## 3 Results and Discussion
### Compressive Strength Analysis  
The compressive strength of cementitious composites is considered to be the major parameter determining their suitability for application in the construction field. The purpose of this section is to summarize the compressive strength properties of the cement composites produced according to the main parameters, such as the type of plastic and the gamma irradiation dose. The compressive strength properties at three curing periods of 3, 7, and 28 days were analyzed for cementitious composites containing five types of nonirradiated plastic aggregates in order to first select suitable plastics as aggregates for cementitious composites before using gamma-irradiated plastic aggregates. In Figure 20, it can be seen that the compressive strength values varied from 16.3 to 22.9 MPa for cementitious composites with nonirradiated plastic aggregates; these were lower than the value of 42.7 MPa obtained for the control specimen, and it was discovered that the use of plastic aggregates as a fine aggregate replacement reduced the compressive strength of the cementitious composites. The compressive strength of cementitious composites with plastic aggregates depends on the density of the raw plastics. To examine this in detail, the density-stress relationship in raw plastics for each type of plastic was compared, and the results are shown in Figure 21.  
Figure 19: Variation in density and unit weight according to gamma irradiation. Figure 20: Compressive strength of cementitious composites according to plastic type. Since plastic aggregates have different densities, it is necessary to check whether the compressive stress is linearly improved in proportion to the density of the raw plastics. As shown in Figure 21, all plastic aggregates except for PET showed better stress efficiency compared to density than fine aggregates. Among them, PE and ABS showed the best efficiency of compressive stress compared to density because they were the farthest from the trend line formed based on natural sand. Given that the main aim of this study was to reduce the weight of cementitious composites and ensure a certain level of compressive strength, we hypothesized that the use of PE and ABS as optimal aggregates would be the most effective in improving the compressive strength of the cementitious composites. In order to prevent the reduction in compressive strength while using plastic aggregates as a substitute for fine aggregates, the plastic aggregates were modified using gamma irradiation, and it was necessary to analyze the compressive behavior characteristics of the cementitious composite to which they were applied. Table 7 shows the results of measuring the compressive strength of cementitious composites. As shown in Figure 22, the compressive strength increased linearly and gently as the gamma irradiation dose increased. In particular, when the PE aggregate irradiated with a 75 kGy dose was used, the compressive strength increased by 85% compared to the nonirradiated specimen, and the strength enhancement efficiency by gamma irradiation was the best of all the tested conditions. Figure 21: Density–stress relationship in cementitious composites by plastic aggregate type. Figure 22: Compressive strength of cementitious composites according to irradiation dose. Such compressive strength results can be attributed to the effects of factors such as the bond strength between the aggregate and cement paste being enhanced by the physical and chemical changes of the plastic aggregate caused by gamma irradiation, as presented in Sections 3.1 and 3.2. However, when irradiated with a certain dose (100 kGy) or more, the energy supplied by the high dose further deteriorates the surface of the plastic aggregate, and some chain scission occurs, causing the partial destruction of the plastic surface particles, resulting in decomposition; consequently, the strength decreases.