Influence mechanism of Nano-SiO2 on geopolymer recycled concrete: Change mechanism of the microstructure and the anti-carbonation mechanism

To improve the durability of geopolymer recycled aggregate concrete based on waste concrete powder (WCP), and realize its application and promotion, this study utilized WCP, fly ash (FA), and ground granulated blast-furnace slag (GGBS) as raw materials, and recycled concrete aggregate (RCA) as aggregate, with Nano-SiO2 as the modifying material to prepare WCP-based geopolymer recycled concrete (WCPGRC). The carbonation depth, uniaxial compressive strength (UCS), and dynamic elastic modulus (DEM) of WCPGRC without Nano-SiO2 (P-WCPGRC) and with Nano-SiO2 (NS-WCPGRC) were investigated. The phase composition, microstructure morphology, and pore distribution characteristics of the two WCPGRCs were investigated using XRD, SEM, and MIP tests. The mechanism of influence of Nano-SiO2 on the carbonation effect and microstructure of WCPGRC was revealed. The findings demonstrate that: (1) The carbonation depth of P-WCPGRC was higher than that of NS-WCPGRC, and both increased linearly with the carbonation times. (2) Both UCS and DEM of P-WCPGRC were lower than those of NS-WCPGRC, and both showed a two-phase change, with the inflection point from a rapid to a slow increase in UCS and DEM occurred on day 7 for P-WCPGRC, while the inflection points for NS-WCPGRC occurred on the day 14. (3) The addition of Nano-SiO2 generated more geopolymer gels, changed the microstructure, reduced the porosity, changed the pore size distribution, and enhanced the physical and mechanical properties and carbonation resistance of WCPGRC. It is feasible to improve the durability of WCPGRC by adding Nano-SiO2.