## 3 Results and Discussion
### Variable Humidity
#### 3.3.1 Scanning Electron Microscopy  
In this part, ordinary concrete and graphene concrete specimens without carbonation test and with 71% and 85% humidity exposure to carbonation experiment (carbonation time was 0, 21, and 35 days, respectively) were selected for SEM tests to study the micro morphology of two kinds of concrete before and after the humidity change. The SEM images of concrete at different humidity are shown in Figures 8 and 9.  
Figure 7: Relationship between concrete carbonation depth, time, and temperature.  
Figure 8a shows that there are a lot of fibrous C-S-H gels in the inner structure of the no corrosion concrete. The gels are closely arranged, and there are few fine pores. The square plate Ca(OH)\({}_{2}\) crystal and the needle rod ettringite Aft crystal overlap with each other, and the whole structure is relatively uniform. As shown in Figure 8b, for the ordinary concrete, after 21 days of carbonation (at 71% humidity), more spherical calcium carbonate crystals are generated, and square plate Ca(OH)\({}_{2}\) crystals and intercalated cementitious C-S-H gel are observed. Compared with Figure 8a, there are many holes and some interconnections between them. With the extension of the carbonation time and the increase in humidity, Figure 8c shows that more spherical calcium carbonate crystals are filled inside the concrete, but the CH crystals are reduced and some of them are connected with each other to form flakes. We also observe that the number of calcium carbonate and CH crystals in Figure 8b is obviously more than that in Figure 5b, which indicates that the influence of humidity on the carbonation of concrete may be greater than that of temperature under the condition of the same concentration of carbon dioxide and the same carbonation time (21 days).  
Compared with Figure 9a, Figure 9b shows that there are a small amount of spherical calcium carbonate crystals and needle rod ettringite (Aft) crystals, and more six square plate Ca(OH)\({}_{2}\) crystals and hydrated calcium silicate gels can be observed, and these gels accumulate with each other. This means that the carbonation reaction is slow, and the products of carbonation are less. Compared with Figure 8b, the graphene concrete after 21 days of carbonation (at 71% humidity) has more Ca (OH)\({}_{2}\) crystals and little CaCO\({}_{3}\) crystals, which indicates that the carbonation reaction of graphene concrete is slower than ordinary concrete in the same environment; that is, graphene delays the carbonation reaction. It can be seen in Figure 9c that there are still more square plate-shaped Ca(OH)\({}_{2}\) crystals, the amount of spherical calcium carbonate in graphene concrete is significantly increased compared with  
Figure 8: SEM images of the plain concrete in different corrosion humidity: (**a**) no corrosion, (**b**) humidity 71% (time 21 days), and (**c**) humidity 85% (time 35 days).  
Figure 9: SEM images of the graphene concrete in different corrosion humidity: (**a**) no corrosion, (**b**) humidity 71% (time 21 days), and (**c**) humidity 85% (time 35 days).  
that in Figure 9b, and the microstructure becomes loose and coral like, with large voids, which means that more hydration products participate in the carbonation reaction.