## 5 Conclusions  
RPC is a newly emerging and promising concrete material. It has an improved microstructure that results in higher compressive and flexural strengths and better durability. This research focused on the optimization of mixture parameters (WB and SF) to achieve the RPC with high compressive,  
Figure 15: Test and simulation results comparisons: (**a**) different SF content; (**b**) different WB. flexural strengths, and low drying shrinkage under normal and warm temperature curing conditions. The following conclusions were drawn based on the experiments and analysis results:  
1. Except for a plain mixture (0.22 WB-0 SF) with a relative lower SP content (1%), the relative flowability of RPC decreased as the SF content increased for the mixtures with a constant WB. At the same time, the relative flowability of RPC decreased as the WB decreased for the mixtures with constant SF content. 2. For the RPC with a certain amount of SF, the penetration resistance increased as the SF content increased for mixtures with constant WB. For a fixed SF content, the penetration resistance increased as the WB decreased. 3. Both the initial and final setting times increased with the increase in the WB, but a higher WB plays a more critical role in controlling the setting time in the RPC mixture. At a high dosage of SF content, a further increase of SF content has no significant effect on both the initial and final set times, although increasing the SF content slightly decreases setting. 4. As the WB increased from 0.18 to 0.22, the compressive and flexural strengths of the RPC mixture increased, but a further increase of WB from 0.22 to 0.26 led to a significant drop in both strengths. 5. During the early age of curing, the compressive and flexural strengths of the RPC cured at the warm temperature are greater than that of concrete cured at a normal temperature. However, after the age of 7 days, the strength development trend of RPC became reversed. 6. Increasing the SF content from 15% to 20% increased the compressive and flexural strengths of RPC. Further increasing the SF content from 20% to 25% decreased the strength. 7. For one-day air-curing condition, plain concrete has less drying shrinkage than RPC. However, for seven-day moist-curing conditions, RPC has less drying shrinkage than plain concrete, despite its higher paste content. 8. Based on the RSM analysis, the optimum WB and SF content to achieve the highest strength was determined to be WB = 0.213 and SF = 20.0%, and the corresponding compressive strength was 84.91 MPa. This combination is similar to the tested mixture design of the WB = 0.22 and SF = 20%, which had an average strength of 84.80 MPa. 9. The RPC studied in this paper generally has a tensile strength that is higher than the maximum circumferential tensile stress demand generated from air pressure in the energy storage pile. It indicates that the RPC is potentially suitable for the energy storage pile application, since the considerable high tensile stress is a major concern for the safety of the pile. In order to conclusively state all of the findings explained above and to validate the overall response model, it is necessary to conduct a series of experiments using the optimized mix proportions and select some percentage variables (WB and SF content). The measurement of the compressive and flexural strengths and drying shrinkage will be needed under different curing regimes. Such further experimental works can confirm the findings obtained from this study. Data curation, U.B. and E.S. ; Formal analysis, U.B. and E.S. ; Funding acquisition, D.Z. and J.R.K. ; Investigation, C.-S.S. and D.Z. ; Methodology, C.-S.S. and D.Z. ; Project administration, J.R.K. ; Supervision, C.-S.S. and D.Z. ; Visualization, U.B. ; Writing--original draft, U.B. ; Writing--review & editing, C.-S.S., D.Z. and J.R.K.