Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers

In this study, we evaluate the effects of chemistry and curing and aging conditions on water loss kinetics, porosity, and the structure of geopolymers. Geopolymer samples were prepared by mixing metakaolin precursor with K or Na silicate solutions having SiO2/A(2)O(3) molar ratios from 2.5 to 4 and H2O/(SiO2/Al2O3) molar ratios from 2 to 4. The samples were cured in sealed and unsealed molds at 60 degrees C for 24 h and then aged at room temperature in open containers. The weight changes were monitored during curing and aging until a steady-state weight was observed. This study shows conclusively that the amount of water [H2O/(SiO2 + Al2O3)] in the initial geopolymer mixture is the most dominant factor affecting density and open porosity of geopolymers after curing and extended aging. The results also indicate that regardless of the amount of water in the initial mixture and curing conditions (i.e., in sealed or unsealed molds), similar to 610-wt% of all K-activated samples after aging for 21 days is water whereas in the case of all Na-activated samples that amount ranges between similar to 15 and 20 wt%. Furthermore, the results demonstrate that SiO2/Al2O3 molar ratio does not have a direct effect on density and open porosity of geopolymers.