Carbonation Performance of Hybrid Ternary Binders Containing Low-Calcium Fly Ash and Sodium Sulfate

Hybrid binders emerge as a solution to further decrease the clinker factor in cementitious systems without unfavorable slow strength development. The Portland cement reduction not only produces greener systems, but also makes them more sensitive to carbonation, especially due to the very limited amount of portlandite available. Therefore, the pore structure is expected to be affected differently by carbonation. This paper aims to clarify the behaviour of different ternary binders containing 50% FA and 8% Na2SO4 as activator. SCMs/total binder ratio was 0.7, w/b was 0.45. Results of accelerated carbonation (1% CO2) on mortars are presented. Mixes were cured for 28 days and subjected to a preconditioning process of 28 more days. The carbonation rate of different mixes suggests a synergy effect occurring among FA, GGBFS, and sodium sulfate. A reference mix containing quartz instead of fly ash was also studied in order to isolate the contribution of the former to the carbonation resistance. GGBFS and Na2SO4 addition refined the pore structure. The microstructural damage due to carbonation process was also addressed by MIP analysis and revealed a more dense matrix than the one for ternary mixes without sodium sulfate after carbonation.