## 2 State-of-Art Review
### The Effect of Varying RFA Incorporation Ratio on Economic Impact of Concrete  
It is significant to consider the economic impact of a material if the objective is to assess its sustainability. The cost of construction materials is the second most influential factor (after functionality) in the decision-making process regarding selection of alternatives [25]. According to [PERSON] et al. [95], the eco-costs/value ratio of conventional concrete could be compared to that of other Green concrete mixes on the bases of the net present value considering that the study takes into consideration a replacement ration (N) that compares the service life of the alternatives being considered as well as the interest rate and the expected inflation rate during the time period of the forecasted cashflow [97]. A simpler way of quantifying the economic burden of concrete is to calculate its market price and compare it to the other alternatives using Equation (6) [106]. \[ \begin{array}{ll}\text{Cost of concrete}&\text{alternative per unit volume }(\text{x})\\ &=\ \sum\limits_{Y=1}^{n}\text{mass of constituent Y per unit volume}\ \star\text{ market price of }Y\text{ per unit mass}\end{array}  \tag{6}\]  
[PERSON] [102] concluded that using limestone instead of granite aggregates results in 50% cost reduction while using recycled aggregates results in 80% savings. However, similar to the findings from 2.3, the economics of replacing sand with RFA was found to be linearly proportional with the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Baseline CML Method**} & \multicolumn{3}{c}{**Cumulative Energy Demand**} \\ \cline{2-7}
**Lorry/Maximum Capacity (Tonnes)** & **ADP** & **GWP** & **ODP** & **POCP** & **AP** & **EP** & **P X10-NRe** \\ \cline{2-7}  & **kg Sb eq** & **kg CO2 eq** & **kg CFC-11 eq** & \begin{tabular}{c} **kg C2H4** \\ **eq** \\ \end{tabular} & **kg SO2 eq** & \begin{tabular}{c} **kg PA4** \\ **eq** \\ \end{tabular} & **Mg JO** \\ \hline \
\begin{tabular}{c} Articulated-lorry \\ transport/27 t \\ \end{tabular} & \(1.98\times 10^{-12}\) & \(4.98\times 10^{-5}\) & \(1.01\times 10^{-13}\) & \(1.59\times 10^{-8}\) & \(2.24\times 10^{-7}\) & \(5.14\times 10^{-8}\) & \(6.73\times 10^{-4}\) \\ \hline
\begin{tabular}{c} Lorry–transport/17.3 t \\ \end{tabular} & \(2.62\times 10^{-12}\) & \(6.57\times 10^{-5}\) & \(1.33\times 10^{-13}\) & \(2.24\times 10^{-8}\) & \(3.11\times 10^{-7}\) & \(7.20\times 10^{-8}\) & \(9.27\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Impact–assessment results to transport one kg-km (ELCD-core database V3.0). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Environmental Indicators**} & \begin{tabular}{c} **NA** \\ **(Average)** \\ \end{tabular} & **NA/km** & \begin{tabular}{c} **RFA** \\ **(Average)** \\ \end{tabular} & **RFA/km** & \begin{tabular}{c} **Equation for Y (Impact for 1 kg of Fine** \\ **Aggregates Concrete Based on \%** \\ **Incorporation OO of RFA** \\ \end{tabular} \\ \hline \begin{tabular}{c} ADP \\ \end{tabular} & \(\
\begin{tabular}{c} kg Sb eq\\ \end{tabular}\) & \(1.01\times 10^{-9}\) & \(9.47\times 10^{-12}\) & \(2.12\times 10^{-10}\) & \(1.70\times 10^{-12}\) &
\begin{tabular}{c} Y = (D2\times 1.7\times 10^{-12}-D1\times 9.47\times \\ \(10^{-12}\)) X + D1 \(\times 9.47\times 10^{-12}\) \\ Y = (D2\times 2.76\times 10^{-5}-D1\times 1.15\times 10^{-4}\) \\ X + D1 \(\times 1.15\times 10^{-14}\) \\ Y = (D1\times 1.15\times 10^{-12}\) \\ Y = (D2\times 1.78\times 10^{-12}-D1\times 7.97\times 10^{-9}\) \\ \(10^{-13}\) Y + D1 \(\times 1.77\times 10^{-9}\) \\ Y = (D2\times 8.24\times 10^{-9}-D1\times 4.59\times 10^{-4}\) \\ \(10^{-14}\)) X + D1 \(\times 1.59\times 10^{-4}\) \\ \(10^{-13}\)) X + D1 \(\times 1.28\times 10^{-6}\) \\ \(10^{-14}\)) X + D1 \(\times 1.28\times 10^{-6}\) \\ \(10^{-13}\)) X + D1 \(\times 1.28\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Predictive models of different mid-point environmental indicators for the % of incorporation X of RFA in concrete.