## 4 Conclusions  
The study effectively combined silica fume and rice husk ash in UHPC production, revealing several key findings:  
* RHA improves workability but can increase entrapped air at higher percentages (15-20%).
* Replacing 50% of SF with RHA (SF10 RHA10) enhances compressive strength beyond that of 100% SF. The compressive strength increases by 15.7% compared to the sample with only 10% SF and by 4% compared to the sample with 20% SF.
* A mixture of 10% RHA and 10% SF enhances both the workability and compressive strength of UHPC. The optimal RHA-SF ratios lead to improved flexural and tensile strengths, with tensile strength increasing by approximately 20% compared to UHPC samples containing 10% SF and about 5% compared to those with 20% SF.
* The stress-strain analysis shows increased ductility and fracture energy with 5% to 15% RHA. This suggests that RHA can effectively substitute a portion of SF.  
This study emphasizes the potential of RHA as a partial substitute for SF, optimizing mechanical properties and promoting the sustainable development of UHPC using locally sourced materials in Vietnam. To advance the research, we propose investigating the effects of RHA and SF on UHPC's microstructure, analyzing their carbon footprint and energy consumption, and assessing long-term durability through environmental exposure tests (salt, sulfate, acid). Further studies should focus on optimizing RHA-SF ratios, comparing other materials, and exploring the scalability of RHA-based UHPC in construction projects. A life cycle assessment (LCA), alongside numerical simulations and experimental testing, will provide a comprehensive evaluation of the environmental and economic benefits, ensuring the long-term effectiveness and sustainability of UHPC.  
It is important to note that this study evaluates sustainability only for UHPC mixtures with RHA in specific regions of Vietnam. Consequently, the weighting factors and evaluation criteria may need to be adjusted to reflect local conditions and requirements. To verify the generalizability and broader applicability of the conclusions and findings, further case studies should be conducted using the proposed sustainability assessment model.  
Conceptualization, T.V.N., V.B.T., B.H.L., M.Q.T., J.M., and S.N.D.; methodology, T.V.N., V.B.T., B.H.L., M.Q.T., J.M., and S.N.D.; software, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; validation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.; formal analysis, J.M. and S.N.D.; investigation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.; resources, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; data curation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; writing--original draft preparation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.;writing--review and editing, M.Q.T., J.M., and S.N.D.; visualization, T.V.N., V.B.T., B.H.L., H.T.D., and S.N.D.; supervision, J.M. and S.N.D.; project administration, T.V.N., J.M., and S.N.D.; funding acquisition, J.M. and S.N.D. All authors have read and agreed to the published version of the manuscript.  
This study received no external funding.  
Not applicable.  
The data presented in this study are available at the request of the corresponding author due to the host institution's rules.  
[PERSON] acknowledges the support by the doctoral grant reference PRT/BD/154268/2022, financed by the Portuguese Foundation for Science and Technology (FCT) under the MIT Portugal Program (2022 MPP2030-FCT).  
The authors declare no conflicts of interest.