# Internal Curing Effect of Waste Glass Beads on High-Strength Cement Composites
## 2 Materials and Methods  
### Experimental Procedure  
CEM Type 1 (SangYong C&E, Seoul, Korea) of ASTM C 150-85 and silica fume (Elkem Silicones) were used as binders for the materials used to prepare the specimens. Silica powder (SAC Materials, Pure silica S-SIL 30) was used as a filler, and a polycarboxylate superplasticizer for UHPC (FLOWMIX 3000S) was used for effective mixing. WGB, which was used as an internal moisture supply medium to reduce unreacted hydrates inside the matrix, had a particle diameter of 2.00 to 6.00 mm and a dry density of 0.89 g/cm\({}^{3}\), as shown in Figure 1. WGB is fabricated through a special heat treatment process at 900 \({}^{\circ}\)C or higher after the waste glass is collected and crushed into a finer powder. In this instance, the Si component of waste glass causes internal expansion, resulting in numerous pores inside. This fabrication process aims to recycle resources; the pores generated in the process lead to a higher water absorption rate of 13.04%. WGB was soaked in water for 48 h, and the saturated surface dry (SSD) state was used. Table 1 lists the physical and chemical properties of WGB. In this study, reinforcing fibers and silica sand, which may act as variables, were excluded to effectively observe the formation of hydration products in specimens as a function of curing time. Table 2 lists the chemical compositions of the binders used to prepare specimens, and Figure 2 shows the particle sizes of raw materials. The particle distribution of the material used for HSCCs is presented to understand the materials constituting the dense microstructure in HSCCs, which exhibit a dense internal structure with nanostructured material. Table 2 lists the analysis results performed with the Bruker (D8 ADVANCE) equipment of Chungnam National University. This study aimed to examine the improvement in internal curing by identifying the changes in the tightly closed internal structure as a result of mixing WGB with the internal structure in which several nanosized materials exist.  
Table 3 lists the mix proportions of specimens. Notations are listed to identify each specimen according to the curing condition and mixing ratio of WGB. WGB mixing ratios of 5%, 10%, and 20% were selected to activate and investigate internal curing while minimizing strength degradation that occurs when a large amount of lightweight aggregate is mixed. This was carried out by referring to a previous study [26]. A study conducted by [PERSON] et al. produced self-compacting concrete using diatomite aggregate and observed significant deterioration in the mechanical properties and durability when the aggregate was replaced at 5%, 10%, 15%, and 20% levels. The binders were subjected to an anhydride mix for 5 min. Next, water and admixtures were added; moreover, additional mixing was performed for 2 min to ensure complete mixing. WGB was then added and mixed for another 2 min. Upon completion of the mixing process, the fresh mixture was poured into a \(\varnothing\)100 \(\times\) 200 mm mold. A 50 \(\times\) 50 \(\times\) 50 mm test piece was additionally prepared to confirm the cross-section of the specimen. The mold was sealed with a film and cured under natural conditions for 24 h. SP, S5, S10, and S20 specimens (hereinafter called \"S-group specimens\") were subjected to steam curing at 80 \({}^{\circ}\)C for 72 h [27] and then curing at room temperature (Temp. 20 \({}^{\circ}\)C, Hum. 55%), whereas RP, R5, R10, and R20 specimens (hereinafter called \"R-group specimens\") were subjected to room temperature curing from the beginning. In this study, steam curing was performed to study the activation of internal curing of UHPC, a high-strength and high-performance concrete, which was compared with room temperature curing. Two types of curing environments were applied to examine the applicability of PC products in the actual field later.  
Figure 2: Particle size distribution and cumulative passing rate of binders and WGBs.