## 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]:  
* 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.