## 4 Results and Discussion  
### Drying Strinkage  
#### 4.1.1 Effect of Slag Content on Drying Shrinkage  
Figure 5 shows the drying shrinkage of RPSG with different slag contents within 28 days. It can be seen that the drying shrinkage decreases with the increase of slag content. The 28-day drying shrinkage of RP-S0-M1.3N6, RP-S15-M1.3N6, RP-S30-M1.3N6 and RP-S45-M1.3N6 are 2.98%, 2.57%, 0.93% and 0.70%, respectively. The drying shrinkage of RPSG with slag content of 15 wt.%, 30 wt.% and 45 wt.% are 13.53%, 68.69% and 76.32% lower than that of RPSG without slag. The drying shrinkage value of RPSG decreases with the increase of Ca/Si, which is contrary to the traditional conclusion that the drying shrinkage value of alkali-activated cementitious materials increases with the increase of Ca/Si [57], such as alkali-activated fly ash/slag, Kaolin/slag, silica fume/slag and other cementitious systems. It is noted that the RPSG with 60 wt.% slag content had a large drying shrinkage value, and cracking occurred on the third day after forming. The capillary pressure theory shows that the essence of shrinkage is that the free water evaporates from the saturated mesoporous pores in a dry environment, and the surface tension in the pore solution forms capillary pressure. Thus, the curved moon surface of liquid-gas was formed, resulting in the reduction of slurry volume. This mechanism is applicable to alkali-activated cementitious materials [58]. The pore size distribution is the main factor affecting its drying shrinkage. However, further analysis is required for the cementitious system with two or more precursors, and a variety of factors should be considered [59]. According to the Mackenzie Bentz equation, the parameters affecting the drying shrinkage of RPSG mainly include bulk modulus, bulk modulus of solid material, pore saturation and capillary tension of liquid phase [60]. Capillary tension is mainly composed of surface tension, contact angle and Kelvin radius. The surface tension and contact angle of the pore solution are the same for different mixtures under the same humidity and are mainly affected by the pore diameter. According to the above theory, the effective factors of drying shrinkage caused by capillary tension are pore diameter, pore saturation and bulk modulus [2]. Slag can promote the production of hydrophilic products such as NASH and CASH, making the slurry denser, so as to improve the bulk modulus of the slurry and the ability of the slurry to resist drying shrinkage and deformation. Slag can refine pore structure, reduce pore size and increase capillary tension. The particle size of slag particles is smaller than that of RP. Slag particles are dispersed to the surface of RP particles during mixing. After hydration, the products can fill pores and form closed pores, improving pore saturation [59]. It can be seen from the above analysis that the drying shrinkage result is the compound action of the bulk modulus, pore diameter and pore saturation. The results show that bulk modulus is the dominant factor affecting the drying shrinkage of RPSG when the slag content is small. Therefore, slag can reduce the drying shrinkage of RPSG. When the slag content is high, the dominant factors affecting the drying shrinkage are pore diameter and pore saturation [61]. Figure 5: Drying shrinkage results of RPSG with different slag contents. The activity of RP under alkaline conditions is much lower than that of slag. After slag replaces RP, the consumed water through chemical reaction and the active SiO\({}_{2}\) in sodium silicate solution increase, and the residual water and active SiO\({}_{2}\) in the slurry decrease. Relevant studies [30] show that silica gel with high water content is easy to dehydrate under dry conditions, resulting in larger shrinkage, while the addition of slag can reduce the water content in the slurry and consume some sodium silicate, thus reducing the drying shrinkage.