# Graphene Nanoplatelets Impact on Concrete in Improving Freeze-Thaw Resistance
## 3 Results and Discussion  
### Scanning Electron Microscopy  
A microstructural morphology on the control concrete and graphene concrete specimens was carried out using scanning electron microscopy. Figure 2 shows the SEM micrographs of the specimen before F-T tests. It can be observed that the sample GC0 has larger volume porosity and contains a lot of acicular and rod cement hydration products, such as AFt, AFm, Ca(OH)\({}_{2}\), and C-S-H gel. The graphene nano-particles were reported that it could fill the voids and promote the growth of the hydration products, change the shape and size of hydration crystal, but did not change its type through reacting with cement and graphene [9]. As can be seen from Figure 2, compared with the ordinary concrete specimens, the hydration products of the specimens with GNP are more compact and the microstructure is more uniform, the intersecting microcrystals make the mechanical properties of concrete improved obviously, which would change the freeze-thaw resistance of concrete. However,it should be noted that when the content of GNP exceeds 0.3%wrt, GNP and other materials of concrete cannot be well mixed, their distribution cannot be uniform, and GNP could be aggregated, which was also found in the process of mixing materials, forming weak areas in the concrete, leading to stress concentration [8]. Therefore, the continuous increase of GNP mass fraction cannot further improve the mechanical strength and F-T resistance of concrete, such as GC0.4. Nevertheless, when the mass fraction of GNP is less, graphene nanoplatelets exist in isolated or small collective forms, with larger particle spacing and less lap joints, which have little effect on the mechanical properties of the concrete. It can be inferred that when the mass fraction of GNP is within a certain range, graphene would plays a role in strengthening and toughening of concrete.  
The SEM images of the specimens after 30 F-T actions are shown in Figure 3. Extensive cracking, 0.6-4.7 \(\upmu\)m, can be found in the control sample. The GC0.3 and GC0.4 concrete sample exhibited more cracks than the GC0.2 sample, GC0.3 concrete contained 0.4-3.5 \(\upmu\)m cracks, but from Table 2 macro-crack cannot be found on these samples surface. Meanwhile, no micro-crack was observed in the GC0.1 concrete sample, this means that the mass fraction of GNP might have a certain range which graphene will play a role in strengthening microstructure of concrete.  
After 200 F-T cycles, from the Figure 4 we can observe that the micro-cracks became wide and long (8.6-24.7 \(\upmu\)m) in the GC0 samples, tiny micro-cracks began to appear in the GC0.05 (1.3-7.8 \(\upmu\)m) and GC0.1 (0.8-9.2 \(\upmu\)m) concrete sample. And in the GC0.2, GC0.3 and GC0.4 concrete samples we found that the micro-cracks became wide and more, the crack lengths are 3.4-16.9 \(\upmu\)m, 2.7-17.3 \(\upmu\)m and 3.3-23.1 \(\upmu\)m, respectively, and the internal structure became poriferous, which lessened the durability and mechanical performance of the concrete. This indicates that the perennial F-T cycles could change the microstructure of the concrete and lead to a more porous. The SEM micrographs gives an illustration for that the compressive strength of the GC0.05 sample is maximum in all specimens during the F-T actions.  
Figure 3: SEM images of the specimens after 30 freeze-thaw tests.  
Figure 2: SEM images of the specimens before freeze-thaw tests.