Effects of Nano-SiO2 on the Pore Structure and Crack Behavior of Basalt Fiber-Reinforced Coal Gangue–Slag Geopolymer Concrete
To improve the mechanical performance and crack resistance of coal gangue–slag geopolymer concrete, basalt fiber-reinforced geopolymer concrete was prepared using calcined coal gangue powder and slag as composite precursors. The effects of nano-SiO2 dosage on strength, water absorption, and crack evolution under splitting tensile loading were investigated. Digital image correlation (DIC), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were further employed to characterize the crack evolution and microstructural changes associated with nano-SiO2 incorporation. The results show that the performance of the specimens was strongly dependent on the nano-SiO2 dosage, with the NS0.5 mixture (0.5% nano-SiO2) exhibiting the most pronounced improvement. Compared with the reference mixture, the 28 d compressive strength and splitting tensile strength of NS0.5 reached 86.80 MPa and 6.02 MPa, corresponding to increases of 17.67% and 39.97%, respectively; meanwhile, the 24 h water absorption decreased from 4.78% to 4.25%. The DIC results indicate that 0.5% nano-SiO2 delayed the localization of maximum principal strain and the penetration of the main crack, reducing the peak crack width from 0.487 mm to 0.222 mm, with a reduction of 54.4%. The microstructural results show that an appropriate nano-SiO2 dosage reduced the total porosity from 13.93% to 5.03%, shifted the pore structure from macropore-dominated to fine-pore-dominated, and decreased the proportion of coarse connected pores and crack-like pores. In contrast, the reduced enhancement observed at higher nano-SiO2 dosages may be associated with poorer nanoparticle dispersion and increased local heterogeneity, although particle agglomeration was not directly verified in this study. Overall, the improvements obtained with 0.5% nano-SiO2 are directly consistent with matrix densification and pore-structure refinement. Micro-filling, heterogeneous nucleation, and additional gel formation are proposed as plausible contributing mechanisms rather than directly verified processes.