Thermal performance optimization of hydrated lime composites: Influence of graphite addition and porosity on heat transfer efficiency

This study investigates the thermal performance of hydrated lime composites enhanced with graphite, with a focus on the combined effects of porosity and moisture content. While previous research has demonstrated improvements in thermal conductivity of gypsum- and lime-based composites with carbon additives, the influence of porosity evolution and residual water on heat storage and transfer remains insufficiently explored. To address this gap, an experimental test bench with a helical coil heat exchanger was developed to measure temperature profiles, energy storage, and specific heat under controlled heating and cooling cycles. Graphite addition (20 wt.%) reduced heating and cooling times by up to 18 °C and increased the liquid–solid temperature differential, confirming improved heat transfer efficiency. Conversely, increased porosity and water loss during drying diminished energy storage capacity, while higher initial moisture contents enhanced thermal performance, highlighting the fundamental role of water in defining the thermophysical behavior of the composites. The results provide new insights into the design of low-cost, lime-based thermal energy storage materials and underline the importance of optimizing graphite fraction, porosity, and moisture control. These findings hold potential for applications in passive building cooling, thermal storage units, and sustainable heat management technologies.