## 4 Discussion and Conclusions  
Based on the above results, it can be indicated that the creation of colloidal SiO\({}_{2}\) particles is the main factor which leads to the cementation effect in tonalite specimens, especially the tonalite specimens subjected to the coupled effects. Without freeze-thaw cycles, chemical corrosion can only cause a slight decrease in Young's moduli. The Young's moduli of tonalite specimens subjected to the combined effect (G4) are distinctly lower compared with those of G2 and G3. Chemical corrosion does further damage to specimens with freeze-thaw cycles. The results of XRD and SEM can prove that the SiO\({}_{2}\) particles are absorbed into cracks and voids. The results show that UCS of tonalite specimens subjected to the combined effect (G4) is not lower compared with that of G3. As more cracks and pores are generated in the specimens due to freeze-thaw cycles, SiO\({}_{2}\) colloidal particles can move into specimens with the acid solution. In addition, the result of peak strain shows that SiO\({}_{2}\) colloidal particles can increase the peak strain of tonalite specimens subjected to freeze-thaw cycles.  
This paper presented the laboratory experiments such as the [PERSON]'s modulus test, the uniaxial compression test, three-point bending test, XRD and SEM test of the tonalite samples subjected to the combined effect of freeze-thaw cycles and chemical corrosion. The following conclusions can be drawn from the results presented in this study:  
1. The [PERSON]'s modulus of several tonalite specimens subjected to the combined effect of chemical corrosion and freeze-thaw weathering is far less than that found for the specimens subjected only to either chemical corrosion or freeze-thaw weathering. Based on Figure 3, nitric acid solution amplifies the damage caused by freeze-thaw cycles.  
Figure 12: SEM images of G4 tonalite specimens under different scales: (**a,b**) G4, under 20 \(\upmu\)m; (**c**) G4, under 500 \(\upmu\)m.  
2. The peak strain of tonalite specimens subjected to the combined effect decreases with decreasing freezing temperature. Moreover, the peak strain of tonalite specimens subjected to the coupled effect is the highest. While the UCS of tonalite specimens submitted to the coupled effect decreases with decreasing freezing temperature, the values are not significantly different at each temperature. It can be shown that the nitric acid solution has a softening effect on the tonalite specimen.
3. The KIC of tonalite specimens decreases with the increase of soaking time in HNO3 (pH = 1), but the decline is almost the same from 60 days to 90 days. It can also be seen that the KIC of G4 declined less than the KIC of G3 compared with the control group.
4. Based on the results of SEM, it was verified that colloidal particles are generated when tonalite specimens are subjected to nitric acid. Following the combined effect, a number of pores and cracks will be filled by colloidal particles. This process enhances the cohesion of the tonalite specimen and decreases the damage that results from freeze-thaw weathering.
5. The main failure mode in tonalite specimens is intercrystalline crack when the specimens are subjected to both chemical corrosion and freeze-thaw weathering.  
[PERSON] designed the experiment, [PERSON] performed the experiment and wrote the paper, [PERSON] revised the paper, [PERSON] supervised it. Useful suggestions were given by [PERSON] from University of Exeter and [PERSON] from RWTH Aachen University are also acknowledged.  
The financial support from Project of Shanghai Soft Science Research Plans (18692106100), China Scholarship Council is gratefully acknowledged.  
The authors declare no conflict of interest.