# Meta-Analysis of the Performance of Previous Concrete with Cement and Aggregate Replacements
## 6 Conclusions, Lessons Learnt, and Way Forward  
This work reviewed the impact of replacing cement and aggregates with different materials on the mechanical, hydraulic, and durability properties of produced PC. A meta-analysis considering the changes in compressive strength, tensile strength, flexural strength, porosity, permeability, and abrasion resistance was employed to elucidate the feasible range of the replacement percentage and the number of materials used to replace cement and the aggregates. Based on these, the following conclusions can be drawn:  
* Most studies on PC made with cement and aggregate replacement focused on density and compressive, tensile, and flexural strengths for mechanical performance evaluation. Permeability and porosity were examined to characterize hydraulic performance. The least evaluated criterion was the durability performance, for which abrasion resistance was the most tested property. Additionally, most of the relevant studies were conducted in Asia (China and India) and the USA. * The mechanical properties were generally maintained or improved with single or binary cement replacement. Of the various materials used to replace cement, FA and SF were the most commonly investigated materials. While the replacement percentage was limited to 30% to maintain adequate performance, it was also possible to produce 100% cement-free geopolymer PC using FA or SG, with comparable properties to the cement-based counterpart. * The hydraulic performance of PC was generally maintained with single or binary cement replacement. However, improvement was recorded upon the incorporation of POFA, RF, CR, SG, PU, and SCBA. Based on the analysis, the recommended partial replacement limit in hydration-based PC was up to 40%. However, complete replacement (100%) in geopolymer PC could provide comparable results to those of the cement-based control. While limited studies examined the durability performance, i.e., abrasion resistance, of PC with cement replacement, it was found that partially replacing cement had a detrimental effect. * The most common material used for aggregate replacement was RA, while the most widely used IWA were PU, WG, SS, CR, CS, EAFS, and RAP. RA replacement gave conflicting strength results, but it increased permeability and porosity. Conversely, the replacement of NA with EAFS, SS, and CS improved the mechanical properties of PC. Nevertheless, their incorporation into PC negatively impacted hydraulic performance. In fact, the replacement percentage was recommended to be limited to 50% in order to maintain adequate mechanical and hydraulic properties. * RA-based PC had the highest dispersion in strength results due to the inert variation of these aggregates, while IWA-based PC had a variability similar to that of OPC-basedPC. Moreover, the dispersion in the results of hydraulic properties decreased with IWA replacement and increased with RA replacement. Between the two alternatives, IWA served as a better replacement for NA than RA. However, RA-based PC still exhibited adequate properties for use in low-traffic pavement applications. * The mechanical and hydraulic properties of PC made with cement and aggregate replacement (combined) were generally comparable to or better than the NA-cement-based control counterparts. In fact, it was possible to improve the performance of PC upon the combined replacement of cement and aggregates; on the other hand, inferior PC performance was noted for mixes with the individual replacement of cement or the aggregates. In addition, the combined replacement of NA and cement reduced the dispersion and variability of the results.