Effect of water content on microstructure and reaction process of metakaolin-based geopolymers

The preparation of functional mesoporous geopolymers with a water templating method has attracted recent attention in geopolymer field. In order to clarify the effect of water content on the size and quantity of mesopores as well as geopolymerization process, metakaolin-based mesoporous geopolymers were prepared by a water templating method. The microstructure and geopolymerization process of mesoporous geopolymers were characterized by mercury intrusion porosimetry, field emission scanning electron microscopy, isothermal conduction calorimetry and X-ray diffraction, respectively. The results show that the compact degree of both primary particles and secondary particles of N-A-S(H) gel, which determines the most probable pore size and porosity of mesoporous geopolymers, is significantly affected by water content. When the n(H₂O):n(Na₂O) (mole ratio) increases from 15.5 to 19.5, the most probable pore size increases from 15.0 nm to 28.7 nm and the porosity increases from 28.8% to 40.6%. The water content has little influence on the pH value of water glass buffer solution and the initial dissolution rate and degree of metakaolin. With the increase of water content, the total dissolution degree of metakaolin increases due to the easier transmission of alkali ions through the porous coating layer and the polycondensations between the silicate and aluminate monomers as well as aluminosilicate oligomers are also inhibited.