Optimization of the mechanical properties of concrete using graphite tailings, steel fibers, and nano-silica based on RSM-BBD

Abstract

As modern civil engineering places increasing demands on concrete materials for high performance and environmental sustainability, the limitations of ordinary concrete in terms of resource consumption and performance enhancement have become increasingly apparent. Currently, the mix design of composite systems incorporating graphite tailings (GT), nanosilica (NS), and steel fibers relies heavily on empirical methods, lacks systematic quantitative optimization, and the mechanisms of synergy among these factors remain unclear, thereby limiting the engineering application of modified eco-concrete. This study employs a Box-Behnken design to systematically investigate the effects of GT, NS, and steel fibers on the 28-day compressive, split tensile, and flexural strengths of concrete. Combined with scanning electron microscopy (SEM) characterization to reveal the microstructural mechanisms, the study verifies the optimal mix proportions through model optimization. The results indicate that the effects of all three factors on the mechanical properties of concrete follow a quadratic nonlinear pattern. The strength of the main effects varies: for compressive strength, NS > steel fibers > GT; for split tensile and flexural strengths, steel fibers > NS > GT. Among these, steel fibers were the core dominant factor in enhancing the tensile and flexural properties of concrete (F-values of 1285.31 and 410.88, respectively). At the same time, NS was the dominant factor in improving compressive strength (F = 447.43), and the optimal replacement rate for graphite tailings was approximately 20%. Interaction analysis revealed significant synergistic effects between GT and NS for compressive strength, between NS and steel fibers for split tensile strength, and between GT and NS as well as NS and steel fibers for flexural strength (interaction terms P < 0.05). The comprehensive optimal mix ratio obtained through response surface model optimization was GT 21.79%, NS 1.48%, and steel fibers 1.49%. The measured 28-day compressive, split tensile, and flexural strengths reached 58.43 MPa, 6.74 MPa, and 10.82 MPa, respectively. Compared to the reference group, these values increased by 38.43%, 39.54%, and 44.65%, respectively, with the relative errors between the measured values and the model predictions all controlled within 5%. SEM characterization revealed that the cement matrix in the GNS4 and GNS18 groups exhibited significantly higher densification than the reference group. The transition zone at the interface between the steel fibers and the matrix exhibited tight bonding, providing reliable mechanical interlocking and chemical bonding that effectively suppressed crack initiation and propagation.