The impact of colloidal nano silica (CNS) and polypropylene fibers (PP) on the properties of recycled concrete aggregate self-compacting concrete (SCC)

The findings of an experimental examination of the rheological, mechanical, and durability characteristics of 15 self-compacting concrete (SCC) mixes containing recycled concrete aggregates (RCA), colloidal nanosilica (CNS), and polypropylene fibers (PP) are presented in this paper. Fine recycled aggregate (FRA) was substituted with fine natural aggregate (FNA) at 25 %, 50 %, 75 %, and 100 %, while coarse recycled aggregate (CRA) was partially substituted with coarse natural aggregate (CNA) at 20 %. The effect of 3.8 % CNS as a partial replacement of cement and 0.91 kg/m3 PP on SCC performance was investigated. Marble waste powder (MWP) was also utilized as filler to improve the properties of the SCC. In all concrete mixes, a constant water/binder ratio of 0.38 was used. It was found that using more than 50 % FRA significantly reduces the workability, strength and durability of the concrete. Regarding the use of CNS and its pozzolanic effect, it was observed that it enhances the strength and durability of SCC with RCA by offsetting the decline in its properties relative to control SCC. In this way, it improved the interfacial transition zone (ITZ) in samples with RCA, leading to a 20 % increase in strength and a 40 % decrease in water absorption. PP increased the tensile strength and ductility of SCC by inhibiting cracks through the bridging effect. The synergistic effects of CNS and PP improve the microstructure, mechanical properties, and durability of SCC with RCA by enhancing density, hydration, and bonding between aggregates and cement paste, making SCC with RCA a sustainable alternative to traditional concrete without compromising performance. • The use of RCA, CNS, and PP reduced the workability of SCC. • Increased RCA substitution lowered concrete strength and durability further. • The CNS pozzolanic reaction formed C-S-H gel, enhancing concrete's strength and durability despite RCA-related deficiencies. • The PP bridging effect enhanced concrete's tensile, flexural, and durability weaknesses caused by RCA use. • Best results achieved with simultaneous use of CNS and PP.