This study aimed to investigate the mechanical characteristics and microstructure of alkali-activated slag (AAS)
paste modified with various proportions of fly ash (FA) and magnesium oxide (MgO). Firstly, ground granulated
blast furnace slag (GGBFS) was partially replaced with FA at 10%, 20%, 30%, and 50% to produce the alkaliactivated slag/fly ash blending (AASF). Then, the AASF paste mixture containing 30% FA was modified by
adding 5%, 10%, and 15% MgO by its mass. The flowability test and hydration heat analysis were conducted to
observe the fresh properties of paste samples, while the hardened properties of paste samples were examined by
compressive strength, length change, crack performance, and carbonation depth tests. The experimental results
illustrated that the workability of paste samples was significantly improved by using FA to replace GGBFS and
reduced by adding MgO. Meanwhile, the hydration heat results exhibited a higher temperature with lower FA
content and greater MgO content. Using 10–20% FA improved the compressive strength and carbonation
resistance of AASF paste samples, but the higher FA content decreased the strength and carbonation resistance
result. Adding MgO improved the compressive strength and the carbonation resistance but exacerbated the
cracks and expansion of paste samples. C-(M)-S-H gel and hydrotalcite-like (Ht) phase were the main hydration
products of AASF paste specimens, which were observed through the utilization of scanning electron microscopy
(SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction analysis. The higher MgO content
accelerated the hydration production formation of paste samples