High-Early-Strength Concrete Optimized with Hybrid Waste-Derived Nanomaterials: RSM-Based Design and Microstructural Analysis

Microstructure and performance of sustainable geopolymer concrete incorporating recycled materials activated by precursors

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

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.

Synergistic Effects of Silica and Calcium‐Based Dual Admixtures on the Flowability, Microstructure, and Strength Development in Low‐Carbon Concrete

Eco‐friendly construction practices have emerged as a cornerstone of modern development. Due to the substantial carbon emissions from cement production, there is a growing demand for supplementary cementitious materials. This study investigated the synergistic effects of two different wastes, silica‐rich rice husk ash (RHA) and calcium‐rich seashell powder (SSP), on the flowability, microstructure, strength development, and reduction in carbon emissions. RHA (0%–12.5%) and SSP (0%–12.5%) were incorporated in equal proportions, replacing cement at levels ranging from 0% to 25%. The fresh and hardened properties of the concrete were evaluated through slump, compressive strength, and split tensile strength tests. Microstructural characteristics were analyzed using scanning electron microscopy (SEM) and energy‐dispersive X‐ray spectroscopy (EDS). In addition, the environmental performance was assessed based on global warming potential (GWP) and eco‐strength efficiency (ESE). The results revealed that incorporating RHA and SSP reduced both the workability and the unit weight of the hardened concrete. Replacing 5%–10% of the cement with these ash powders initially had a negative impact on mechanical performance. However, beyond 10% replacement, gradual improvements were observed. At 25% replacement, the modified concrete achieved superior compressive strength, along with enhanced split tensile strength, modulus of elasticity, and cement efficiency. Moreover, the blended concrete demonstrated notable environmental benefits by reducing carbon emissions. Overall, the combined use of RHA and SSP offers a promising approach for sustainable waste management and eco‐efficient construction practices.

Mechanical and microstructural assessment of sustainable geopolymer concrete incorporating waste glass powder and tire fiber aggregates

This study investigates the combined effects of waste glass powder (WGP) and waste tire fiber aggregate (TFA) on the mechanical and microstructural performance of fly ash (FA)–ground granulated blast furnace slag (GGBS)-based geopolymer concrete (GPC). Previous studies generally examined WGP and TFA separately, therefore, limited information is available regarding their combined influence on geopolymer concrete. The novelty of this research lies in evaluating the synergistic effects of WGP and TFA within a unified experimental framework to enhance sustainability and mechanical performance. In this research, the optimal WGP content was determined using slump and compressive strength tests at 3%, 6%, 9%, and 12% replacement levels. Subsequently, TFA contents of 5%, 10%, 20%, and 30% were incorporated into the optimum FA–GGBS–WGP blend as partial fine aggregate replacement. An alkaline activator consisting of Na2SiO3 and NaOH at a 2:1 ratio with a 12 M concentration was used. The results showed that 9% WGP achieved optimal performance, increasing slump and improving 28-day compressive strength by 6% compared with the control mix. Incorporation of Low TFA contents (5–10%) enhanced both tensile and flexural strengths, with 10% TFA producing the highest improvements of 27% in tensile strength and 30% in flexural strength, due to improved crack-bridging behavior and fiber reinforcement. However, higher TFA contents (20–30%) substantially reduced workability and mechanical properties because of poor compaction and weak interfacial bonding. At 30% TFA, slump decreased by 48%, while compressive, tensile, and flexural strengths decreased by 58%, 31%, and 17%, respectively. SEM, XRD, and FTIR analyses provided evidence of N–A–S–H and C–A–S–H gel formation, which may have contributed to enhanced matrix integrity and microstructural development. Overall, the combined use of waste glass powder and tire fiber aggregate demonstrates strong potential for developing sustainable geopolymer concrete, where low TFA contents improve tensile and flexural performance, while also causing a moderate reduction in compressive strength.

Experimental investigation on the performance of concrete incorporating nano-silica and nano-titanium dioxide

The incorporation of nano-sized mineral additives has emerged as an effective approach to enhance the microstructure and mechanical performance of cementitious materials; however, comparative information on the individual effects of nano-silica (NS) and nano-titanium dioxide (NT) across different concrete grades remains limited. Therefore, this study investigates the influence of NS and NT as partial cement replacements on the mechanical properties of M20, M30, and M40 concrete. Concrete mixes incorporating 2%, 4%, and 6% NS and NT were evaluated for workability and mechanical properties after 7, 28, and 90 days of curing. The results indicated that both nano-additives enhanced concrete performance, with an optimum replacement level of 4%. Compared with the control concrete, the optimum NT and NS mixes improved the 90-day compressive strength by approximately 15.6% and 11.8%, respectively. The improved performance was attributed to enhanced particle packing, pore refinement, and accelerated hydration, resulting in a denser cement matrix. However, increasing the nano-additive content to 6% reduced workability and mechanical performance due to particle agglomeration, which hindered uniform dispersion and hydration. Overall, both NS and NT enhanced concrete performance, with NT exhibiting slightly greater improvements than NS at the optimum replacement level.

Multi-response optimization of mechanical, chemical durability, and microstructural performance of sustainable hybrid fiber-reinforced high-strength concrete incorporating pyrolyzed coffee grounds using RSM

The use of agricultural waste materials and hybrid fiber reinforcement in concrete offers a sustainable approach for producing high-performance construction materials with improved mechanical properties and durability. This study investigated the mechanical, chemical durability, thermal, and microstructural performance of high-strength concrete incorporating 15% pyrolyzed coffee grounds (PCG) produced at 350 °C as a partial fine-aggregate replacement. Hooked-end steel fibers and alkali-treated banana fibers were used as hybrid reinforcements, while Response Surface Methodology (RSM) based on Central Composite Design (CCD) was employed to optimize the effects of fiber dosage and steel–banana hybridization ratio. Mechanical performance was evaluated through compressive, splitting tensile, and flexural strength tests, while durability was assessed under 10% NaCl, 5% HCl, and 5% HNO 3 exposure. Microstructural characterization was conducted using SEM/EDX, XRD, and TGA/DTG analyses. The results showed that the combined incorporation of PCG and hybrid fibers enhanced concrete performance through improved crack-bridging, matrix densification, and pore refinement. The optimum mixture, containing 1.25%–1.50% total fiber dosage and a steel-to-banana fiber ratio of 80:20, achieved a compressive strength of 69.6 MPa, splitting tensile strength of 9.0 MPa, and flexural strength of 14.0 MPa. The same mixture exhibited superior chemical durability, with minimum mass losses of 2.45%, 3.90%, and 4.40% under NaCl, HCl, and HNO 3 exposure, respectively. Microstructural analyses confirmed a denser matrix, stronger fiber–matrix bonding, reduced pore connectivity, and enhanced hydration-product formation, while TGA/DTG results indicated improved thermal stability. Validation experiments closely matched model predictions, confirming the reliability of the CCD-RSM models. Overall, the synergistic use of pyrolyzed coffee grounds and hybrid steel–banana fibers produced a durable, high-strength, and environmentally sustainable concrete suitable for structural applications in aggressive environments while promoting the valorization of coffee-processing waste within a circular economy framework.