## 4 Conclusions  
By studying the effects of nano-silica on the autogenous shrinkage, internal relative humidity, internal temperature, and compressive strength of UHSCs, the following conclusions were reached:  
1. The isothermal calorimetric data showed that due to the significantly delayed effect of polycarboxylate superplasticizers, the heat flow peaks of each hydration heat appeared in the order of nS0 > nS2 > nS4. Three peaks (dissolved peak of tricalcium aluminate, the exothermic peak of C3S and other topochemical reactions, and sulphate-type-AFm and C3A rehydration) were found in the heat flow curves in all UHSCs. Isothermal calorimetry is used to determine the setting time approximately. The setting time of UHSC specimens was delayed due to the addition of polycarboxylate superplasticizers. A greater amount of polycarboxylate superplasticizers led to more noticeable delayed effects.
2. Total shrinkage increased with an increase in the nano-silica content. All specimens expanded at the beginning of the aging (1.2-6.7 h). The expansion of nS2 was more pronounced compared to that of nS0. However, the expansion of nS4 did not show up more noticeably as the amount of nS increased.
3. The trends in the autogenous shrinking of UHSC were divided into two stages: the variable temperature stage (Stage I) and the room temperature stage (Stage II). The dividing point between the two stages occurred at approximately 2 days. In stage I, the hydration rate values decreased as the nS content increased. The maximum IT values decreased as the nS content increased. The IRH curve peaks of all specimens showed distinct downward peaks in a short period of time, and the timing at which these peaks appeared corresponded to the location of the highest peak of IT. However, in stage II, the IRH of all specimens continued to decrease. The final IRH value of UHSC after 7 days decreased as the nS content increased.
4. At 28 days, XRD analysis showed that compared with nS0, the CH intensity of the nS2 and nS4 specimens decreased. This was due to the pozzolanic activities of the added nano-silica and CH, which consumed the CH produced by cement hydration.
5. Firstly, at 3 days, a linear relationship between the compressive strength and cumulative heat of hydration was clearly observed. Secondly, at 28 days, the addition of 2 wt.% and 4 wt.% nano-silica increased the strength of the specimen by 26.31% and 30.32%, respectively. As the amount of nS added increased, the ability of nS to enhance the compressive strength of UHSCs decreased significantly.  
Figure 10: The linear regression results of heat released and the compressive strength of UHSCs at 3 days of age.  
**Author Contributions:** Data curation, G.-Z.Z. and X.-Y.W.; Formal analysis, X.-Y.W.; Funding acquisition, X.-Y.W.; Investigation, G.-Z.Z. and X.-Y.W.; Methodology, G.-Z.Z.; Project administration, G.-Z.Z.; Resources, H.-K.C.; Software, H.-K.C. and X.-Y.W.; Supervision, G.-Z.Z.; Validation, H.-K.C.; Visualization, H.-K.C.; Writing--original draft, G.-Z.Z.; Writing--review & editing, X.-Y.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (No. 2015R1A5A1037548), and an NRF grant (NRF-2020R1A2C4002093).  
**Conflicts of Interest:** The authors declared that they have no conflict of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.