Abstract

Objective The approach for preparing a coal gangue-mineral powder-based geopolymer mortar using coal gangue powder and mineral powder as precursors， with water glass and NaOH as alkali activators， mitigates coal gangue accumulation and environmental pollution. However， coal gangue-mineral powder-based geopolymers have inherent defects such as high brittleness， low toughness， and susceptibility to cracking. To improve the fracture characteristics of this geopolymer mortar， nano-SiO₂ is introduced to modify basalt fiber-reinforced coal gangue-mineral powder-based geopolymer mortar （BGSG）.

Methods The effects of nano-SiO₂ on the fluidity， rheology， compressive strength， flexural strength， uniaxial tensile strength，and fracture performance of the geopolymer mortar were evaluated by adjusting the content of nano-SiO₂. A control group without nano-SiO₂ was used for comparison. The impact of different nano-SiO₂ contents （1%，2%，3%） on the performance of the geopolymer mortar， primarily composed of coal gangue powder and mineral powder， was investigated. The mechanisms by which nano-SiO₂ affected the geopolymer mortar properties were explored using energy criteria， X-ray diffraction， infrared spectroscopy， and thermogravimetric analysis.

Results and Discussion With the increase in nano-SiO₂ content， the fluidity of BGSG decreased. When the nano-SiO₂ content reached 3%， the fluidity of the geopolymer was 110 mm， which was 38. 5% lower than that of the control group （179 mm）.The thixotropic loop area of BGSG expanded with higher nano-SiO₂ content. At nano-SiO₂ contents of 1%，2%， and 3%， the loop areas were 3 140，4 080， and 5 000 Pa·s ^-1 ， respectively， showing increases of 21. 2%，57. 5%， and 93. 1% compared to the control group （2 590 Pa·s ^-1 ）.

The yield stress and plastic viscosity of the slurry increased with higher nano-SiO₂ content， with yield stress rising from 31. 5 Pa to 72. 7 Pa， and plastic viscosity from 6. 95 Pa·s to 11. 45 Pa·s. The optimal toughening effect was observed at a 3% nano-SiO₂ content. After 28 days of curing， the compressive and flexural strengths of the nano-SiO₂ modified BGSG were 22. 3 MPa and 6. 8 MPa， respectively， which increased by 29. 1% and 39. 5% compared to the control group.The P-COMD curve became fuller with increasing nano-SiO₂ content， showing noticeable improvements in the slope of the rising segment and the area under the curve. Compared to the control group， the fracture energy and ductility index of the specimen with 3% nano-SiO₂ increased by 150. 7% and 70. 1%， respectively. Additionally， the tensile performance of the specimens improved with the addition of nano-SiO₂. At a 3% nano-SiO₂ content， the ultimate tensile strength （σut） was 5. 75 MPa， and the instability toughness （K_IC^un ） was 0. 534 MPa·m^1/2 ， increasing by 36. 9% and 47. 8%， respectively， compared to the control group. Microstructural analysis revealed that the incorporation of nano-SiO2 did not change the types of hydration products in the geopolymer.

Conclusion Nano-SiO₂ enhances the geopolymer mortar performance through filling， pozzolanic， and nucleation effects. Aggregate filling and polymerization reactions improve the pore structure of the matrix and the gaps between the matrix and fibers，thereby increasing the structural density. The addition of nano-SiO₂ reduces inherent defects within the geopolymer， strengthens the bonding between the matrix and fibers， and inhibits the initiation and propagation of internal cracks in the geopolymer mortar， thereby improving the toughness and ductility of the coal gangue-mineral powder-based geopolymer mortar. Furthermore，the incorporation of nano-SiO₂ significantly enhances the tensile and fracture properties of the geopolymer.

Keywords：geopolymer； coal gangue powder； nano-silica； rheology； toughening mechanism