*3.5. CH Content*

*3.5. CH Content*  The main hydration products of cement after 110 °C are CH and C–S–H gel. TG and DTG analyses can express the mass loss resulting from the decomposition of the hydration products. TG and DTG curves of hardened pastes under standard curing conditions at different ages are presented in Figure 10. It is obvious that the mass loss of sample SC was lower than that of samples SS25 and SS45 at the same ages, especially at early ages (14 d and 28 d). However, the gap between samples SS25 and SS45 was small. As seen from the DTG curves, there are two distinct peaks. One peak represents the dehydration of the C– S–H gel before 200 °C, and the other is associated with the decomposition of CH within a temperature range of 400 °C–500 °C [46,47]. It is worth noting that a smaller peak can be found at approximately 800 °C, especially at 90 d. This peak is related to the decomposition of CaCO3 originating from the carbonation of CH. The total CH content was calcu-The main hydration products of cement after 110 ◦C are CH and C–S–H gel. TG and DTG analyses can express the mass loss resulting from the decomposition of the hydration products. TG and DTG curves of hardened pastes under standard curing conditions at different ages are presented in Figure 10. It is obvious that the mass loss of sample SC was lower than that of samples SS25 and SS45 at the same ages, especially at early ages (14 d and 28 d). However, the gap between samples SS25 and SS45 was small. As seen from the DTG curves, there are two distinct peaks. One peak represents the dehydration of the C–S–H gel before 200 ◦C, and the other is associated with the decomposition of CH within a temperature range of 400 ◦C–500 ◦C [46,47]. It is worth noting that a smaller peak can be found at approximately 800 ◦C, especially at 90 d. This peak is related to the decomposition of CaCO<sup>3</sup> originating from the carbonation of CH. The total CH content was calculated based on the mass losses, corresponding to the decomposition of CH and CaCO3.

lated based on the mass losses, corresponding to the decomposition of CH and CaCO3.

(**a**) (**b**)

90 d.

**Figure 9.** The pore size distribution of hardened pastes under temperature-matching curing conditions: (**a**) at 28 d; (**b**) at

The main hydration products of cement after 110 °C are CH and C–S–H gel. TG and DTG analyses can express the mass loss resulting from the decomposition of the hydration products. TG and DTG curves of hardened pastes under standard curing conditions at different ages are presented in Figure 10. It is obvious that the mass loss of sample SC was lower than that of samples SS25 and SS45 at the same ages, especially at early ages (14 d and 28 d). However, the gap between samples SS25 and SS45 was small. As seen from the DTG curves, there are two distinct peaks. One peak represents the dehydration of the C– S–H gel before 200 °C, and the other is associated with the decomposition of CH within a temperature range of 400 °C–500 °C [46,47]. It is worth noting that a smaller peak can be found at approximately 800 °C, especially at 90 d. This peak is related to the decomposition of CaCO3 originating from the carbonation of CH. The total CH content was calculated based on the mass losses, corresponding to the decomposition of CH and CaCO3.

*3.5. CH Content* 

**Figure 10.** Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of hardened pastes under standard curing conditions: (**a**) at 14 d; (**b**) at 28 d; (**c**) at 90 d. curing conditions: (**a**) at 14 d; (**b**) at 28 d; (**c**) at 90 d.

Therefore, the CH content of hardened paste at different ages under standard curing conditions is shown in Figure 11. It is clear that the CH content declined as the incorporation of GSP increased at early and late ages, which can be primarily attributed to the reduction in the cement content. Furthermore, unlike cement, GSP does not produce CH, but consumes a certain amount of CH due to its pozzolanic reaction. In order to better explain the influence of GSP on the cement system, the CH contents of plain cement systems were calculated using reduction factors of 0.75 (25% GSP) and 0.55 (45% GSP), respectively. The calculation results are also marked in Figure 11. At 14 d, compared to those of the cement after the calculation with reduction factors of 0.75 and 0.55, the CH contents of samples SS25 and SS45 were significantly higher. This indicates that the promoting effects of GSP on the cementitious system exceeded the pozzolanic effect of GSP. These promoting effects on cement hydration can be attributed to the dilution effect and the nuclecontent in composite systems at 90 d. **Figure 10.** Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of hardened pastes under standard Therefore, the CH content of hardened paste at different ages under standard curing conditions is shown in Figure 11. It is clear that the CH content declined as the incorporation of GSP increased at early and late ages, which can be primarily attributed to the reduction in the cement content. Furthermore, unlike cement, GSP does not produce CH, but consumes a certain amount of CH due to its pozzolanic reaction. In order to better explain the influence of GSP on the cement system, the CH contents of plain cement systems were calculated using reduction factors of 0.75 (25% GSP) and 0.55 (45% GSP), respectively. The calculation results are also marked in Figure 11. At 14 d, compared to those of the cement after the calculation with reduction factors of 0.75 and 0.55, the CH contents of samples SS25 and SS45 were significantly higher. This indicates that the promoting effects of GSP on the cementitious system exceeded the pozzolanic effect of GSP. These promoting effects on cement hydration can be attributed to the dilution effect and the nucleation effect, which forms a higher effective water/cement ratio and a larger growth space for hydration

ation effect, which forms a higher effective water/cement ratio and a larger growth space for hydration products. At an early age, hydration of cement in the cementitious system

sponding hardened paste containing GSP showed little difference. However, with further hydration, the pozzolanic reaction of GSP consumed more CH, leading to a lower CH

products. At an early age, hydration of cement in the cementitious system is dominant. At 28 d, the CH contents in the cement paste after calculation and the corresponding hardened paste containing GSP showed little difference. However, with further hydration, the pozzolanic reaction of GSP consumed more CH, leading to a lower CH content in composite systems at 90 d. *Crystals* **2021**, *11*, x FOR PEER REVIEW 12 of 18

**Figure 11.** CH content of hardened pastes under standard curing conditions. **Figure 11.** CH content of hardened pastes under standard curing conditions.

The TG and DTG curves of hardened pastes at different ages under temperaturematching curing conditions are presented in Figure 12. As shown in Figure 12, the trend in mass losses was obviously different from that observed under standard curing conditions (Figure 10). The gaps between samples MC, MS25 and MS45 were relatively small at all ages. There are also two endothermic peaks on the DTG curves at approximately 100 °C and 450 °C, corresponding to the sequential decomposition of C–S–H gel and CH crystals. Another absorption peak is located at approximately 650 °C, which represents the decomposition of CaCO3. The TG and DTG curves of hardened pastes at different ages under temperaturematching curing conditions are presented in Figure 12. As shown in Figure 12, the trend in mass losses was obviously different from that observed under standard curing conditions (Figure 10). The gaps between samples MC, MS25 and MS45 were relatively small at all ages. There are also two endothermic peaks on the DTG curves at approximately 100 ◦C and 450 ◦C, corresponding to the sequential decomposition of C–S–H gel and CH crystals. Another absorption peak is located at approximately 650 ◦C, which represents the decomposition of CaCO3.

The CH contents of different hardened pastes, including measured values and normalization marks, under temperature-matching curing conditions are indicated in Figure 13. At 14 d, the CH contents in the cement paste after calculation and the corresponding hardened paste containing GSP showed little difference. This indicates that the CH content remained balanced between production from cement hydration and consumption due to the pozzolanic reaction of GSP. Currently, the promoting effect of GSP on the cementitious system can compensate for its negative effect on CH content due to the pozzolanic effect. At 28 d, compared to those of the cement after calculation with reduction factors of 0.75 and 0.55, the CH contents of samples SS25 and SS45 were significantly lower. At 90 d, the gap in CH contents between the cement paste and hardened paste containing GSP became wider. This indicates that the pozzolanic effect of GSP proves its importance with further hydration. Compared to that observed under standard curing conditions, the gap in the CH content at 14 d was obviously smaller under temperature-matching curing condition. Elevated curing temperature has a greater impact on the pozzolanic reaction of GSP, consuming more CH.

**Figure 11.** CH content of hardened pastes under standard curing conditions.

decomposition of CaCO3.

The TG and DTG curves of hardened pastes at different ages under temperaturematching curing conditions are presented in Figure 12. As shown in Figure 12, the trend in mass losses was obviously different from that observed under standard curing conditions (Figure 10). The gaps between samples MC, MS25 and MS45 were relatively small at all ages. There are also two endothermic peaks on the DTG curves at approximately 100 °C and 450 °C, corresponding to the sequential decomposition of C–S–H gel and CH crystals. Another absorption peak is located at approximately 650 °C, which represents the

**Figure 12.** TG and DTG curves of hardened pastes under temperature-matching curing conditions: (**a**) at 14 d; (**b**) at 28 d; (**c**) at 90 d. **Figure 12.** TG and DTG curves of hardened pastes under temperature-matching curing conditions: (**a**) at 14 d; (**b**) at 28 d; (**c**) at 90 d.

### The CH contents of different hardened pastes, including measured values and nor-*3.6. Non-Evaporable Water Content*

malization marks, under temperature-matching curing conditions are indicated in Figure 13. At 14 d, the CH contents in the cement paste after calculation and the corresponding hardened paste containing GSP showed little difference. This indicates that the CH content remained balanced between production from cement hydration and consumption due to the pozzolanic reaction of GSP. Currently, the promoting effect of GSP on the cementitious system can compensate for its negative effect on CH content due to the pozzolanic effect. At 28 d, compared to those of the cement after calculation with reduction factors of 0.75 and 0.55, the CH contents of samples SS25 and SS45 were significantly lower. At 90 d, the gap in CH contents between the cement paste and hardened paste containing GSP became wider. This indicates that the pozzolanic effect of GSP proves its importance with further hydration. Compared to that observed under standard curing conditions, the gap in the CH content at 14 d was obviously smaller under temperature-The non-evaporable water contents of hardened paste under two curing conditions are presented in Figure 14a,b, respectively. The lowest non-evaporable water content of hardened paste exceeded 12% under both curing conditions. Under standard curing condition, the non-evaporable water content of hardened paste containing GSP was slightly higher than that of sample SC. In particular, hardened paste containing 25% GSP showed the highest non-evaporable water content. This indicates that the promoting effects, including the dilution effect and the nucleation effect of GSP on the cement hydration, were obvious at early and late ages. The results agree with the results of the adiabatic temperature rise of concrete (Figure 1). In theory, the hydration of Portland cement per gram produces 0.25 g of non-evaporable water, but the pozzolanic reaction of slag per gram generates 0.30 g of non-evaporable water [48]. Meanwhile, increasing the fineness of active powder has a more positive effect on the pozzolanic reaction [48]. Therefore, the faster reaction of GSP results with a higher temperature rise rate was observed for sample SS25 (Figure 1), along with the

matching curing condition. Elevated curing temperature has a greater impact on the poz-

zolanic reaction of GSP, consuming more CH.

higher non-evaporable water content of sample SS25. However, the gap in non-evaporable water contents between samples SC and SS45 was relatively small, resulting from a sharp reduction in cement content. Combined with the results of compressive strength under two curing conditions, sample SS45 had the highest compressive strength. The compressive strength of concrete depends not only on the amount of hydration products, it is also closely related to the pore structure. Sample SS45 had the lowest total porosity and volume of harmful pores, which resulted in the highest compressive strength. *Crystals* **2021**, *11*, x FOR PEER REVIEW 14 of 18

**Figure 13.** CH content of hardened pastes under temperature-matching curing conditions. **Figure 13.** CH content of hardened pastes under temperature-matching curing conditions.

compressive strength under two curing conditions, sample SS45 had the highest compressive strength. The compressive strength of concrete depends not only on the amount of **Figure 14.** Non-evaporable water content of hardened pastes: (**a**) under standard curing conditions; (**b**) under temperature-matching curing conditions. **Figure 14.** Non-evaporable water content of hardened pastes: (**a**) under standard curing conditions; (**b**) under temperaturematching curing conditions.

hydration products, it is also closely related to the pore structure. Sample SS45 had the lowest total porosity and volume of harmful pores, which resulted in the highest compressive strength. Under temperature-matching curing conditions, the trend of the non-evaporable water content was similar to that under standard curing conditions (Figure 14a). However, the non-evaporable water content of hardened paste became higher than that under standard curing conditions. Increasing the early curing temperature evidently improves the Under temperature-matching curing conditions, the trend of the non-evaporable water content was similar to that under standard curing conditions (Figure 14a). However, the non-evaporable water content of hardened paste became higher than that under standard curing conditions. Increasing the early curing temperature evidently improves the hydra-

hydration rate of the plain cement and composite binder. Meanwhile, the highest nonevaporable water content was observed for sample SS45 at 90 d. Replacing cement with

matching curing conditions. It is worth noting that the non-evaporable water content of hardened paste containing GSP under two curing conditions reached or exceeded the content of cement paste after 14 d of curing. A previous study on ordinary slag showed that the same non-evaporable water content with replacement rates of 50%–70% can be achieved after 60 d of curing [28,49]. This indicates that the fineness of slag plays an important role in cement hydration and the pozzolanic reaction of mineral admixtures.

(1) Adding 25% GSP increases the adiabatic temperature rise of high-strength concrete due to the promoting effects on cement hydration, whereas adding 45% GSP decreases the adiabatic temperature rise, which can be attributed to a reduction in ce-

(2) Compared to the compressive strength of plain cement concrete, the growth rates of strength at different ages due to the addition of GSP under temperature-matching curing conditions are higher than those under standard curing temperature. Temperature-matching curing conditions have a positive effect on the development of the

(3) Compared to plain cement systems, cementitious material systems containing GSP tend to have lower total porosity and a lower volume of harmful pores. The dense pore structure of the GSP system leads to better chloride ion penetrability resistance of the concrete, which is more distinct under early temperature-matching curing conditions. Increasing the curing temperature has a greater influence on high-strength

late compressive strength of GSP concrete.

concrete mixed with 25% GSP.

**4. Conclusions** 

ment content.

tion rate of the plain cement and composite binder. Meanwhile, the highest non-evaporable water content was observed for sample SS45 at 90 d. Replacing cement with GSP markedly increased the late non-evaporable water content under temperature-matching curing conditions. It is worth noting that the non-evaporable water content of hardened paste containing GSP under two curing conditions reached or exceeded the content of cement paste after 14 d of curing. A previous study on ordinary slag showed that the same non-evaporable water content with replacement rates of 50%–70% can be achieved after 60 d of curing [28,49]. This indicates that the fineness of slag plays an important role in cement hydration and the pozzolanic reaction of mineral admixtures.
