## 3 Analysis and Discussion of Test Results
### Analysis of Capillary Water Absorption Properties of Recycled Concrete
#### 3.2.3 Analysis of Data Calculations  
According to the experimental data, the water absorption rate can be divided into three stages, which are the pre-fast absorption stage (T\({}^{1/2}\) = 0-84.8 s), the secondary slow absorption stage (T\({}^{1/2}\) = 84.8-207.8 s), and the late gentle absorption stage (T\({}^{1/2}\) = 207.8-777.7 s). The initial water absorption rates of the RAC, WRAC, GRAC, and WGRAC groups can be obtained from the linear fit of the preliminary phase (T\({}^{1/2}\) = 0-84.8 s) of the cumulative water absorption curve per unit time, and its relationship with the number of freeze-thaw cycles in different media is shown below. As can be seen in Figure 6a, at 50 freeze-thaw cycles, the water absorption of the WRAC, GRAC, and WGRAC groups decreased by 4.82%, 8.1%, and 13.8% compared to the RAC group. At 200 freeze-thaw cycles, the water absorption of the WRAC, GRAC, and WGRAC groups decreased by 19.4%, 24.6%, and 33.8% compared to the RAC group. As can be seen in Figure 6b, at 50 freeze-thaw cycles, the capillary water absorption of the WRAC, GRAC, and WGRAC groups decreased by 9.3%, 13.2%, and 18.1%, which are lower than that of the RAC group. At 200 freeze-thaw cycles, the pre-absorption rates of the WRAC, GRAC, and WGRAC groups decreased by 15.38%, 23.14%, and 34.83%, respectively, compared with the RAC group. As can be seen in Figure 6c, at 50 freeze-thaw cycles, the water absorption of the WRAC, GRAC, and WGRAC groups decreased by 8.1%, 13.3%, and 16.3% compared with that of the RAC group, and when 200 freeze-thaw cycles were performed, the water absorption of the WRAC, GRAC, and WGRAC groups decreased by 14.4%, 23.8%, and 38.6% compared with that of the RAC group, respectively. The water absorption of the specimens did not change much at the low number of freeze and thaw cycles in the clear water freezing and thawing environment. The reason for this is that in the early stage of freezing and thawing, the specimen is more complete and dense, and the damage caused by freezing and thawing is mainly concentrated in the surface layer, which has less effect on the interior of the concrete. With the increase in the number of freeze-thaw cycles, the freeze-thaw damage gradually increases, destroying the concrete surface mortar; the internal closed pores of the concrete are gradually destroyed and connected; the water infiltration into the internal channels of the concrete is gradually increased; the freeze-thaw damage is gradually diffused to the internal concrete; and therefore, the amplitude of the change in the capillary absorption rate of the concrete is gradually increased. At a low number of freeze-thaw cycles in a sulfate freezing and thawing environment, the capillary water absorption rate of each group of specimens in  
Figure 6: Capillary water absorption before the number of freezing and thawing cycles in different media. (**a**) Freshwater freeze–thaw cycle; (**b**) sulfate freeze–thaw cycle; (**c**) chlorine salt freeze–thaw cycle. the early stage has a certain increase compared with that of clear water, but the growth rate is relatively slow because the hydration reaction between sulfate and concrete internally generates crystals of calcium alumina, gypsum, etc., which fill the pores and make the concrete relatively dense, allowing it to resist part of the freezing and thawing damage. With the increase in the number of freezing and thawing cycles, the specimen's surface erosion spalling becomes more and more serious, and the sulfate solution is more likely to penetrate into the concrete interior and constantly crystallize and expand, under the pressures of crystallization and infiltration, to accelerate the specimen's internal freezing and thawing damage, so that the specimen's water absorption ability is accelerated.