Porous geopolymer from cyanobacterial sludge and coal fly ash as shape-stabilizing matrix for thermal energy storage using phase change paraffin

The consumption of fossil fuels drives both the energy crisis and greenhouse gas emissions, while the accumulation of solid wastes such as coal fly ash (CFA) and cyanobacterial sludge (CS) exerts considerable ecological pressure. This study explores the utilization of CS and CFA as raw materials, employing egg white powder as a surfactant and hydrogen peroxide as a blowing agent to fabricate a highly porous geopolymer (CCG). The organic fibers in CS act as pore templates and reinforcing agents, while their partial degradation in the alkaline environment participates in the gelation process. The optimal formulation comprises a 9 mol/L sodium hydroxide solution, 3 wt% egg white powder, 7 wt% hydrogen peroxide, a CS:CFA mass ratio of 2:8, and a sodium silicate to sodium hydroxide solution ratio of 2.5:1. Following synthesis, CCG underwent vacuum impregnation with phase change paraffin (PCP). The capillary forces within the porous matrix effectively encapsulate the molten PCP, yielding a shape-stabilized phase change composite (CCGP). Relative to CCG the CCGP exhibits a thermal conductivity enhancement of 0.081–0.168 W/(m·K). The solidification and melting enthalpies of the CCGP were 67.58 J/g and 69.23 J/g, respectively, with remarkable thermal stability at temperatures up to 600 °C. This study not only achieves high-value utilization of CS and CFA, but also provides new insights into protein-assisted foaming and fiber pore formation in geopolymer systems, offering guidance for the design of shape-stabilized phase change composites with optimized encapsulation efficiency and long-term thermal stability.