## 3 Studies on Using Organic PCM in Concrete for Thermal Energy Storage
Since fully impregnating the AAC samples with paraffin would reduce the performance of insulation and raise the paraffin seepage risk during melting, it was proposed that intermediate paraffin contents, around two-thirds of the level of saturation, are suitable for the whole specimens. By analysing the fresh and hardened characteristics of multiple sets of mixes generated with variable cement and free water levels, [PERSON] and [PERSON] (2019) [55] investigated the viability of using cross-linked polyethylene (XLPE) waste materials in concrete. Up to 8% of the cement mass was made up of shredded XLPE of variable sizes. The results of the tests revealed that the workability and air content of the concrete were not significantly changed by the addition of XLPE, but the unit weight was reduced because of the density difference between the aggregates and the XLPE shreds. Water permeability is another property where XLPE additions improve concrete performance. As XLPE content and particle size increase,  
Figure 4: Variation of anticipated thermal conductivity as a function of paraffin content for AAC-600-based composite samples at (**a**) 20 \({}^{\circ}\)C and (**b**) 35 \({}^{\circ}\)C [54]. concrete strength and shrinkage decrease somewhat. Reducing the water-to-cement ratio, however, may help make up for these drawbacks. The residual compressive strengths of XLPE-modified concrete mixes decrease due to the thermal breakdown of XLPE after exposure to heat. To successfully encapsulate the aggregate holding phase transition components, [PERSON] et al. (2019) [56] developed a nano-refined epoxy paste employing optimal portions of the future of nano-silica as well as the powder of graphite. By enhancing workability, decreasing consumption, and strengthening adherence to the formed interfacial bond, the improved recipe provided postponents in the setting to 3-3.5 h and a 15-fold decrease in the coated layer thickness. Thermal stability, chemical compatibility, and thermal dependability were all observed in the macro-encapsulated phase transition materials. With a latent heat storage capacity of 12.6 J/g, the melting and freezing points were calculated to be 17.12 \({}^{\circ}\)C and 32.9 \({}^{\circ}\)C, respectively. With a compressive strength of over 15 MPa, the thermocrete developed through 100% integration of macro-encapsulated aggregates owed its potential energy savings to its lower internal temperature of 6.4 \({}^{\circ}\)C, to the resistance to elevated fluctuations in the temperature while keeping a limited range of 22-30.2 \({}^{\circ}\)C, and to the ability to shift loads of energy through the highest times of 15 min. The thermal characteristics of paraffin-impregnated burned clay aggregate (PIA) concrete were investigated by [PERSON] et al. (2019) [57]. Incorporating paraffin as a PCM carrier into the burned clay aggregates required the application of heat and pressure. The concrete's compressive and flexural strengths, as well as the concrete panels' ability to insulate against heat loss, were evaluated. Tests showed that PIA in traditionally used burned clay aggregate concrete increased its strength and thermal insulation. It was also indicated that the time to peak temperature and peak temperature decrease were both higher for the PIA-treated panels than for the control panel. The novel Geopolymer-Coated Expanded Clay-phase Change Material (GP-L-PCM) macrocapsules were supplemented to the GPC at 25%, 50%, and 75% volume ratios to create GP-L-PCM slabs, as detailed by [PERSON] et al. (2019) [58]. The production of LECA slabs required a similar quantity of Lightweight Expanded Clay Aggregates (LECA). A comparison was made between a reference GPC slab's thermal and structural performance and those of LECA and GP-L-PCM slabs. It was found that the maximum surface temperatures on the LECA and GP-L-PCM slabs are lower than those of the GPC slab, indicating a decreased heat transfer (a drop of 5.6 \({}^{\circ}\)C and 8.0 \({}^{\circ}\)C, respectively).