# Meta-Analysis of the Performance of Previous Concrete with Cement and Aggregate Replacements
## 5 Results and Discussion
### Aggregate Replacement in Previous Concrete  
#### 5.3.1 Alternative Materials  
Figure 7 shows the materials used to replace aggregates in PC production. These include recycled aggregates (RA), pumice (PU), waste glass (WG), steel slag (SS), copper slag (CS), electric arc furnace slag (EAFS), and polyethylene terephthalate bottles (RAP). RA has been investigated as an aggregate replacement in 40% of the reviewed articles. As the global human population increases, the demand for new infrastructure has augmented alongside the ongoing demolition of old structures [32]. The produced construction and demolition waste (CDW) have been stockpiled and landfilled, causing environmental pollution. With limited land space and an ever-increasing amount of CDW, it has become necessary to dispose of CDW properly. This explains the high number of publications investigating the use of RA as a replacement for NA. Nevertheless, the replacement of NA by RA in concrete decreased the mechanical properties due to its low density and high porosity, coupled with the weak transition zone between RA and cement paste [33; 50; 51; 52; 53; 54; 55]. Indeed, RA has adverse effects on most concrete strengths except impact resistance [56]. Furthermore, industrial wastes, including PU, WG, SS, CR, CS, EAFS, and RAP, have received significant attention in recent years; research has focused on means of recycling these industrial by-products rather than wastefully disposing of them [35]. When NA is replaced by an industrial waste aggregate (IWA), the resulting PC exhibits adequate compressive and flexural strength and hydraulic performance. Additionally, by using these waste materials, PC with good wear resistance can be achieved [26; 44; 46; 57; 58]. Despite the negative impact of using 100% RA on the properties of PC, the achieved strength can satisfy the minimum requirement stated by various transportation authorities. A study conducted by [PERSON] et al. revealed that PC produced with 100% RA had a compressive strength of 10.5 MPa, which was 275% higher than the minimum required for PC [10]. Similarly, another study showed that PC compressive and flexural strengths of up to 27.6 and 3.8 MPa were reported with adequate hydraulic performance [5]. The higher strength in the latter study may be due to the nature and composition of the RA. The size of the RA also influenced the strength of the PC. A study showed that 100% RA that was 19 mm in size produced a 3.0 MPa PC, while a mix incorporating 100% RA 9.5 mm in size had a strength of 4.1 MPa. An opposite trend was observed in hydraulic properties, where 19 mm RA had better performance [17; 22; 50]. Compared to NA-based PC, 100% SS incorporation resulted in similar hydraulic performance, improved mechanical performance, superior carbonation resistance, and better abrasion resistance [26]. Meanwhile, mixes made with 50% AP experienced reduced strengths, low abrasion resistance, and improved hydraulic performance owing to AP's  
Figure 7: Distribution of materials used as a replacement for aggregates in the literature. high porous nature [46]. The potential use of WG was also explored, where the obtained permeability and compressive strength of PC incorporating WG were within the minimum requirements of the Japanese Standard Association (JSA) standard, JIS A 5371. Furthermore, 60% CS replacement in PC increased the compressive, tensile, and flexural strengths by 31, 19, and 18% as compared to PC exclusively made with OPC [58]. Based on these results, it can be deduced that the performance of PC incorporating RA depends on the size and nature of the RA and the composition of the mix. Furthermore, 100% RA replacement can be viable in PC depending on its purpose and application. Similarly, IWA, including SS, WG, and CS, were found to be promising aggregate replacement materials in PC at 50 and 100% replacement levels, but for specific applications [59; 60].