# Model-Based Methods to Produce Greener Metakaolin Composite Concrete
## 3 The Sustainability Evaluation Model and Methods for Achieving Sustainability
### Methods for Achieving Sustainability  
As shown in the Figure 5, the steps for the analysis of the sustainability of concrete can be summarized as follows:  
First, based on the composite hydration model, the strength of metakaolin composite concrete can be determined using Equation (1). Second, based on concrete mixtures, the total CO\({}_{2}\) emissions, energy consumption, and resource consumption for 1 m\({}^{3}\) of concrete can be determined using Equation (2). Third, the sustainability factors--normalized CO\({}_{2}\) emissions, normalized energy consumption, and normalized resource consumption--can be determined using Equation (3). Finally, feasible methods for greener composite concrete can be found based on the results of parameter studies. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Items** & **CO\({}_{2}\) (kg/kg)** & **Energy (MJ/kg)** & **Resource (kg/kg)** \\ \hline Metakaolin & 0.4 & 3.48 & 1.0 \\ Cement & 0.83 & 4.727 & 1.73 \\ Sand & 0.001 & 0.022 & 1.0 \\ Coarse aggregate & 0.007 & 0.113 & 1.0 \\ Water & 0.0003 & 0.006 & 0 \\ Superplasticizer & 0.72 & 18.3 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The CO\({}_{2}\) emissions, energy consumption, and resource consumption of concrete components. Adapted with permission from ref. [15]. Copyright 2015 Elsevier. Figure 6 shows the results of sustainability for the water/binder ratio 0.65 group. From 28 to 90 days, the strength of concrete increased (Figure 6a,b). Figure 6c,d shows that as the replacement percentage of metakaolin increases, the normalized CO2 emissions and resource consumption decreased. Moreover, as the design age changed from 28 to 90 days, the normalized CO2 emissions and resource consumption decreased. Figure 6e shows the normalized energy consumption. When the replacement ratio of metakaolin was less than 0.15, normalized energy consumption showed reductions as the content of metakaolin increased. Once 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. Moreover, as shown in the Table 1, as the content of metakaolin increased, the content of superplasticizer significantly increased. Figure 6f shows the effect of superplasticizer on normalized energy consumption. The difference between accounting for and not accounting for the superplasticizer is more obvious for mixtures with high metakaolin content, which contained amounts of high superplasticizer. Moreover, if the contribution of the superplasticizer is ignored, we can find a different trend; i.e., the normalized energy consumption continuously decreases as the replacement percentage of metakaolin increases. Hence, it is important to consider the energy consumption from the superplasticizer for a mixture with high metakaolin content. Figure 7 shows the results of sustainability for the water/binder ratio 0.45 group. Generally, the fundamental trends of the water/binder ratio 0.45 group are like those of the water/binder ratio 0.65 group. As the replacement percentage of metakaolin increased, the normalized CO2 emissions and normalized resource consumption decreased, and the normalized energy consumption showed rapid reduction, followed by a slight increment. Moreover, raising the design age from 28 to 90 days is effective at lowering suitability scores, such as normalized CO2 emissions, energy consumption, and resource consumption. Figure 5: Flowchart of sustainability analysis. Figure 8 shows the influence of the water/binder ratio on sustainability. As the water/binder ratio decreased from 0.65 to 0.45, the sustainability factors, such as normalized CO\({}_{2}\) emissions, energy consumption, and resource consumption, showed reductions. This agrees with [PERSON] et al. 's study [12]. Figure 6: Sustainability analysis of the water/binder ratio 0.65 group. (**a**) strength on day 28 of water/binder ratio 0.65 group.