Porous geopolymer monoliths: synthesis, characterization, and performance as a novel bulk solid adsorbent for CO2 capture

This study introduces a novel geopolymer system based on Spent Potlining (SPL), an industrial waste byproduct, used as an aluminosilicate precursor (in combination with metakaolin, MK) and as a gas-generating agent for carbon dioxide (CO 2 ) capture. Five geopolymer mixes (SM1-SM5) were synthesized by varying the SPL/MK ratio to investigate its effect on pore structure and CO 2 adsorption performance. Geopolymer formation was confirmed through XRD, 29 Si and 27 Al MAS NMR, and FTIR analyses. BET surface area and total pore volume progressively increased across the sample series (SM1-SM5), with SM5 achieving the highest surface area (15.0 m 2 /g) and pore volume (0.107 cm 3 /g). SEM revealed cohesive and homogeneous microstructures, indicating effective geopolymerization, while micro-CT imaging showed enhanced macroporosity and interconnected pore networks with increasing SPL content. Notably, SM1 (100% SPL) exhibited rapid gas release and expansion, forming a highly open, interconnected macroporous structure with the highest total porosity (76.2%). Pore morphology analysis using Fiji-ImageJ indicated predominantly spherical pores. Thermal conductivity increased from 0.074 W/mK in SM1 to 0.113 W/mK in SM5, correlating with increasing density and decreasing porosity. SM5 also demonstrated the highest adsorption capacity (0.069 mmol/g), surpassing both pure SPL (0.035 mmol/g) and pure MK (0.036 mmol/g), highlighting the synergistic effect of combining SPL and MK.