# Model-Based Methods to Produce Greener Metakaolin Composite Concrete
## 5 Conclusions  
This study presented an integrated strength-sustainability evaluation framework to produce greener metakaolin composite concrete. The framework consists of a strength model and a sustainability model. The input parameters are concrete mix proportions and curing temperature; and the outputs are strength, normalized CO\({}_{2}\) emissions, resource consumption, and energy consumption.  
First, based on a hydration model, the reaction extents of metakaolin and cement are determined. Moreover, a linear equation is proposed for evaluation of concrete strength development. Using the strength model, the strength development at various ages is determined. The coefficients of the strength model do not vary for different mix combinations and ages. The strength development model reflects the strength crossover between composite concrete and plain concrete, and accounts for the fact that strength starts after a threshold age, not immediately. The hydration strength model has a simple format and high evaluation accuracy.  
Second, the calculation results of the strength model are used as input parameters for sustainability evaluation models. Based on the results of the sustainability analysis, some methods for achieving greener concrete were found: extending the design ages and lowering the water/binder ratios are effective ways to reduce normalized CO\({}_{2}\) emissions, resource consumption, and energy consumption. Increasing the replacement ratio of metakaolin can lower the normalized CO\({}_{2}\) emissions and resource consumption.  
Third, when the replacement ratio of metakaolin was less than 0.15, normalized energy consumption showed reductions as the content of metakaolin increased. When the replacement ratio of metakaolin was higher than 0.15, normalized energy consumption showed slight increments as the content of metakaolin increased. This is because the energy consumption of 1 kg of superplasticizer is far higher than that of the rest of a concrete mixture. It is important to consider the energy consumption of the superplasticizer for a mixture with high metakaolin content.  
In summary, it is helpful to use the proposed integrated strength-sustainability evaluation framework in the production of greener metakaolin composite concrete.  
**Author Contributions:** Conceptualization, K.-B.P., R.-S.L.; methodology, Y.H., X.-Y.W. and R.-S.L.; validation, K.-B.P., Y.H., R.-S.L. and X.-Y.W.; writing--original draft preparation, K.-B.P. and X.-Y.W.; writing--review and editing, K.-B.P., Y.H. and X.-Y.W.; visualization, Y.H. and X.-Y.W.; supervision, K.-B.P. and X.-Y.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was supported by 2018 Research Grant (PoINT) from Kangwon National University. This study was supported by the National Research Foundation of Korea (NRF-2015R1A5A1037548, NRF-2020R1A2C4002093, and NRF-2020R1A2C1102871). This study was supported by BK21 FOUR (Fostering Outstanding Universities for Research).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available from the corresponding author upon reasonable request.  
**Conflicts of Interest:** The authors declare no conflict of interest.