Preparation of ceramic foams from metakaolin-based geopolymer gels

The average pore diameter of hardened K₂O·Al ₂O₃·4SiO₂·11H₂O gcopolymer gel was measured by standard porosimetry techniques and determined to be extremely small (6.8 nm in diameter). On heating the geopolymer, significant capillary pressure (> 21 MPa) was expected due to vaporization of water from small pores. These capillary forces were enough to cause cracking and failure of monolithic geopolymer bodies. To avoid this problem, foaming agents, including spherical Al powder and hydrogen peroxide, were added to the geopolymer paste to engineer controlled porosity into the material and to shorten the diffusion distance for entrapped water to leave the samples. In order to control the internal pressure, the mixed pastes were cast into sealed metal dies and cured at elevated temperatures. Armoloy®coating on steel dies was found to be effective at improving the mold life and preventing geopolymer from sticking to the mold after curing. Foamed samples made using 0.5 and 1.5 wt% H₂O₂ and curing at 200°C for 7 h, had good machinability and high compressive strengths (44-77 MPa), but did not produce crack-free ceramics on heating. Mercury intrusion porosimetry and SEM results suggested that these samples did not obtain a percolating network of porosity due to hydrogen peroxide addition. Samples made using 60 wt% spherical Al as a foaming agent had pores of irregular shape with a larger pore size distribution, and were successfully converted to crack-free ceramics on heating. The Al foamed samples appeared to have attained a percolating pore network and exhibited minimal shrinkage on heating.