Microstructure, strength development mechanism, and CO2 emission analyses of alkali-activated fly ash-slag mortars

A comprehensive investigation was conducted to examine the influence of fly ash/slag mass ratio on the strength and microstructure of alkali-activated fly ash-slag (AAFS) mortars. NaOH and Na2SiO3 were employed as activators for different fly ash/slag ratios. The results revealed that AAFS mortar with a fly ash/slag ratio of 25/75 exhibited the highest strength, reaching 104 MPa. The porosity and gel content of AAFS showed a weak correlation with compressive strength, emphasizing the significant impact of the generated gel type on AAFS strength. The comprehensive evaluation of AAFS strength incorporates factors related to the structure and chemical composition of reaction products. A strength index derived from AAFS mortar is proposed and validated against seven additional literature datasets from different systems, demonstrating a robust correlation between the calculated index and the compressive strength. Furthermore, carbon emission analysis reveals that, compared to OPC, FA100 exhibits 7.9%–21.2% lower carbon emissions, while FA25BF75 shows 0.1%–97.5% lower CO2 intensity than OPC, owing to the higher strength of AAFS. In general, AAFS exhibits improved properties and environmental benefits, indicating significant potential for practical applications as a complete replacement for OPC.