## 4 Results and Discussion
### Drying Strinkage
#### 4.1.2 Effect of Alkali Equivalent on Drying Shrinkage  
Figure 6 shows the drying shrinkage of RPSG with different alkali equivalents within 28 days. The test results show that the drying shrinkage does not have a certain positive correlation with the alkali equivalent. The 28-day drying shrinkage of RP-S30-M1.3N3, RP-S30-M1.3N6, RP-S30-M1.3N9, RP-S30-M1.3N12 and RP-S30-M1.3N15 are 0.40%, 0.93%, 0.83%, 0.57% and 0.18%, respectively. When the alkali equivalent is small (In this paper, the alkali equivalent is less than 6 wt.%), drying shrinkage increases with the increase of alkali equivalent. This is consistent with some current studies on alkali-activated binders. It can be explained that the increase of alkali concentration increases the proportion of pores and gel pores in the slurry, reduces the content of large pores [62] and increases the capillary tensile stress. In addition, a certain alkaline environment is the basis of slag hydration, and an appropriate increase in alkali equivalent can promote slag hydration [63]. Therefore, for the low alkali equivalent specimens, slag hydration is incomplete, and part of the slag that is not hydrated fills the large-sized pores of the recycled brick mix microfine, which slows down water evaporation. At the same time, slag that is not hydrated has a \"microskeleton\" effect, which can also reduce the drying shrinkage performance of the specimen. These reasons together contribute to the low alkali equivalent specimen drying shrinkage being small. When the alkali equivalent is large, drying shrinkage decreases with the increase of alkali equivalent. The specimens with high alkali equivalent are unstable, as more alkali will promote the precipitation of Ca(OH)\({}_{2}\) and the formation of low Ca/Si gel, reduce mesoporous proportion of the matrix and reduce drying shrinkage of the specimen. For example, the drying shrinkage of RP-S30-M1.3N9, RP-S30-M1.3N12 and RP-S30-M1.3N15 decreased by 11.33%, 39.27% and 80.71%, respectively, compared with RP-S30-M1.3N12. In addition, the porosity of the RPSG system is higher than that of the traditional alkali-activated cementitious system, and there are more macropores in the slurry. Although it has higher activity under high alkali equivalent, it cannot actually consume the excessive alkali component in the system, and the excessive alkali will generate white crystals in the macropores in the slurry and on the surface of the specimen under a dry environment. After crystallization is generated in the internal pores of the slurry, a certain amount of crystallization pressure will be generated on the pore wall to offset the capillary tensile stress caused by the mesoporous pores so that the drying shrinkage value of the specimen is reduced and even expanded with the extension of the curing age. It can be well verified from the drying shrinkage results of the specimen RP-S30-M1.3N15. It can be seen from Figure 6 that the specimen RP-S30-M1.3N15 with 15 wt.% alkali equivalent has a large shrinkage in the early stage, but the shrinkage value decreases with the increase of age. It can be observed that white alkali crystals are separated from the slurry of RPSG, and some visible cracks appear on the surface of the specimen. These cracks are not shrinkage cracks caused by capillary tension but expansion cracks caused by pressure formed by free alkali crystallization. However, specimens with high alkali equivalents are very unstable and prone to large cracks under ambient conditioning conditions, contributing to a reduction in specimen strength. Therefore, increasing the alkali equivalent is not a good measure to reduce the drying shrinkage of RPSG.