Carbonation resistance of fly ash-metakaolin-based geopolymer pastes regulated by slag and carbide slag

Geopolymer is a new type of low-carbon and green cementitious material, and carbonation behavior is closely related to the calcium content within geopolymers. Therefore, investigating the influence of calcium content on the carbonation resistance of geopolymers is an urgent issue to address. To this end, this study utilized low-calcium fly ash and metakaolin as raw materials to prepare fly ash-metakaolin-based geopolymer pastes (FMGP). Additionally, high-calcium geopolymer slag and carbide slag were used to partially replace fly ash in FMGP. Standard curing and rapid carbonation tests were conducted, comparing them with ordinary Portland cement pastes. The study analyzed the effects of different slag and carbide slag contents on the microstructures, mechanical properties, phase composition, and carbonation resistance of FMGP before and after carbonation. The results revealed that the introduction of slag or carbide slag significantly enhanced the compressive strength of FMGP before carbonation, with a maximum increase of up to 39.29%. However, after carbonation, there was a noticeable decrease in compressive strength, with reductions exceeding 25%. In addition, FMGP incorporating slag or carbide slag maintained a carbonation rate of approximately 3.20 mm/d1/2 or less, and the pH of the pore solution after 28 days of carbonation remained between 11 and 11.5. Moreover, carbonation of FMGP containing slag or carbide slag resulted in pore refinement, though changes in gel properties had a more significant impact on mechanical properties than alterations in pore structure. FMGP incorporating slag or carbide slag developed a less stable Aragonite crystalline phase due to decalcification of the gel structure after carbonation. Additionally, the incorporation of slag or carbide slag promoted the hydration reactions of FMGP and the formation of C-(A)-S-H gel.