## 3 Results and Discussion
### Variable Temperature
#### 3.2.1 Scanning Electron Microscopy  
SEM observation experiments in which the temperature reached 29, 37, and 45 \({}^{\circ}\)C (carbonation time was 7, 21, and 35 days, respectively) were conducted to study the microstructure of two kinds of concrete before and after carbonation.  
The microstructure of ordinary concrete is shown in Figure 5. When the temperature reaches 29 \({}^{\circ}\)C, from Figure 5a, it can be seen that there are a lot of fibrous C-S-H gels that overlap with each other. Square plate CH crystals and a little calcium carbonate crystal can also be observed. We can see that the microstructure is relatively loose and there are many interconnected pores. When the temperature reaches 37 \({}^{\circ}\)C, compared with Figure 5a, small spherical calcium carbonate crystals, fibrous C-S-H crystals, and plate-shaped CH accumulated more compactly, filling the voids and improving the compactness of the microstructure. This means the carbonation reaction is accelerated. We can also see that the number of calcium carbonates in Figure 2b is significantly higher than that in Figure 5b, and the number of CH crystals in Figure 5b is lower than that in Figure 2b, which indicates that the influence of carbon dioxide on the carbonation degree of concrete is significantly greater than that of temperature under the same humidity and carbonation time (21 days). From Figure 5c, more spherical calcium carbonate crystals accumulate in the concrete, which reduces the internal voids and makes the structure denser. From Figure 5, with the extension of the temperature and carbonation time, it can be inferred that the calcium carbonate crystals generated gradually increased in this test. However, compared with Figure 2, there are not many cracks and holes in the concrete, which indicates that the influence of the carbon dioxide concentration is greater than that of the temperature rise on the carbonation resistance of ordinary concrete.  
Figure 4: Relationship between concrete carbonation depth, time, and CO\({}_{2}\) concentration.  
As shown in Figure 6a, when the temperature reaches 29 \({}^{\circ}\)C (carbonation time is 7 days), it can be seen that the rod C-S-H gel and CH crystal are interconnected with each other, and a small amount of spherical CaCO\({}_{3}\) crystal is formed. When the temperature reaches 37 \({}^{\circ}\)C, it can be observed that there are more spherical calcium carbonate crystals and AFt crystals and C-S-H gel generated in Figure 6b, and the pores are few and short. Hence, in this stage, the internal structure of GNP concrete becomes compact. With the increase of the temperature and carbonation time, it can be found that the spherical calcium carbonate increased obviously compared with Figure 6b. Moreover, there is an extended flower cluster structure, which extends one cluster after another and connects closely.  
Compared with Figures 5a and 6a, there is little difference between them. There is more rod C-S-H gel and CH crystal dislocation from each other. From Figures 5b and 6b, it can be seen that the amount of CaCO\({}_{3}\) in graphene concrete is more than that of ordinary concrete. Comparison between Figures 5c and 6c shows that there are more spherical calcium carbonate crystals in graphene concrete, but it is not obvious in ordinary concrete, which means that the carbonation reaction of graphene concrete is more sufficient. Therefore, the increase in temperature may cause graphene to accelerate the carbonation reaction of concrete, which may be because graphene has good thermal conductivity. Therefore, the temperature sensitivity of graphene concrete is higher than that of ordinary concrete.