The role of internally incorporated nano-silica in recycled aggregate concrete: Modification of transport properties

Abstract

This paper conducted an experimental investigation on the compressive strength, transport properties behavior and its microstructure of recycled aggregate concrete (RAC) internally modified with nano-SiO2 (NS). The capillary water absorption and chloride penetration resistance of RAC incorporating 0, 30%, 50% and 100% weight replacement of RCAs (namely, R0, R30, R50 and R100) with different NS dosages (0, 1%, 2% and 3% of cementitious materials) were experimentally investigated. The coupled action of NS and RCAs on the resistance to water absorption and chloride penetration into concrete were further discussed. Mercury intrusion porosimetry (MIP) was used to determine the pore structure of mortar in RAC and scanning electronic microscopy-energy dispersive spectrum (SEM-EDS) was selected to characterize microstructural properties of ITZs and mortar. The results indicate that the optimum dosage of NS was determined to be 3%. The 28-day compressive strength of R100 with 3% NS addition was nearly 1.15 time that of R100 without NS addition, and the initial sorptivity and chloride diffusion coefficient of R100 with 3% NS addition was 23.1% and 37.10% lower, respectively, than that of R100 without NS addition. This phenomenon can be attributed to filling effect, nucleation effect and pozzolanic effect of NS. The coupled action of the RCA content and NS dosages has a slightly influence on the sorptivity and chloride diffusion coefficient. The initial sorptivity of R100 with 3% NS addition was about 1.47 time as much as that of R0 without NS addition. The pore structure of specimens is refined by NS according to the obtained MIP results, and a proportion of harmful pores and less harmful pores are transformed into harmless pores. In addition, the microscale analyses suggest that NS promises an optimization of ITZ structures and compacts the mortar matrix, thus improving the mechanical and durability-related properties of RAC.