## 3 Results and Discussion
### Flexural Strength of Geopolymer  
The flexural strength of the five ratios of water and slag powder of 0.24, 0.26, 0.28, 0.30, and 0.32 at different ages is shown in Figure 6. As shown in Figure 6a, as the ratio of water to slag powder increased, the flexural strength of the geopolymer gradually decreased, with the maximum reduction in flexural strength at 28 d. The geopolymer with a ratio of water to slag powder of 0.24 had the highest flexural strength, with strengths of 2.70 MPa, 3.60 MPa, and 6.50 MPa at 3 d, 7 d, and 28 d, respectively. However, when the ratio of water to slag powder was 0.32, the flexural strength was the smallest, with strengths of 1.93 MPa, 2.73 MPa, and 4.10 MPa for the three ages, respectively. As shown in Figure 6b, the growth rate of flexural strength was the fastest within 1 to 3 days; it gradually slowed down within 3 to 7 days. The flexural strength of the geopolymer with a ratio of water to slag powder of 0.32 had the highest growth rate in 1-3 d and 3-7 d, the strength ratios of 3 d to 28 d and 7 d to 28 d were 47% and 67%. The other ratios of water to slag powder with faster growth rates of flexural strength were 0.26 and 0.28, respectively.  
Figure 5: Compressive strength and ratio of geopolymer at different ratios of water to slag powder. (**a**) Compressive strength; (**b**) Compressive strength ratio.  
Figure 6: Flexural strength and ratio of geopolymer at different ratios of water to slag powder. (**a**) Flexural Strength; (**b**) Flexural Strength ratio.  
As a road grouting material, geopolymer could fill the void of the road and reinforce the road base, requiring good mechanical properties. Under traffic loads, the geopolymer should have not only a certain compressive and flexural strength, but also a certain degree of flexibility. The ratio of compressive strength to flexural strength of the geopolymer reflected the brittleness of the material. The higher the ratio, the greater the brittleness of the geopolymer. If the ratio is opposite, the flexibility of the geopolymer is better. In order to reflect the flexibility of the geopolymer, the ratio of compressive strength to flexural strength was used. The compressive and flexural strength ratio of the five ratios of water to slag powder of the geopolymer at different ages of 3 d, 7 d, and 28 d are shown in Figure 7. As shown in Figure 7, the compressive-flexural ratio of the five ratios of water to slag powder of geopolymer mostly increased and then decreased with age. The main reason was that the growth rate of the 7 d geopolymer compressive strength was significantly higher than that of the 7 d flexural strength. The 3 d compressive-flexural ratio of the water to slag powder ratio of 0.28 geopolymer was the smallest, while the 28 d compressive-flexural ratio of the water to slag powder ratio of 0.24 geopolymer was the smallest. However, the 28 d compressive-flexural ratio of the water to slag powder ratio of 0.28 geopolymer was only slightly higher than that of the water to slag powder ratio of 0.24. The geopolymer belonged to the early strength type, and the growth rate of early strength was relatively fast. When it was used to reinforce the road base, it required not only high compressive strength and flexural strength, but also good flexibility. From the analysis of compressive strength, flexural strength, and compressive-flexural ratio, it can be seen that the geopolymer with a ratio of water to slag powder of 0.28 had better mechanical properties.