Pore Structure Characteristics of Metakaolin-based Geopolymers by Nitrogen Adsorption Method

To reveal the microscopic pore structure of metakalin-based geopolymers and explore the effective ways to prepare mesoporous geopolymers and regulate its pore structure, the adsorption-desorption isotherms, microscopic pore structure of metakaolin-based geopolymers (i.e., total pore volume, specific surface area, morphology and pore size distribution) were investigated by nitrogen adsorption method. The influences of soluble sodium silicate modulus and water content on the pore structure of geopolymers were discussed. The results show that the adsorption-desorption isotherms of metakaolin-based geopolymers belong to type IV, and its adsorption hysteresis loop is a mixture of H1 and H3. The total pore volume and specific surface area are 0.1418-0.3136 cm³/g and 28.87-53.25 m²/g, respectively. The pore size distribution range is from 2 nm to 92 nm, among which, mesopores with the sizes of 2-50 nm are dominant, accounting for 97.82% of the total pore volume, and 98.87% of the pore surface area. Moreover, massive pores in geopolymers are mainly the two-side open cylindrical pores and parallel wall pores, while a small amount of pores are one side opened pores, like cylindrical pores, slited-shaped pores with parallel or ink bottle-shaped pores. The mesopore structure can be modulated by changing sodium silicate modulus and water content in a certain range. The total pore volume of metakalin-based geopolymers decreases from 0.2253 cm³/g to 0.1418 cm³/g, while the most probable pore diameter varies from 13.91 nm to 19.56 nm with the increase of sodium silicate modulus from 1.2 to 1.8. The total pore volume increases from 0.2219 cm³/g to 0.3136 cm³/g, while the pore size distribution varies from single peak at water content of 15.5 to double peaks, and eventually broadens to disappear at water content of 18.5 with the increase of water content from 15.5 to 18.5. Water content shows a stronger effect than the modulus of water glass in modulating the mesopore structure of metakaolin-based geopolymers.