## 4 Conclusions  
This study validates suggestions related to the benefits of using gamma-irradiated plastic aggregates, compared with nonirradiated plastics, to recover some of the lost strength when raw plastic aggregates are added to cementitious composites as a substitute for natural fine aggregates. Based on the detailed review of the presented literature and the test results collected in this study, the following conclusions can be derived:  
Analysis of the effect of plastic type on the cementitious composites showed that their compressive strength decreased when plastic aggregate was added, regardless of the type of plastic, and the extent of change depended on the raw plastic density, as reported. In this case, plastic aggregates are inherently smooth and hydrophobic and characterized by poor bond formation in the interfacial transition zone (ITZ) between plastic aggregates and cement paste, dissimilarly to natural aggregates; consequently, the inclusion of plastic aggregates in cementitious composites results in decreased compressive strength. However, when analyzing the effect of gamma irradiation on the compressive strength of the cement composites, the compressive strength of the cementitious composites in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen** & **Irradiation Dose** & **Compressive Strength** & **Increase Rate Compared to** \\  & **(kGy)** & **(MPa)** & **Nonirradiated Specim (\%)** \\ \hline PP0 & 0 & 19.09 & - \\ PP25 & 25 & 25.09 & 31.4 \\ PP50 & 50 & 25.69 & 34.8 \\ PP75 & 75 & 33.02 & 73.0 \\ PP100 & 100 & 32.55 & 70.5 \\ \hline PE0 & 0 & 21.31 & - \\ PE25 & 25 & 24.02 & 12.7 \\ PE50 & 50 & 25.76 & 20.9 \\ PE75 & 75 & 39.43 & 85.0 \\ PE100 & 100 & 32.67 & 53.3 \\ \hline PC0 & 0 & 22.02 & - \\ PC25 & 25 & 28.81 & 30.8 \\ PC50 & 50 & 30.3 & 37.6 \\ PC75 & 75 & 32.54 & 47.8 \\ PC100 & 100 & 30.02 & 36.3 \\ \hline ABS0 & 0 & 22.89 & - \\ ABS25 & 25 & 28.6 & 25.0 \\ ABS50 & 50 & 29.81 & 30.2 \\ ABS75 & 75 & 34.94 & 52.6 \\ ABS100 & 100 & 32.52 & 42.1 \\ \hline PET0 & 0 & 16.32 & - \\ PET25 & 25 & 19.98 & 22.4 \\ PET50 & 50 & 20.42 & 25.1 \\ PET75 & 75 & 23.62 & 44.7 \\ PET100 & 100 & 22.69 & 39.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Compressive strength for cementitious composites with irradiated plastic aggregates. creased in proportion to the increase in gamma irradiation dose up to a specific dose (75 kGy). In particular, when the PE aggregate was irradiated with 75 kGy, the compressive strength was increased by 85% compared to the nonirradiated specimen, which was the maximum compressive strength observed in this study. However, it was found that the compressive strength decreased when irradiating with the highest dose (100 kGy). Changes in strength properties of cementitious composites with gamma-irradiated plastic aggregates are justified by SEM and FT-IR analysis. The SEM analysis results show the roughness of the surface of the irradiated plastic compared to the normal plastic sample. The strength enhancement mechanism through the morphological change of the plastic aggregate according to gamma irradiation observed through SEM is the modification of the essentially smooth surface of raw plastic through gamma irradiation, and as the surface of the plastic aggregate becomes rough, it can be explained that the strength of the cementitious composite is improved by improving the adhesion in the interfacial transition zone (ITZ) between the cement pastes  
The mechanism of strength enhancement through the chemical change of plastic aggregates according to gamma irradiation observed through FT-IR can be explained in two ways. First, in the case of crosslinkable plastics such as PE aggregate, the structure of the plastic becomes dense as crosslinking occurs between adjacent molecules, and thus it has superior mechanical properties compared to raw plastic aggregates.