural lightweight aggregate concrete. *Appl Energy* 2015;139:43-55.

[82] Xu B, Li Z. Paraffin/diatomite composite phase change material incorporated cement-based composite for thermal energy storage. *Appl Energy* 2013;105:229-37.

[83] Kong X, Zhong R, Rong X, Min C, Qi C. Building energy storage panel based on paraffin/expanded perlite: preparation and thermal performance study. *Mater* 2016;9.

[84] Sun Z, Zhang Y, Zheng S, Park Y, Frost RL. Preparation and thermal energy storage properties of paraffin/calcined diatomite composites as form-stable phase change materials. *Thermochim Acta* 2013;558:16-21.

[85] Nepomuceno MCS, Silva PD. Experimental evaluation of cement mortars with phase change material incorporated via lightweight expanded clay aggregate. *Constr Build Mater* 2014;63:89-96.

[86] Li M, Wu Z, Tan J. Heat storage properties of the cement mortar incorporated with composite phase change material. *Appl Energy* 2013;103:393-9.

[87] Xu B, Ma H, Lu Z, Li Z. Paraffin/expanded vermiculite composite phase change material as aggregate for developing lightweight thermal energy storage cement-based composites. *Appl Energy* 2015;160:358-67.

[88] Memon SA, Lo TY, Cui H, Barbhuiya S. Preparation, characterization and thermal properties of dodecanol/cement as novel form-stable composite phase change material. *Energy Build* 2013;66:697-705.

[89] Wang Q, Zhang J, Ding Z, Qiu L. Study on properties of paraffin phase change energy storage concrete. *Energ Procedia* 2012;16:365-70.

[90] Chen Z, Qin M. Preparation and hydrothermal properties of composite phase change humidity control materials. *Appl Therm Eng* 2016;98:1150-7.

[91] Sun D, Wang L. Utilization of paraffin/expanded perlite materials to improve mechanical and thermal properties of cement mortar. *Constr Build Mater* 2015;101:791-6.

[92] Ricklefs A, Thiele AM, Falzone G, Sant G, Pilon L. Thermal conductivity of cementitious composites containing microencapsulated phase change materials. *Int J Heat Mass Transf* 2017;104:71-82.

[93] Kheradmand M, Castro-Gomes J, Azenha M, Silva PD, de Aguiar JLB, Zoorob SE. Assessing the feasibility of impregnating phase change materials in lightweight aggregate for development of thermal energy storage systems. *Constr Build Mater* 2015;89:48-59.

[94] Zhang Q, Wang H, Ling Z, Fang X, Zhang Z. RT100/expand graphite composite phase change material with excellent structure stability, photo-thermal performance and good thermal reliability. *Sol Energy Mater Sol Cell* 2015;140:158-66.

[95] Ramakrishnan S, Sanjayan J, Wang X, Alam M, Wilson J. A novel paraffin/expanded perlite composite phase change material for prevention of PCM leakage in cementitious composites. *Appl Energy* 2015;157:85-94.

[96] Li H, Chen H, Li X, Sanjayan JG. Development of thermal energy storage composites and prevention of PCM leakage. *Appl Energy* 2014;135:225-33.
[97] Lucas S, Senff L, Ferreira V, Barroso de Aguiar JL, Labrincha JA. Fresh state characterization of lime mortars with PCM additions. Appl Rheol 2010;20:63162-7.

[98] Djamai ZI, Salvatore F, Si Larbi A, Cai G, El Mankibi M. Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars. Cement Concr Res 2019;119:51-63.

[99] Cunha S, Aguiar J, Pacheco-Torgal F. Effect of temperature on mortars with incorporation of phase change materials. Constr Build Mater 2015;98:89-101.

[100] Cao VD, Pilehvar S, Salas-Bringas C, Szczotok AM, Rodriguez JF, Carmona M, et al. Microencapsulated phase change materials for enhancing the thermal performance of Portland cement concrete and geopolymer concrete for passive building applications. Energy Convers Manag 2017;133:56-66.

[101] Ramakrishnan S, Wang X, Sanjayan J, Wilson J. Thermal energy storage enhancement of lightweight cement mortars with the application of phase change materials. Procedia Eng 2017;180:1170-7.

[102] Frølich L, Wadsø L, Sandberg P. Using isothermal calorimetry to predict one day mortar strengths. Cement Concr Res 2016;88:108-13.

[103] Kim Y-R, Khil B-S, Jang S-J, Choi W-C, Yun H-D. Effect of barium-based phase change material (PCM) to control the heat of hydration on the mechanical properties of mass concrete. Thermochim Acta 2015;613:100-7.

[104] Li M, Hui L. Study on the phase change cement based material. Int J Struct Civ Eng 2016;5:136-9.

[105] Šavija B, Schlangen E. Use of phase change materials (PCMs) to mitigate early age thermal cracking in concrete: theoretical considerations. Constr Build Mater 2016;126:332-44.

[106] Sharifi NP. Application of phase change materials to improve the thermal performance of buildings and pavements [PhD Thesis] Faculty of the WORCESTER POLYTECHNIC INSTITUTE; 2016.

[107] Sharifi NP, Sakulich A. Application of phase change materials to improve the thermal performance of cementitious material. Energy Build 2015;103:83-95.

[108] Bentz DP, Turpin R. Potential applications of phase change materials in concrete technology. Cement Concr Compos 2007;29:527-32.

[109] Sakulich AR, Bentz DP. Incorporation of phase change materials in cementitious systems via fine lightweight aggregate. Constr Build Mater 2012;35:483-90.

[110] Cunha S, Aguiar J, Ferreira V, Tadeu A. Mortars based in different binders with incorporation of phase-change materials: physical and mechanical properties. Eur J Environ Civ Eng 2015;19:1216-33.

[111] Pania M, Yunping X. Effect of phase-change materials on properties of concrete. Mater J.109.

[112] Lucas SS, Ferreira VM, Aguiar JLBd. Latent heat storage in PCM containing mortars—study of microstructural modifications. Energy Build 2013;66:724-31.

[113] Cellat K, Beyhan B, GÜngör C, Konuklu Y, Karahan O, Dündar C, et al. Thermal enhancement of concrete by adding bio-based fatty acids as phase change materials. Energy Build 2015;106:156-63.

[114] Xu T, Chen Q, Zhang Z, Gao X, Huang G. Investigation on the properties of a new type of concrete blocks incorporated with PEG/SiO2 composite phase change material. Build Environ 2016;104:172-7.

[115] Wang Z, Su H, Zhao S, Zhao N. Influence of phase change material on mechanical and thermal properties of clay geopolymer mortar. Constr Build Mater 2016;120:329-34.

[116] Xie J, Li Y, Wang W, Pan S, Cui N, Liu J. Comments on thermal physical properties testing methods of phase change materials. Adv Mech Eng 2013;5:695762.

[117] Cabeza LF, Barreneche C, Martorell I, Miró L, Sari-Bey S, Fois M, et al.

Unconventional experimental technologies available for phase change materials (PCM) characterization. Part 1. Thermophysical properties. Renew Sustain Energy Rev 2015;43:1399-414.

[118] Inés Fernández A, Solé A, Giró-Paloma J, Martínez M, Hadjieva M, Boudenne A, et al. Unconventional experimental technologies used for phase change materials (PCM) characterization: part 2 – morphological and structural characterization, physico-chemical stability and mechanical properties. Renew Sustain Energy Rev 2015;43:1415-26.

[119] Suttaphakdee P, Dulsang N, Lorwanishpaisarn N, Kasemsiri P, Posi P, Chindaprasirt P. Optimizing mix proportion and properties of lightweight concrete incorporated phase change material paraffin/recycled concrete block composite. Constr Build Mater 2016;127:475-83.

[120] Wang X, Yu H, Li L, Zhao M. Research on temperature dependent effective thermal conductivity of composite-phase change materials (PCMs) wall based on steady-state method in a thermal chamber. Energy Build 2016;126:408-14.

[121] Kong X, Yao C, Jie P, Liu Y, Qi C, Rong X. Development and thermal performance of an expanded perlite-based phase change material wallboard for passive cooling in building. Energy Build 2017;152:547-57.

[122] Kim HB, Mae M, Choi Y, Kiyota T. Experimental analysis of thermal performance in buildings with shape-stabilized phase change materials. Energy Build 2017;152:524-33.

[123] Sage-Lauck JS, Sailor DJ. Evaluation of phase change materials for improving thermal comfort in a super-insulated residential building. Energy Build 2014;79:32-40.

[124] Kong X, Lu S, Huang J, Cai Z, Wei S. Experimental research on the use of phase change materials in perforated brick rooms for cooling storage. Energy Build 2013;62:597-604.

[125] Guichard S, Miranville F, Bigot D, Malet-Damour B, Boyer H. Experimental investigation on a complex roof incorporating phase-change material. Energy Build 2015;108:36-43.

[126] Kusama Y, Ishidoya Y. Thermal effects of a novel phase change material (PCM) plaster under different insulation and heating scenarios. Energy Build 2017;141:226-37.

[127] Cerón I, Neila J, Khayet M. Experimental tile with phase change materials (PCM) for building use. Energy Build 2011;43:1869-74.

[128] Arce P, Castellón C, Castell A, Cabeza LF. Use of microencapsulated PCM in buildings and the effect of adding awnings. Energy Build 2012;44:88-93.

[129] Gómez MA, Álvarez Feijoo MA, Comesaña R, Eguía P, Míguez JL, Porteiro J. CFD simulation of a concrete cubicle to analyze the thermal effect of phase change materials in buildings. Energies 2012;5.

[130] Cellat K, Beyhan B, Konuklu Y, Dündar C, Karahan O, GÜngör C, et al. 2 years of monitoring results from passive solar energy storage in test cabins with phase change materials. Sol Energy 2019. (in press).

[131] Gallé C, Peycelon H, Bescop PL. Effect of an accelerated chemical degradation on water permeability and pore structure of cementbased materials. Adv Cem Res 2004;16:105-14.

[132] Hall C, Hoff WD, Taylor SC, Wilson MA, Yoon B-G, Reinhardt HW, et al. Water anomaly in capillary liquid absorption by cement-based materials. J Mater Sci Lett 1995;14:1178-81.

[133] Liu X, Meng H, Wang Y, Katayama Y, Gu J-D. Water is a critical factor in evaluating and assessing microbial colonization and destruction of Angkor sandstone monuments. Int Biodeterior Biodegrad 2018;133:9-16.