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Scale effects on the mechanical behavior of rock joints have been extensively studied in rocks and rock-like materials. However, limited attention has been paid to understanding scale effects on the shear strength of rock joints in relation to normal stress σn applied to rock samples under direct shear tests. In this research, a two-dimensional particle flow code (PFC2D) is adopted to build a synthetic sandstone rock model with a standard joint roughness coefficient (JRC) profile. The manufactured rock model, which is adjusted by the experiment data and tested by the empirical Barton’s shear strength criterion, is then used to research scale effects on the shear strength of rock joints caused by normal stresses. It is found that the failure type can be affected by JRC and σn. Therefore, a scale effect index (SEI) that is equal to JRC plus two times σn (MPa) is proposed to identify the types of shear failure. Overall, shearing off asperities is the main failure mechanism for rock samples with SEI > 14, which leads to negative scale effects. It is also found that the degree of scale effects on the shear strength of rock joints is more obvious at low normal stress conditions, where σn < 2 MPa.

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Extensive research has demonstrated the advantageous utilization of medium–low temperature fast pyrolysis (FP) for biomass, yielding high–grade liquid–phase chemicals or fuels. However, the field of FP–based high–performance solid biochar research still presents several gaps. Herein, a one–step versus two–step method for biomass H3PO4 activation under FP was comparatively analyzed for the first time, and efficiently activated carbons (ACs) for dye removal were successfully synthesized at a low temperature (723 K). Investigation of methylene blue (MB) adsorption revealed that the one–step sample P–H–0.5, possessing a specific surface area of 1004 m2·g−1, exhibited a remarkable adsorption capacity of 695.54 mg·g−1 with an ultra–high removal rate (99.94%, C0 = 150 mg·L−1). The two–step sample P–2–H–2, a modified byproduct of FP, achieved efficient dye adsorption in the shortest time (2 min, 383.91 mg·g−1). This originated from the well–developed surface macropores and elevated group content derived from phosphorus (P)—modification. Both adsorption data were well–fitted with pseudo–second–order kinetics and the Langmuir model, revealing the presence of chemical effects and the dominance of monolayer adsorption. A more detailed kinetic study suggested intrapore transport primarily governed the adsorption process on P–H–0.5, whereas P–2–H–2 relied on surface diffusion. FTIR and XPS revealed notable differences in the active sites between the two methods. Aside from –OH, –COOH with C–O–P, the P elements of P–H–0.5 were classified as C–P–O3 and C2–O–P2, demonstrating the ability of one–step FP to introduce heteroatoms into carbon defects. The basic interactions of ACs with MB were π–π stacking and hydrogen bonding established by –OH–containing groups. At a suitable pH (>5), most H+ was removed from the surface, and the electrostatic attraction became the strongest linking force. Both ACs exhibited exceptional reusability, with removal rates surpassing 90% of the initial rate after four cycles of regeneration.