Partially fly ash and nano-silica incorporated recycled coarse aggregate based concrete: Constitutive model and enhancement mechanism

Abstract

The single use of nano-silica (NS) or fly ash (FA) can improve the strength of recycled aggregate concrete (RAC). However, the studies on NS-FA-modified RAC (NFRAC) are limited, and further research is required to understand the mechanical behaviors of NFRAC over different curing ages. In this study, to further improve the properties of RAC, these two supplementary cementitious materials (SCMs) were utilized in RAC together: NFRAC. The workability as well as the compressive and split tensile properties were determined by an orthogonal test along with the effects of various factors, including curing age, recycled coarse aggregate (RCA) replacement rate, and SCM content on the performance of NFRAC. NFRAC was found to have adequate mechanical properties, both in the early and later curing ages. Based on NFRAC with 100% RCA replacement rate as the main research object, the strain ductility, energy consumption, and constitutive model were further explored. Scanning electron microscopy was performed to explore the modified mechanism owing to the incorporation of NS and FA. Both the SCMs can successfully accelerate the pozzolanic reaction, thereby improving the mechanical properties. In addition, NS can also fill the micropores in RCAs and react with the CH of mortar to form C–S–H gels, thereby improving the properties of ITZ. However, NS weakens the workability that can be compensated by incorporating spherical FA. Finally, 10% FA and 2% or 3% NS are recommended as SCMs for RAC.