## 3 Results and Discussion
### Development of Autogenous Shrinkage, Internal Relative Humidity, and Internal Temperature
#### 3.2.2 Development of the Internal Relative Humidity, Shrinkage, and Internal Temperature  
Autogenous shrinkage occurs without any loss of moisture from the concrete, and it is a microvolume change. Many past studies have introduced the capillary tension theory to explain shrinkage behavior [37]. Capillary water is subjected to surface tension through the curved moon surface, causing volume changes and deformation of the concrete. The decrease in relative humidity in the pore structure causes capillary tension to cause shrinkage stresses, a mechanism that causes the autogenous shrinkage of concrete [38; 39]. Figures 5-7 show the relationships among the shrinkage strain, internal temperature, and internal relative humidity of all specimens curing to 7 days. The experimental data of IRH and AS of UHSC were analyzed. 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 after approximately 2 days. Firstly, in stage I, Figure 2a shows that the maximum hydration rates of nS0, nS2, and nS4 were 3.22, 2.78, and 2.60 mW/g, respectively. As mentioned in the previous section, in order to ensure the same workability, UHSC added with a higher amount of nano-silica requires more superplasticizers, which suppress binder hydration. This trend of the hydration rate values decreasing with an increasing nS content is shown by the IT values (the data can be seen in Figures 5a, 6a and 7a). The maximum IT values of nS0, nS2, and nS4 were 30.51, 29.58, and 28.02 \({}^{\circ}\)C (IT\({}_{\text{nS0}}\) \(>\) IT\({}_{\text{nS2}}\) \(>\) IT\({}_{\text{nS4}}\)), respectively. Figure 5: Development of the internal relative humidity (IRH) and internal temperature (IT) (**a**) and autogenous shrinkage (**b**) with the age of the nS0 specimen. Linear regression results (**c**) of the IRH and autogenous shrinkage of the nS0 specimen in stage II. Secondly, in stage I, the IRH curve peaks of all specimens showed distinct downward peaks in a short time after the final setting time, and the timing at which these peaks appeared corresponded to the highest peak of IT. The three main reasons for this behavior are as follows: The continued occurrence of binder hydration led to a decrease in IRH. Meanwhile, at a higher temperature, water in the ink bottle spaces was released and redistributed as condensed water with the passage of time, resulting in an increase in IRH [40]. The last factor was the greater decrease in the surface tension of the interstitial fluid due to the increased superplasticizer content. The reduction in surface tension resulted in an increase in the IRH [41]. Therefore, the macroscopic expression was a decline in IRH [42]. Thirdly, in stage II, the IRH of all specimens continued to decrease. This behavior was due to the continued binder hydration, which consumed some of the water, resulting in the IRH decreasing. The final IRH value of UHSC at 7 days decreased as the nS content increased, and the final IRH values of nS0, nS2, and nS4 were 88.54%, 87.76%, and 74.19%. The IRH trend of UHSC was consistent with the trend in AS. From the results shown in Figures 5b, 6b and 7b, it is clear that as the content of nS added increased, the AS of UHSC became more significant, and the final IRH value decreased more. The AS values of nS0, nS2, and nS4 were \(-\)738.96, \(-\)879.04, and \(-\)1002.16 \(\mu\)m/m, respectively. In stage I (initial 2 days), a significant reduction in IRH was exhibited due to the dominant effects of binder hydration and PCE addition. Afterwards, binder hydration continued, leading to an increase IT, and the ink bottle effect became dominant, leading to a rebound in IRH. In stage II, the results shown in Figures 5c, 6c and 7c indicate that AS and IRH had great significant linear relevance for UHSC, regardless of the dosage of nS.