When a phase transition occurs (such as from solid to liquid or liquid to gas), phase change materials (PCMs) have the ability to absorb and release significant amounts of thermal energy. These substances are crucial for a number of applications, such as heat management, temperature control, and energy storage. The following elaborates on a few common examples of phase transition materials and their thermal behaviour, stability, physical, and chemical characteristics in relation to their applications ^[1][2][3][1,2,3].

First, paraffin wax (organic) has a well-defined melting point and strong thermal stability, which makes the phase shift process predictable and reproducible. It is generally stable across a broad temperature range and during numerous cycles of melting and solidification. Depending on its composition, paraffin wax commonly melts between 46 °C and 68 °C at relatively low temperatures [4]. Due to its non-toxic and non-reactive chemical properties, it can be used for a variety of purposes, including thermal energy storage, food packaging, and building insulation. The use of paraffin wax as a thermal interface medium in electronics and in passive thermal control systems for buildings is quite popular.

Second, salt hydrates (inorganic) are solid substances that have high latent heat values and acute melting points, which in turn results in effective energy storage and release during phase transitions. They are typically stable when kept in a closed container, although they can deteriorate when exposed to air moisture. Salt hydrates have hygroscopic properties, which means they are prone to absorbing moisture [5]. Water absorption must be avoided because it can impair their effectiveness. In applications for air cooling and solar thermal energy storage, salt hydrates are preferred.

Third, eutectic mixtures are made up of two or more organic compounds that have a certain composition that enables them to melt and solidify at consistent temperatures, resulting in a clearly defined phase shift. Eutectic mixes often display high cycle lives and are stable. These materials come in a variety of shapes, including slabs, powders, and pellets, and their melting points can be adjusted to meet the needs of certain applications [6]. Eutectic PCMs are typically non-toxic and non-corrosive, which makes them appropriate for thermal energy storage in electronics and structures such as portable cooling units and temperature-controlled packaging.

Fourth, metallic PCMs (inorganic) frequently exhibit strong thermal conductivity, allowing for quick heat transfer throughout phase change operations. These materials have great thermal stability and can endure repeated cycles of melting and solidification without suffering serious damage [7]. To best suit certain applications, metallic PCMs might take the form of pure metals or metal alloys with specialised melting points. Although they can differ depending on the particular metal or alloy used, chemical qualities are typically stable and non-toxic. Aerospace applications and high-power electronics cooling frequently use metallic PCMs.

Nowadays, the rising energy needs of buildings and the severity of global warming are significant problems. To address these issues, thermal energy storage (TES) building materials are being prepared by incorporating phase change materials (PCMs) into construction materials ^{[8][9][10][11]}[8,9,10,11]. The ability of PCMs to collect and release thermal energy suggests that they might be used to mitigate the effects of temperature fluctuations on building performance. Thus, PCMs will serve to simultaneously provide thermal comfort and reduce energy consumption in buildings ^{[12][13][14][15][16][17][18][19]}[12,13,14,15,16,17,18,19].

While, in theory, every substance may be called a PCM, not all of them are practical for everyday construction. In fact, PCMs necessitate guaranteeing a minimum of three rudimentary needs to be efficiently incorporated into the components of construction (such as structures and incorporated energy regimes): (1) an elevated melting enthalpy, (2) a suitable temperature of phase change, and (3) a limited volume variation through the phase change. Further features, like safety and heat conductivity, and many methodical as well as economic issues, such as manufacture cost and usage mode, have to be taken into account while selecting the correct PCM ^{[20][21][22][23][24][25]}[20,21,22,23,24,25].

Over the last decade, there has been a significant surge in academic interest in PCMs as a result of the widespread availability of products at affordable prices and potential areas of utilisation in building applications [26]. Walls, ceilings, roofs, and windows are all examples of building components or structural members that can integrate PCMs ^[27][28][29][27,28,29]. To develop lighter architectural structures, PCM integration often reduces the mass density of the final materials. The usage of latent heat storage materials like PCMs was extensively researched by the technical community due to their significant perspective in relation to the enhanced heat of melting ^{[30][31][32][33][34]}[30,31,32,33,34].

Cement, gypsum, concrete, brick, etc. are all viable options for the construction of interior walls. Many efforts have been undertaken in recent years to develop materials for construction that are capable of storing as well as releasing thermal energy. Although a large-scale study failed to demonstrate efficiency, Stoll et al. [35] claimed that passive thermal treatments using PCMs may significantly lower the risk of bridge freezing. Drissi et al. [36] looked at how microcapsules’ decay affected their thermo-physical characteristics. This may happen, for instance, as a result of mechanical stress experienced during the mixing of PCM and concrete. The experiments revealed that the damaged PCMs lost roughly 12% of their fusion heat and 28% of their specific heat capacity. The heat conductivity of porous construction materials like wood [37] and concrete [38] has been the subject of modelling efforts as well, with the goal of developing fractal-based models to make predictions.

Bentz and Turpin [39] conducted a numerical analysis of the thermal reactions of bridge decks at twelve different sites using the CONTEMP computer model and found that the latent heat released by PCM might lower the yearly frequency of freeze-thaw cycles by roughly 30%. According to the research by Sakulich and Bentz [40], incorporating PCMs onto bridge decks might be an effective technique for elongating their useful life. Freeze-thaw management (FCM) using paraffin oil and methyl laurate was studied by Farnam et al. [41]. Using a differential scanning calorimeter (DSC), they compared the efficiency of two PCM impregnation techniques, namely, lightweight aggregate filling and tube filling. Sharshir et al. [42] directed a comprehensive analysis of the most current research on the TES using phase change materials in construction applications. Furthermore, the PCMs for sustainable and energy-efficient construction were the subject of a comprehensive study by Wang et al. [43].

Ling and Poon [44] provided an overview of PCMs, how to include them, and how they affect concrete’s characteristics both while it is fresh and after it is hardened. The PCMs’ thermal performance in concrete was also discussed, as were their stability and any issues encountered while incorporating them into the material.

2. Conceptual Challenges of Utilising PCMs in the Concrete Industry

In order for the use of PCMs in the concrete industry to be effective and have optimum efficiency, a number of related difficulties must be resolved. The following are a few of the most pressing obstacles ^{[45][46][47][48]}[45,46,47,48]:

• It is essential to guarantee PCM compatibility with various concrete mix types. Some PCMs may interact with certain admixtures or additives used in the manufacture of concrete, changing its mechanical properties or posing compatibility problems.
• Over time, some PCMs may experience phase change cycling, which could result in performance loss, leakage, or deterioration. For construction to be sustainable and long-lasting, PCM stability inside the concrete matrix must be guaranteed.
• During phase transitions, some PCMs can experience volume changes, which could cause micro-cracking in the concrete matrix. To prevent damaging impacts on the concrete’s structural integrity, these volume variations must be controlled.
• Due to the super-cooling phenomenon, PCMs can experience a lack of solidification, which reduces their ability to store latent heat and causes an insufficient phase change cycle. As a result, ineffective phase change features of PCMs are anticipated.
• The inside environment of concrete is quite alkaline by nature, and in some circumstances, this high alkali causes the PCMs to degrade. For application in concrete, high-alkali PCMs such as polyethylene glycol ought to be excluded.
• When PCM is incorporated into concrete via the immersion method or the direct mixing method, PCM leakage from the concrete may result. During the mechanical mixing of these techniques with other concrete ingredients, some of the PCM that has been encapsulated may be broken. In order to achieve an efficient phase change of the PCMs while keeping the maximum strength of the concrete, extensive analysis must be performed to choose the appropriate way of PCM inclusion into the concrete.
• The rate of heat absorption and release during the phase transition operation is reduced considering PCM’s poor thermal conductivity. Particularly when the temperature varies quickly, low thermal conductivity PCM is useless for energy storage. Thus, it is crucial to ensure efficient heat transmission during phase change transitions to increase the capacity for energy storage and release.
• In contrast to conventional concrete materials, PCMs that are acceptable for concrete are not always inexpensive or easily obtainable on the market.
• Buildings that use PCM-enhanced concrete might need to comply with specific regulations and requirements. It is crucial to create standards and guidelines concerning PCM applications because they can promote business acceptance and regulatory acceptability. However, the absence of long-term data on how PCMs affect the longevity of concrete has deterred stakeholders from approving their use.

For PCM-enhanced concrete to be successfully implemented and widely used in the building sector, it will be essential to cope with these challenges through ongoing research, better materials, advanced encapsulation methodologies, design optimisation methods, and the creation of appropriate standards.