## 3 Results and Discussion
### Variable CO\({}_{2}\) Concentration
#### 3.1.1 Scanning Electron Microscopy  
SEM observation experiments of when the CO\({}_{2}\) concentration reached 8% and 10% (carbonation time was 21 and 42 days, respectively) were conducted to study the microstructure of two kinds of concrete before and after carbonation.  
The microstructure of the reference sample is shown in Figure 2. From Figure 2a, it can be seen that there are many fibrous C-S-H gels in the inner structure of the no carbonation concrete, and the cementites are closely arranged and the pores are few and short. However, the square plate Ca(OH)\({}_{2}\) crystals (CH crystals) and the needle rod ettringite Aft crystal overlap. When the CO\({}_{2}\) concentration is 8%, square-shaped CH crystals, needle rod ettringite Aft crystal, and C-S-H gels can still be observed as shown in Figure 2b. Compared with that in Figure 2a, the size of the C-S-H gel pores increased significantly, spherical CaCO\({}_{3}\) is filled in the internal space of concrete, and its arrangement is disorderly and irregular, which indicates that carbon dioxide penetrated into the concrete and reacted with Ca(OH)\({}_{2}\). Thus, the carbonation products improve the compactness of concrete. As shown in Figure 2c, when the CO\({}_{2}\) concentration is 10%, it can be found that the internal microstructure of the concrete is well filled, the overall surface of the structure is relatively flat, and there is no material with obvious shape, but there are large convex holes, and the holes wall is thin. Compared with that in Figure 2b, more CaCO\({}_{3}\) is generated, which may be due to more carbon dioxide invading into the concrete. The carbonation reaction is more sufficient, which causes pore expansion of the concrete and makes the internal structure of concrete loosen, and even results in the interface of the paste and aggregate losing its bonding performance.  
According to the existing literature [28; 29; 36], graphene can be filled in the voids of cement-based products; therefore, it can improve the hydration rate and degree of cement, and also change the size and shape of hydration products. However, it is considered that graphene does not participate in the hydration reaction of cement, so the type of cement products will not be changed. As can be seen from Figures 2a  
Figure 2: SEM images of the plain concrete in different CO\({}_{2}\) concentrations. (**a**) No corrosion, (**b**) 8% CO\({}_{2}\) concentration, and (**c**) 10% CO\({}_{2}\) concentration.  
ordinary concrete, GNP was well-dispersed in the cement. The hydration products of graphene concrete have more calcium silicate gels arranged together, and few pores and CaCO\({}_{3}\) crystals inside overlap with each other, and some layer-like hydrated structures were exhibited, which indicates that GNP makes the microstructure of concrete denser. From Figure 3b, it can be seen that when the CO\({}_{2}\) concentration is 8% (carbonation time of 21 days), there are more square plate CH crystals and spherical CaCO\({}_{3}\) in graphene concrete. However, compared with Figure 2a, the number of gel structures C-S-H obviously decreased. We also found that amorphous hydrated calcium silicate gel (C-S-H) is fibrous, and the cementitious materials are closely cross-linked. Furthermore, some holes and a few cracks are observed. Compared with Figure 2c, the inner part of the graphene concrete structure is denser, there are no large holes, and the pore structure is evenly distributed. This indicates that GNP could delay the carbonation process of concrete. The SEM graphs provide an explanation for the difference in the carbonation depth between the graphene concrete and the corresponding specimen.