Performance Evolution and Microstructure Optimization of Recycled Fine Aggregate Rapid-Hardening Sulfoaluminate Cement Mortar by Nano-SiO2 Surface Modification.

In this study, rapid-hardening sulfoaluminate cement (SAC) was used as cementitious material, and recycled fine aggregates (RFAs) were surface pretreated by immersion in nano-SiO2 (NS) suspensions. NS-modified SAC recycled fine aggregate mortars were prepared at three cement-sand ratios (1:1, 1:2 and 1:3) to systematically investigate the regulatory effects of NS concentrations (0%, 1%, 2% and 3%) on macroscopic performance, hydration products and interfacial microstructure. Multi-scale characterizations, including XRD, TG-DTG, SEM-EDS and microhardness tests, were carried out. The testing results show that appropriate NS can optimize SAC hydration by heterogeneous nucleation and the pozzolanic reaction. At a cement-sand ratio of 1:1, the compressive and flexural strengths gradually increase as the NS concentration rises from 0% to 2%. Compared with the control group, the 28 d compressive and flexural strength are enhanced by 19.5% and 16.6%, respectively, the drying shrinkage decreases by 8.0%, and carbonation resistance is obviously improved. Meanwhile, the formation of AFt is promoted, amorphous C-S-H gel accumulates continuously, and the content of Ca(OH)2 is gradually consumed by the pozzolanic reaction of NS. For specimens modified with 2% NS, the maximum microhardness reaches 1326 HV, which greatly benefits the mechanical properties of mortar. However, further increasing the NS concentration to 3% triggers nanoparticle agglomeration and reduces effective reactive silica, leading to a decline in hydration products, deteriorated interfacial compactness and reduced mechanical performance. Therefore, 2% can be determined as the optimal NS concentration which can provide a theoretical basis for high-value resource recycling of recycled fine aggregates in SAC mortar.