## 9 Conclusions  
In the present research, the synergic effect of silica fume SF and nylon fiber NF was studied on the mechanical characteristics and durability aspects of concrete utilizing recycled concrete aggregate RCA. The following conclusions are obtained from the study. 1. RCA concrete major issue is that it has poor mechanical and durability performance could be solved by joint adding of silica fume SF and nylon fibers. Optimum mix regarding mechanical behavior and durability performance and sustainability of RCA concrete was obtained at 50% RCA, 0.5% nylon fibers, and 20% silica fume. 2. Adding 20% silica fume at cement substitute enhanced the later strength of both natural aggregate concrete and recycled concrete aggregate utilizing mixes. Silica \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mix ID** & **Cost (Rs) Per Cum** & **Cost (Rs) Per Cum** & **Cost (Rs) Per Cum** & **Cost (Rs) Per Cum** \\ \hline \multicolumn{4}{c}{Natural aggregate concrete} & \multicolumn{4}{c}{Recycled aggregate concrete} \\  & & RAC & RAC + SF & RAC + SF + NFs \\ RCA0-SF0-PC & 3000 & 2500 & 2800 & 3100 \\ RCA50-SF0-PC & 3000 & 2500 & 2800 & 3100 \\ RCA100-SF0-PC & 3000 & 2500 & 2800 & 3100 \\ RCA0-SF20-PC & 3000 & 2500 & 2800 & 3100 \\ RCA100-SF20-PC & 3000 & 2500 & 2800 & 3100 \\ RCA0-SF0-NFRC & 3000 & 2500 & 2800 & 3100 \\ RCA50-SF0-NFRC & 3000 & 2500 & 2800 & 3100 \\ RCA100-SF0-NFRC & 3000 & 2500 & 2800 & 3100 \\ RCA50-SF0-NFRC & 3000 & 2500 & 2800 & 3100 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Cost Benefit Analysis. time ascribed more to the development of durability and strength in the concrete sample having recycled concrete aggregate. 3. Silica fume reduced chloride penetration, water absorption, and recycled aggregate concrete degradation in the solution of acid. Silica fume also enhances nylon fiber reinforced concrete penetration resistance against chloride penetration and water absorption. 4. Nylon fiber improved the mechanical performance of NAC and RCA incorporating mixes up to 26%. Nylon fiber was useful for flexural and split tensile strength as compared to compressive strength. Nylon fiber increased chloride penetration and water absorption by 18% and 8% respectively but also improved the resistance against acid attack. 5. At a given dose of recycled concrete aggregate, nylon fiber reinforced concrete exhibited marginally more chloride penetration and water absorption than plain concrete but resistance against acid attack was more than plain concrete. 6. Silica fume utilizing nylon fiber reinforced concrete samples displayed more gain in mechanical strength as compared to mixes with no silica fume. 7. With the utilization of nylon fibers in concrete, nylon fibers played a significant role in concrete as crack arrestors, lowering the permeability, reducing the dispersion of cracks, and also improving the strength of concrete. 8. From cost benefit analysis, the small extra cost can be justified as in case of using fibers, RCA and silica fume, this small extra cost improved the concrete performance significantly with better strength and durability in comparison to the normal concrete. 9. Reusing waste materials such as silica fume and recycled aggregate can not only help in preserving the natural deposits of limestone, but it will also add value to the waste materials when it is reused, which promotes circular economy and also helps the environment greatly. The current research showed that the waste/recycled materials such as (silica fume, recycled aggregate) can be effectively utilized in the nylon fiber reinforced concrete, the concrete with waste materials showed improved performance but it was also cost effective, which comes to conclusion that we can help preserve our natural materials from being overly used in construction industry by using these waste/recycled materials, which will bring us one step close to achieving sustainability. Conceptualization--J.A.