Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Abstract

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO2 and Al2O3 component was mixed with sodium silicate (Na2SiO3) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15M, which were used as NaOH activators of geopolymerization. The mass ratio of Na2SiO3/NaOH was fixed at 2.5 with SiO2/Al2O3 at 2.69. After the fly ash and alkali activators were mixed for 1min until homogeneous, the geopolymer paste was cured for 1min using household microwave oven at different output powers of 200, 500, 700, and 850W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1min.