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

Abstract

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 (CO₂) capture. Five geopolymer mixes (SM1-SM5) were synthesized by varying the SPL/MK ratio to investigate its effect on pore structure and CO₂ adsorption performance. Geopolymer formation was confirmed through XRD, ²⁹Si and ²⁷Al 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 (15.04 m²/g) and pore volume (0.1074 cm³/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.