# Internal Curing Effect of Waste Glass Beads on High-Strength Cement Composites
## 4 Conclusions  
This study used WGBs as LWAs to improve the internal hydration degree of HSCCs that use ultrafine particles. Moreover, internal hydration characteristics were investigated for different aging durations. The correlation between the presence of pre-wetted WGBs and internal hydration was established. The results of this study are summarized as follows.  
1. The compressive strength of HSCC specimens decreased as WGB content increased, regardless of the curing environment. This was due to the low mechanical strength of the added WGBs; strength decreased as the volume content of WGBs increased.
2. From XRD pattern analysis results, S-group specimens showed similar reactant peak tendencies regardless of age. In R-group specimens, as WGB content increased according to age, the production of products, such as belite, also increased.
3. From TGA results, S20 specimen exhibited the highest weight loss at the age of 56 d in the section below 200 \({}^{\circ}\)C. Based on these results, NMR spectrum analysis was conducted for S20 and R20 specimens; the extent of pozzolanic reaction increased as the Q1/Q2 ratio decreased.
4. The interactions among the major hydration products inside the matrix were identified through thermodynamic modeling. The volume of the generated hydrates differed depending on the curing environment, and a large difference in the amount of C-S-H gel generated was confirmed at an early age. R20 specimen exhibited hydrate proportions similar to those of S20 specimen with increasing age.
5. The residual moisture trapped inside WGB was confirmed through cross-sectional imaging of the specimen and SEM-EDS analysis. The amount of hydrates produced by S20 and R20 confirmed through EDS analysis was found to have a difference of up to 3 fold in the Si, Ca, and Al content. Moreover, the hydration tendency inside WGB was similar to that in thermodynamic modeling.  
The XRD, TGA, \({}^{29}\)Si MAS NMR, and SEM-EDS results of the HSCC paste mixed with WGBs subjected to steam curing and room temperature curing showcased varying formation of hydrates, including C-S-H gel. These results agreed with the results of thermodynamic modeling. This study also confirmed the effect of internal curing by WGBs. However, the compressive strength tended to decrease with the addition of WGBs owing to the prior damage and poor mechanical properties of WGBs. Therefore, an analysis of the hydration products formed inside WGB micropores is required. Moreover, further research is required in the future to decrease the influence of WGBs on strength by adjusting the geometry and size of WGBs or by selecting a more suitable LWA than WGB. This will help improve the hydration degree in a highly densified matrix, such as UHPC.  
Conceptualization, S.P. and J.N.; methodology, S.P.; investigation, S.P.; formal analysis, G.K. and J.N.; data curation, S.P.; visualization, S.P.; writing--manuscript draft, S.P.; writing--reviewing and editing, S.P., S.L. and J.N.; validation, S.P., G.K., S.L. and J.N.; supervision, G.K., S.L. and J.N. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Research Foundation of the Ministry of Science and ICT of Korea in 2020 (grant number 2020R1C1C101403812).  
Not applicable.  
The authors declare no conflict of interest.