Influence of microcapsule size and shell polarity on thermal and mechanical properties of thermoregulating geopolymer concrete for passive building applications

Microencapsulated phase change materials (MPCM) were added to geopolymer concrete (GPC) for utilization as a thermal energy storage concrete for passive building applications. Three different MPCM were compared to examine the influence of the hygroscopic nature of the MPCM shell, the PCM core/polymer shell ratio, and the MPCM size on the microstructure, thermal properties and compressive strength of GPC. The combination of a hygroscopic nature of the polymer shell, a high core/shell ratio, and a small MPCM size were found to improve the interface bonds between microcapsules and the GPC matrix, increase the energy storage capacity of GPC, and results in a good dispersion of MPCM in the GPC matrix. After adding 5.2 wt% MPCM to GPC, the power consumption for stabilizing the indoor temperature at 23 °C may be reduced by up to 18.5 ± 0.3% for GPC containing PS-DVB/RT27 (paraffin Rubitherm®RT27 core and a shell of polystyrene cross-linked with divinylbenzene), 20.1 ± 0.7% for GPC containing PMMA/PCM26 (paraffin mixture core with a crosslinked polymethyl methacrylate shell) and 25.9 ± 0.3% for GPC containing MF/PCM24 (paraffin mixture core with a melamine-formaldehyde polymer shell). Adding MPCM to GPC induces a higher amount of air pockets, which weaken the compressive strength. Unfortunately, the same parameters that are advantageous for reducing the energy consumption also results in a greater decline of the compressive strength. The compressive strength is further reduced when the microcapsule core is in its liquid state. However, the compressive strength still satisfies the mechanical European regulation (EN 206-1, compressive strength class C20/25) for concrete applications, except for GPC containing 5.2 wt% of MF/PCM24.