## 4 Results and Discussion
### Visual Efflorescence  
#### 4.3.1 Effect of Slag Content on Visual Efflorescence  
Visual efflorescence can intuitively reflect the degree of efflorescence of the specimen. Figure 11 shows the visual efflorescence changes of RPSG with different slag contents at 1 day, 3 days and 7 days. It can be seen from Figure 11 that after 1 day of immersion, the white crystalline substance first appears around the top of the specimen RP-S0-M1.3N6 without slag addition, while there is no obvious change around the top of the specimens RP-S15-M1.3N6, RP-S30-M1.3N6 and RP-S45-M1.3N6. After 3 days of immersion, white  
Figure 10: Capillary porosity of RPSG with different moduli.  
Figure 9: Capillary porosity of RPSG with different alkali equivalents.  
crystalline substances precipitated around and at the top of the specimen RP-S0-M1.3N6, while the specimens RP-S15-M1.3N6 and RP-S45-M1.3N6 only showed a small circle of crystalline substances at the bottom above the immersed part. However, crystallization appeared on the top of the specimen RP-S30-M1.3N6 with a slag content of 30 wt.%. This is due to the joint porosity between the top and bottom of the specimen, which causes the abnormal phenomenon that the surface of the specimen crystallizes prematurely. After 7 days of immersion, the surface of specimen RP-S0-M1.3N6 starts peeling on the surface of the crystallization pressure gauge, and part of the paste swells or falls off. This can be attributed to the fact that crystals are formed when alkaline-rich solutions inside or on the surface of the specimen pores react with CO2 and continue to migrate with the moisture. The crystals continue to grow, generating a crystallization pressure. When the pressure exceeds the internal tensile strength of the specimen, cracks and rupture occurs [67]. The degree of efflorescence of other specimens is not significantly different from that of 3 days. It can be concluded from the test phenomena that the addition of slag can significantly reduce the visual efflorescence of RPSG and reduce the visual efflorescence rate.  
Visual efflorescence can intuitively show the phenomenon of efflorescence when the specimen is immersed in water at the bottom, but this test method cannot quantify the degree of efflorescence in the slurry. Figure 11 shows the mass loss results of RPSG with different slag contents due to efflorescence. The mass loss of RP-S0-M1.3N6 was the highest, reaching 11.17 g, and the results were in good agreement with visual efflorescence. Unlike the decrease in capillary porosity with increasing slag content as shown in Figure 8, specimen RP-S15-M1.3N6 showed minimal visual efflorescence and mass loss. At the same time, there is an obvious macro-crack above the submerged part at the bottom of the specimen. This is because alkali-activated materials can occur not only on the surface of the specimen but also in the larger capillary pores on the surface or cracks in the interior of the specimen. Efflorescence will produce crystallization pressure and force cracks to expand further, which will further enhance the phenomenon of efflorescence [45]. At the same time, the appearance of efflorescence substances will also be different due to the speed of evaporation, so the efflorescence substances cannot be completely scraped out during the test. In addition, the visual image of efflorescence will also have certain differences due to the different shapes of the crystalline substances.  
Figure 11: Visual efflorescence of RPSG with different slag contents.