## 3 Results and Discussion
### Compressive Shear Bonding Strength  
During the compressive shear bonding strength tests, when there was shear dislocation between the rectangular concrete block and the geopolymer cylindrical core, their bonding was considered to be a failure. The compressive shear bonding strength under the five ratios of water to slag powder of 0.24, 0.26, 0.28, 0.30, and 0.32 at different ages of 3 d, 7 d, and 28 d is shown in Figure 10. Under different ages of 3 d, 7 d, and 28 d, as the ratio of water to slag powder increased, the compressive shear bonding strength showed a trend of first increasing and then decreasing. The compressive shear bonding strength reached its maximum at the ratio of water to slag powder of 0.28. As the age increased, the compressive shear bonding strength of the \(\Phi\) 52 mm, \(\Phi\) 57 mm, and \(\Phi\) 62 mm types of geopolymer cylindrical core showed a decreasing trend. The reason was that the slag geopolymer had a certain degree of shrinkage with the increase in age, which would affect the bonding performance of the material. When the age was 28 d, the difference in compressive shear  
Figure 8: Failure surface of new and old bonding beams. (a) 3 d; (b) 7 d; (c) 14 d.  
Figure 9: Flexural strength of bonding beam and recovery rate of flexural strength. (a) Flexural strength of bonding beams; (b) Recovery rate of flexural strength.  
bonding strength among the three diameter geopolymer cylindrical core was small, and the changes in compressive shear bonding strength of the five ratios of water and slag powder were also not significant.  
In order to analyze the effect of shrinkage of the geopolymer on the compressive shear bonding strength, the average of the five ratios of water to slag powder at the same age was calculated, and the variation in compressive shear bonding strength is shown in Figure 11. The compressive shear bonding strength of the geopolymer cylindrical core of \(\Phi\) 62 mm was the smallest, and the strength of the three ages was relatively similar. This was because the bonding area of the geopolymer cylindrical core was the largest, resulting in a large shrinkage and a smaller bonding strength. As the bonding area of the \(\Phi\) 57 mm and \(\Phi\) 52 mm geopolymer cylindrical core decreased, the shrinkage effect of the material decreased, resulting in an increase in bonding strength.  
Compared with flexural strength of new and old bonding beams, the trend of compressive shear bonding strength with the change in ratio of water and slag powder was  
Figure 11: The effect of shrinkage of geopolymer at different ages on bonding strength.  
Figure 10: Compressive shear strengths of geopolymer at different ages. (**a**) Compressive shear bonding strength of 3 d age; (**b**) Compressive shear bonding strength of 7 d age; (**c**) Compressive shear bonding strength of 28 d age.  
basically the same. However, due to the cylindrical shape of the geopolymer core, the shrinkage of the core affects the bonding between the geopolymer material and the concrete block. Therefore, the compressive shear bonding strength decreased with age. When the geopolymer was used for road grouting and base reinforcement, it was mainly used to bond loose particles and fill voids, which corresponds to the flexural strength of new and old bonding beam tests and compressive shear bonding strength tests. However, the voids in the road base were usually small, so the shrinkage of the geopolymer had a relatively small impact on the bonding strength.