**Figure 5.** Results of fast freeze-thaw cycle tests of concretes after 300 cycles: (**a**) relative dynamic elasticity modulus; (**b**) *3.6. Sulphate Attack*

*3.6. Sulphate Attack*  Figure 6 shows the influence of different initial moisture curing times on the 28-d compressive strength. The 28-d compressive strengths of samples C, M1 and M2 for 3-d initial moisture curing are 55.3 MPa, 59.1 MPa and 67.9 MPa, respectively. Compared to sample C, the growth rates of samples M1 and M2 are 6.9% and 22.8%, respectively. The 28-d compressive strengths of samples C, M1 and M2 for 7-d initial moisture curing are 57.4 MPa, 64.7 MPa and 70.8 MPa, respectively. Adding 9% and 15% ultrafine metakaolin increases the 28-d compressive strength by 12.7% and 23.3%, respectively. The strength growth rate of the 28-d compressive strength significantly increases with increasing ultrafine metakaolin content. The growth rate for 7-d initial moisture curing is higher than the growth rate for 3-d initial moisture curing. Meanwhile, compared to that for 3-d initial Figure 6 shows the influence of different initial moisture curing times on the 28-d compressive strength. The 28-d compressive strengths of samples C, M1 and M2 for 3-d initial moisture curing are 55.3 MPa, 59.1 MPa and 67.9 MPa, respectively. Compared to sample C, the growth rates of samples M1 and M2 are 6.9% and 22.8%, respectively. The 28-d compressive strengths of samples C, M1 and M2 for 7-d initial moisture curing are 57.4 MPa, 64.7 MPa and 70.8 MPa, respectively. Adding 9% and 15% ultrafine metakaolin increases the 28-d compressive strength by 12.7% and 23.3%, respectively. The strength growth rate of the 28-d compressive strength significantly increases with increasing ultrafine metakaolin content. The growth rate for 7-d initial moisture curing is higher than the growth rate for 3-d initial moisture curing. Meanwhile, compared to that for 3-d initial moisture curing, the 28-d compressive strength of mortar for 7-d initial moisture curing increases. The growth rates of samples C, M1 and M2 are 3.8%, 9.5% and 4.3%, respectively. The growth rates of samples M1 and M2 are higher than that of sample C. Therefore, prolonging the initial moisture curing time is more favorable for ultrafine metakaolin concrete than plain cement concrete.

moisture curing, the 28-d compressive strength of mortar for 7-d initial moisture curing increases. The growth rates of samples C, M1 and M2 are 3.8%, 9.5% and 4.3%, respectively. The growth rates of samples M1 and M2 are higher than that of sample C. Therefore, prolonging the initial moisture curing time is more favorable for ultrafine metakaolin

concrete than plain cement concrete.

**Figure 6.** The 28-d compressive strength with different initial moisture curing time. **Figure 6.** The 28-d compressive strength with different initial moisture curing time.

The visual appearance of samples C, M1 and M2 exposed to sulphate attack for different ages is shown in Figure 7a–c, respectively. The numbers marked in Figure 7 refer to the initial moisture curing time. Many salt crystals precipitate from the upper parts of the mortars and tend to increase with prolonged semi-immersion time. In the process of semi-immersion, no obvious cracks appear on the surfaces of all mortars. The relative compressive strengths of the mortars after different semi-immersion times are shown in Figure 8a. When the mortars were initially moisture cured for 3 d, the relative compressive strength of all mortars after 28 d of semi-immersion are less than 100%. However, the relative compressive strengths for certain mortars after 56 d and 90 d of semi-immersion are more than 100%, especially for plain cement mortar. When the mortars are initially moisture cured for 7 d, the relative compressive strengths of almost all mortars after semiimmersion are less than 100%. Therefore, from the results for the relative compressive strengths, prolonging the initial moisture curing time has an adverse effect on the sulphate attack resistance. This is unreasonable. Previous studies have shown that prolongation of the initial moisture curing time is beneficial to cement hydration and pore structure development, and sulphate attack resistance should not decrease with prolongation of the initial moisture curing time [23,27]. Therefore, the compressive strength cannot be used as an index to evaluate sulphate attack resistance of rectangular mortars in semi-immersion tests. The flexural strength losses of mortars after different semi-immersion times are shown in Figure 8b. Apparently, the flexural strength losses of mortars decrease with in-The visual appearance of samples C, M1 and M2 exposed to sulphate attack for different ages is shown in Figure 7a–c, respectively. The numbers marked in Figure 7 refer to the initial moisture curing time. Many salt crystals precipitate from the upper parts of the mortars and tend to increase with prolonged semi-immersion time. In the process of semi-immersion, no obvious cracks appear on the surfaces of all mortars. The relative compressive strengths of the mortars after different semi-immersion times are shown in Figure 8a. When the mortars were initially moisture cured for 3 d, the relative compressive strength of all mortars after 28 d of semi-immersion are less than 100%. However, the relative compressive strengths for certain mortars after 56 d and 90 d of semi-immersion are more than 100%, especially for plain cement mortar. When the mortars are initially moisture cured for 7 d, the relative compressive strengths of almost all mortars after semiimmersion are less than 100%. Therefore, from the results for the relative compressive strengths, prolonging the initial moisture curing time has an adverse effect on the sulphate attack resistance. This is unreasonable. Previous studies have shown that prolongation of the initial moisture curing time is beneficial to cement hydration and pore structure development, and sulphate attack resistance should not decrease with prolongation of the initial moisture curing time [23,27]. Therefore, the compressive strength cannot be used as an index to evaluate sulphate attack resistance of rectangular mortars in semi-immersion tests. The flexural strength losses of mortars after different semi-immersion times are shown in Figure 8b. Apparently, the flexural strength losses of mortars decrease with increasing ultrafine metakaolin at any semi-immersion age. When the semi-immersion time is the same, prolonging the initial moisture curing time has little impact on the flexural strength loss of samples C and M1. However, compared to the flexural strength loss for 3-d initial moisture curing, the flexural strength loss of sample M2 for 7-d initial moisture curing is relatively low. The addition of ultrafine metakaolin significantly improves the sulphate attack resistance of mortar, and the sulphate attack resistance also significantly improves with increasing ultrafine metakaolin. Therefore, in the semi-immersion test, it is reasonable to evaluate the sulphate attack resistance of mortar by flexural strength loss.

creasing ultrafine metakaolin at any semi-immersion age. When the semi-immersion time is the same, prolonging the initial moisture curing time has little impact on the flexural strength loss of samples C and M1. However, compared to the flexural strength loss for 3 d initial moisture curing, the flexural strength loss of sample M2 for 7-d initial moisture curing is relatively low. The addition of ultrafine metakaolin significantly improves the sulphate attack resistance of mortar, and the sulphate attack resistance also significantly

**Figure 7.** Visual appearance of mortar exposed to sulphate attack: (**a**) C; (**b**) M1; (**c**) M2. **Figure 7.** Visual appearance of mortar exposed to sulphate attack: (**a**) C; (**b**) M1; (**c**) M2.

**Figure 8.** Results of sulphate attack: (**a**) relative compressive strength; (**b**) flexural strength loss.

### **Figure 8.** Results of sulphate attack: (**a**) relative compressive strength; (**b**) flexural strength loss. **4. Conclusions**

	- sive strength and splitting tensile strength at 28 d by approximately 24% and 33%, respectively. The effect of mixing the same amounts of ultrafine metakaolin and silica (3) Adding ultrafine metakaolin obviously reduces the connected porosity of concrete, which effectively improves its resistance to chloride ion penetration and freeze–thaw cycles. Silica fume has the same effect as ultrafine metakaolin.

cycles. Silica fume has the same effect as ultrafine metakaolin. (4) Prolonging the initial moisture curing time is more favorable for ultrafine metakaolin concrete than plain cement concrete in terms of compressive strength. Simultane-**Author Contributions:** Conceptualization, S.Z.; methodology, Y.Z. and F.H.; software, S.Z. and Y.Z.; validation, J.S. and Y.Z.; writing—original draft preparation, J.S.; writing—review and editing, J.S. and F.H. All authors have read and agreed to the published version of the manuscript.

ously adding ultrafine metakaolin and prolonging the initial moisture curing time can significantly improve the sulphate attack resistance of mortar. **Funding:** This research was funded by the National Natural Science Foundation of China (No. 51908033) and Beijing Natural Science Foundation (No. 8204067).

**Funding:** This research was funded by the National Natural Science Foundation of China (No.

**Institutional Review Board Statement:** Not applicable.

**Author Contributions:** Conceptualization, S.Z.; methodology, Y.Z. and F.H.; software, S.Z. and **Informed Consent Statement:** Not applicable.

Y.Z.; validation, J.S. and Y.Z.; writing—original draft preparation, J.S.; writing—review and editing, **Data Availability Statement:** Data are contained within the article.

J.S. and F.H. All authors have read and agreed to the published version of the manuscript. **Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

doi:10.1016/j.conbuildmat.2020.121285.

**2020**, *265*, 120557, doi:10.1016/j.conbuildmat.2020.120557.

*Conserv. Recycl.* **2020**, *162*, 105037, doi:10.1016/j.resconrec.2020.105037.

**References** 


1. Abedalqader, A.; Shatarat, N.; Ashteyat, A.; Katkhuda, H. Influence of temperature on mechanical properties of recycled asphalt

2. Mei, S.; Wang, Y. Viscoelasticity: A new perspective on correlation between concrete creep and damping. *Constr. Build. Mater.*

3. Wang, D.; Wang, Q.; Huang, Z. Reuse of copper slag as a supplementary cementitious material: Reactivity and safety. *Resour.* 

pavement aggregate and recycled coarse aggregate concrete. *Constr. Build. Mater.* **2021**, *269*, 121285,

