*3.2. Compressive Strength*

The compressive strengths of plain cement concrete and GSP concrete under standard curing conditions are shown in Figure 2a. It can be easily observed that the compressive strength of concrete slightly increased with GSP at all ages. The compressive strengths of concrete at ages of 7 d, 28 d, 90 d and 180 d were more than 70 MPa, approximately 80 MPa, 85 MPa and 90 MPa, respectively. Compared to the 7 d compressive strength, the growth rates of the strength at the ages of 28 d, 90 d and 180 d were approximately 12%, 20% and 24%, respectively. Thus, the growth rates of the compressive strengths of plain cement concrete and GSP concrete showed little difference at the same ages under standard curing conditions. Compared to the compressive strengths of plain cement concrete, the

growth rates in strength due to the addition of GSP were calculated and are presented in Figure 2b. The growth rate of sample SS25 at all ages was relatively low, at no more than 2%. The growth rate of sample SS45 was higher than that of sample SS25 at all ages. In particular, the 28 d growth rate of sample SS45 reached 6%. The growth rate is related to pore structure and hydration products. Compared to sample SS25, sample SS45 has a higher substitution rate and the presence of GSP with finer particles has a more positive influence on early hydration, resulting in a higher growth rate. and are presented in Figure 2b. The growth rate of sample SS25 at all ages was relatively low, at no more than 2%. The growth rate of sample SS45 was higher than that of sample SS25 at all ages. In particular, the 28 d growth rate of sample SS45 reached 6%. The growth rate is related to pore structure and hydration products. Compared to sample SS25, sample SS45 has a higher substitution rate and the presence of GSP with finer particles has a more positive influence on early hydration, resulting in a higher growth rate.

The compressive strengths of plain cement concrete and GSP concrete under standard curing conditions are shown in Figure 2a. It can be easily observed that the compressive strength of concrete slightly increased with GSP at all ages. The compressive strengths of concrete at ages of 7 d, 28 d, 90 d and 180 d were more than 70 MPa, approximately 80 MPa, 85 MPa and 90 MPa, respectively. Compared to the 7 d compressive strength, the growth rates of the strength at the ages of 28 d, 90 d and 180 d were approximately 12%, 20% and 24%, respectively. Thus, the growth rates of the compressive strengths of plain cement concrete and GSP concrete showed little difference at the same ages under standard curing conditions. Compared to the compressive strengths of plain cement concrete, the growth rates in strength due to the addition of GSP were calculated

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*3.2. Compressive Strength* 

**Figure 2.** (**a**) Compressive strength of high-strength concrete under standard curing condition; (**b**) growth rate of compressive strength at different ages. **Figure 2.** (**a**) Compressive strength of high-strength concrete under standard curing condition; (**b**) growth rate of compressive strength at different ages.

The compressive strength and growth rate of compressive strength under temperature-matching curing conditions are presented in Figure 3a,b, respectively. The compressive strength of high-strength concrete significantly increases with GSP at all ages under temperature-matching curing conditions, which is different from the trend under standard curing conditions. Compared to the 7 d compressive strength, the growth rates of the strength at 28 d and 90 d were approximately 11% and 21%, respectively. However, the growth rates of 180 d compressive strength were approximately 22%, 28% and 30%, respectively. The growth rate of GSP concrete at 180 d was higher than that of plain cement concrete. Meanwhile, compared to the compressive strength of plain cement concrete, the growth rates of strength at different ages due to the addition of GSP under temperaturematching curing conditions (Figure 3b) were higher than those under standard curing temperatures (Figure 2b). In addition, the growth rate increased with GSP. In particular, the 180 d growth rates were the highest. This result indicates that the temperature-matching curing conditions have a more positive effect on the development of the late compressive strength of GSP concrete. Elevated temperatures promote the pozzolanic reaction of The compressive strength and growth rate of compressive strength under temperaturematching curing conditions are presented in Figure 3a,b, respectively. The compressive strength of high-strength concrete significantly increases with GSP at all ages under temperature-matching curing conditions, which is different from the trend under standard curing conditions. Compared to the 7 d compressive strength, the growth rates of the strength at 28 d and 90 d were approximately 11% and 21%, respectively. However, the growth rates of 180 d compressive strength were approximately 22%, 28% and 30%, respectively. The growth rate of GSP concrete at 180 d was higher than that of plain cement concrete. Meanwhile, compared to the compressive strength of plain cement concrete, the growth rates of strength at different ages due to the addition of GSP under temperaturematching curing conditions (Figure 3b) were higher than those under standard curing temperatures (Figure 2b). In addition, the growth rate increased with GSP. In particular, the 180 d growth rates were the highest. This result indicates that the temperature-matching curing conditions have a more positive effect on the development of the late compressive strength of GSP concrete. Elevated temperatures promote the pozzolanic reaction of GSP. The pozzolanic reaction of GSP consumes CH and forms C–S–H gel, improving the density of the interfacial transition zone between the cement and aggregates [40,41]. Furthermore, C–S–H gel plays a key role in mechanical performance. When GSP is added to the cementitious system, Al3+ is released from the slag and finally forms a C–(A)–S–H gel, leading to an increase in the Al/Si molar ratio and a decrease in the Ca/Si molar ratio [42,43]. C–S–H with higher Al/Si and lower Ca/Si ratios has a higher bonding capacity and thus improves the compressive strength [40–44].

thus improves the compressive strength [40–44].

**Figure 3.** (**a**) Compressive strength of high-strength concrete under temperature-matching curing conditions; (**b**) growth rate of compressive strength at different ages. **Figure 3.** (**a**) Compressive strength of high-strength concrete under temperature-matching curing conditions; (**b**) growth rate of compressive strength at different ages.

### *3.3. Chloride Ion Penetrability Resistance 3.3. Chloride Ion Penetrability Resistance*

The chloride ion penetrability resistance of concrete at ages of 28 d and 180 d under standard curing conditions are shown in Figure 4. It can be seen in Figure 4 that the chloride ion penetrability grades of sample SC were ''moderate" and "low" at 28 d and 180 d, respectively. However, the penetrability grades of samples SS25 and SS45 fell to the "low" level and the "very low" level at the two ages. Therefore, substitution with GSP can improve the chloride ion penetrability resistance of high-strength concrete, and the effect increases with increasing GSP. This is because the filling effect of grinding slag fills the pore structure of concrete, and the pozzolanic reaction consumes CH in the transition zone, resulting in more C–S–H gel, which refines the pore structure. The chloride ion penetrability resistance of concrete at ages of 28 d and 180 d under standard curing conditions are shown in Figure 4. It can be seen in Figure 4 that the chloride ion penetrability grades of sample SC were "moderate" and "low" at 28 d and 180 d, respectively. However, the penetrability grades of samples SS25 and SS45 fell to the "low" level and the "very low" level at the two ages. Therefore, substitution with GSP can improve the chloride ion penetrability resistance of high-strength concrete, and the effect increases with increasing GSP. This is because the filling effect of grinding slag fills the pore structure of concrete, and the pozzolanic reaction consumes CH in the transition zone, resulting in more C–S–H gel, which refines the pore structure. *Crystals* **2021**, *11*, x FOR PEER REVIEW 7 of 18

GSP. The pozzolanic reaction of GSP consumes CH and forms C–S–H gel, improving the density of the interfacial transition zone between the cement and aggregates [40,41]. Furthermore, C–S–H gel plays a key role in mechanical performance. When GSP is added to the cementitious system, Al3+ is released from the slag and finally forms a C–(A)–S–H gel, leading to an increase in the Al/Si molar ratio and a decrease in the Ca/Si molar ratio [42,43]. C–S–H with higher Al/Si and lower Ca/Si ratios has a higher bonding capacity and

**Figure 4.** Chloride ion penetrability resistance of high-strength concrete under standard curing conditions. **Figure 4.** Chloride ion penetrability resistance of high-strength concrete under standard curing conditions.

The chloride ion penetrability resistance of concrete at 28 d and 180 d under temperature-matching curing conditions are presented in Figure 5. Significantly, as the age increases, the chloride ion penetration resistance of concrete did not change. The chloride The chloride ion penetrability resistance of concrete at 28 d and 180 d under temperaturematching curing conditions are presented in Figure 5. Significantly, as the age increases, the chloride ion penetration resistance of concrete did not change. The chloride ion pene-

ion penetrability grades of sample MC were ''moderate" at the two ages. The penetrability

further hydration. This is because the pozzolanic reaction of GSP mainly occurred at an early age and increasing the early curing temperature promoted the reaction of GSP, which had an adverse effect on the late reaction. In terms of the chloride ion penetration resistance, combined with the results under standard curing conditions, increasing the curing temperature has a greater influence on high-strength concrete mixed with 25%

**Figure 5.** Chloride ion penetrability resistance of high-strength concrete under temperature-

GSP.

matching curing conditions.

trability grades of sample MC were "moderate" at the two ages. The penetrability grades of both samples MS25 and MS45 fell to the "very low" level at the same time. The GSP content has little effect on the chloride ion penetration resistance of concrete with further hydration. This is because the pozzolanic reaction of GSP mainly occurred at an early age and increasing the early curing temperature promoted the reaction of GSP, which had an adverse effect on the late reaction. In terms of the chloride ion penetration resistance, combined with the results under standard curing conditions, increasing the curing temperature has a greater influence on high-strength concrete mixed with 25% GSP. GSP content has little effect on the chloride ion penetration resistance of concrete with further hydration. This is because the pozzolanic reaction of GSP mainly occurred at an early age and increasing the early curing temperature promoted the reaction of GSP, which had an adverse effect on the late reaction. In terms of the chloride ion penetration resistance, combined with the results under standard curing conditions, increasing the curing temperature has a greater influence on high-strength concrete mixed with 25% GSP.

**Figure 4.** Chloride ion penetrability resistance of high-strength concrete under standard curing

The chloride ion penetrability resistance of concrete at 28 d and 180 d under temperature-matching curing conditions are presented in Figure 5. Significantly, as the age increases, the chloride ion penetration resistance of concrete did not change. The chloride ion penetrability grades of sample MC were ''moderate" at the two ages. The penetrability grades of both samples MS25 and MS45 fell to the "very low" level at the same time. The

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**Figure 5.** Chloride ion penetrability resistance of high-strength concrete under temperaturematching curing conditions. **Figure 5.** Chloride ion penetrability resistance of high-strength concrete under temperature-matching curing conditions.
