## 3 Results and Discussions
### Microstructure Analysis of FA after Incorporation  
The hydration of Portland cement produces many chemical compounds, including the C-S-H, the calcium aluminate hydrate gel (C-A-H), the lamellar calcium hydroxide (CH) crystals, the Aft, and the irregular petal-shaped calcium sulfide aluminate hydrate (AFm) [41]. Figure 7 shows the SEM images of the standard curing for 3 days and 7 days of concrete without FA. Figure 5: XRD patterns of the hydration products of concrete after 3 days of the standard curing: (**a**) concrete without FA; (**b**) concrete mixed with 20% FA. Figure 6: XRD patterns of the hydration products of concrete after 7 days of the standard curing: (**a**) concrete without FA; (**b**) concrete mixed with 20% FA. The early hydration rate of pure cement concrete specimens is faster. In Figure 7a, there are a large number of hexagonal plate calcium hydroxide crystals, large cracks can be seen in the structure of the specimen, and the structure is loose. In Figure 7b, hydration has occurred in most of the composite Portland cement clinker minerals in the structure at the standard curing for 7 days, forming a large number of calcium hydroxide crystals, which is significantly more than at the standard curing for 3 days, and the internal structure of the concrete is denser. Figure 8 shows the SEM images of concrete at the standard curing for 3 days and 7 days when 20% FA is added. A large number of fibrous, network, sheet, and flocculent C-S-H gels appeared in the specimen structure, and a large number of rod-like ettrifugite was generated in Figure 8a. At this time, the calcium hydroxide was not obvious, and most of them were interwoven C-S-H gel and AFt. In Figure 8b, the active substance in the FA is rehydrated with calcium hydroxide, and the interface between cement colloid and FA tends to be dense. The FA in the concrete structure presents a state of approximately spherical particles, and the pores of the structure are accompanied by FA particles without hydration reaction (active reactants appear on the surface) and cube crystals. There are more rod-like ettrifugite in the interior. With the increase in curing time, the hydration reaction of concrete structure specimens is continuously strengthened, and the activity of FA is gradually stimulated. In Figure 8, it can be clearly seen that the specimens of concrete structures at the standard curing for 7 days contain a large number of needle-rod substances, which are denser than those at the standard curing for 3 days. In conclusion, with the increase of FA content, the volcanic ash effect is continuously intensified, spherical particles of concrete microstructure surround a large number of hy  
Figure 8: SEM images of FA mixed with 20% concrete after the standard curing for (**a**) 3 days and (**b**) 7 days. Figure 7: SEM images of concrete without FA after the standard curing for (**a**) 3 days and (**b**) 7 days. dration products, and the specimen structure becomes loose. However, with the extension of the standard curing age, the types of hydration products increase, and there are unhydrated FA particles, a large number of needle-like ettrinigite, and a small amount of calcium hydroxide. From the analysis of the microstructure test results, it can be seen that the internal structure density of the standard curing 7 day concrete in this study is better than that of ordinary concrete of the same age after adding 20% FA. With most FA as the concrete admixture of the micro test study, it is concluded that the holes and cracks in the internal structure of concrete are effectively filled, forming a density skeleton system, its standard curing of 7 and 28 days concrete structure is consistently denser. In order to reduce the footprint of cement production, the cement was partially replaced by FA. Calculations based on carbon emissions in the carbon footprint suggest that the higher the FA content, the higher the low-carbon response of the concrete.