Modification Effect of Ca(OH)₂ on the Carbonation Resistance of Fly Ash-Metakaolin-Based Geopolymer

Compared with Portland cement, geopolymers have poor carbonization resistance, which will greatly limit the application their application. To improve the carbonization resistance of geopolymers, firstly, the carbonization behavior of the fly ash-metakaolin-based geopolymer was studied through accelerated carbonization tests. Secondly, different amounts of Ca(OH)₂ were introduced into the composite system, and the modification effect of the carbonization resistance of the modified geopolymer was studied. Finally, the modification effect of Ca(OH)₂ on the fly ash-metakaolin-based geopolymers was analyzed, and the modification mechanism was explored. It was found that adding Ca(OH)₂ to the fly ash-metakaolin-based geopolymer could significantly improve its initial compressive strength, but its strength after carbonization remained basically unchanged; meanwhile, the compressive strength of the terpolymer after carbonization clearly decreased after adding Ca(OH)₂. Compared with ordinary Portland cement, the carbonization rate of fly ash-metakaolin-based geopolymer is faster, and the addition of Ca(OH)₂ can inhibit the development of its carbonization depth. With increased carbonization age, the alkalinity of the geopolymer decreased, and the addition of Ca(OH)₂ inhibited the decrease in the alkalinity of the geopolymer. The addition of Ca(OH)₂ improved the microstructure of the geopolymers, the pore structure became denser, and the pore size became smaller size after carbonization. The hydration products of fly ash-metakaolin-based geopolymer are mainly amorphous silicaluminate gel and C-S-H gel, and Ca(OH)₂ forms in the hydration products of terpolymer with the incorporation of Ca(OH)₂, which is conducive to improving the carbonization resistance. In summary, Ca(OH)₂ can play a good role in modifying the carbonization resistance of fly ash-metakaolin-based geopolymers.