Effects of Nano-SiO2 on the Pore Structure and Crack Behavior of Basalt Fiber-Reinforced Coal Gangue–Slag Geopolymer Concrete

Impacts of Water Saturation on the Mechanical Behavior of Basalt/Glass Fiber-Reinforced Recycled Aggregate Concrete Under Varying Stresses: Insights from Macro and Micro Perspectives

Recycled aggregate concrete (RAC) offers an effective approach to reducing the environmental burden associated with construction and demolition waste. In this study, a fiber-reinforced RAC was developed by replacing part of the cement with fly ash and ground granulated blast-furnace slag, while glass fibers or basalt fibers were incorporated as reinforcing materials. A systematic experimental program was conducted to evaluate the mechanical behavior of the proposed concrete under different saturation conditions. The results show that the best toughness performance was achieved in the natural moisture state. In comparison, compressive and flexural strengths reached their maximum values under dry conditions, whereas splitting tensile strength peaked in the natural state. Based on the experimental data, prediction equations were established for the splitting tensile and flexural strengths by considering both saturation degree and fiber content. A stress–strain model under uniaxial compression was also developed. In addition, scanning electron microscopy (SEM) was employed to examine the fiber–matrix interface and hydration products, thereby clarifying the microstructural characteristics of the concrete at different saturation levels.

Study on Mechanical Properties and Crack Evolution of Basalt Fiber-Reinforced Desert Sand High-Strength Concrete Based on DIC

This study investigates the strength development, crack evolution and toughening mechanism of basalt fiber-reinforced desert sand high-strength concrete. An L9(33) orthogonal design was first used to optimize the reference mixture, after which basalt fibers with volume fractions of 0, 0.3%, 0.4% and 0.5% were incorporated. Mechanical testing, digital image correlation, SEM, XRD, TG and FTIR were combined to clarify the relationship among fiber dosage, crack propagation and microstructural reinforcement mechanisms. The optimized matrix mixture was obtained with a water-to-binder ratio of 0.32, a desert sand replacement ratio of 40% and a fly ash content of 20%. The incorporation of basalt fiber had little influence on the 28 d compressive strength, whereas the splitting tensile strength was markedly improved. The highest splitting tensile strength was observed in the 0.4% fiber group, reaching 5.46 MPa, which was 12.81% higher than that of the reference mixture. DIC results showed that basalt fiber reduced strain localization and limited crack opening. The 0.5% group had the lowest COD, while the 0.4% group showed a better balance among tensile strength, strain redistribution and crack-opening control. SEM observations showed fiber bridging and fiber–matrix interaction near the fracture region. Meanwhile, XRD, TG-DTG and FTIR showed no obvious changes in the main phases or functional groups, indicating that the improvement was mainly related to the physical crack-control effect of basalt fibers rather than chemical modification of the matrix. Overall, 0.4% basalt fiber was identified as the preferred dosage for the present system.

Strength and Crack Resistance Synergy and Microstructural Mechanism in Industrial By-Product Geopolymer–Fiber Stabilized Soil

Analysis of Splitting Tensile Damage Behavior and Deterioration Mechanism of Aluminate Cement-Based Ultrahigh-Performance Concrete under Simulated Fire Temperatures

Traditional portland cement (PC)-based ultrahigh-performance concrete (UHPC) is prone to spalling under fire temperatures due to its dense microstructure, accompanied by severe performance degradation, which hinders its widespread application in engineering. Therefore, this study developed an aluminate cement-based UHPC (AUHPC) with excellent high-temperature resistance and antispalling performance, and systematically investigated the effects of steel fiber type, content, and exposure temperature on the compressive and splitting tensile properties. The results indicate that steam and dry-heat curing effectively inhibit the strength retrogression caused by the transformation of CAH 10 to C 3 AH 6 . Meanwhile, with the increase in temperature, recycled brick powder (RBP) participates in the hydration reaction and forms stable C 2 ASH 8 . At 400°C, the compressive and splitting tensile strengths of AUHPC specimens containing 5% RBP reach the maximum, with the strength of RSF2 specimens increasing by 37.2% and 46.3%, respectively, compared with that at room temperature. When the temperature exceeds 600°C, the number of pores on the specimen surface decreases gradually, while the cracks increase and widen. At 800°C, the splitting tensile strength of NSF2 and RSF2 series specimens decreases by 44.6% and 7.9%, respectively, compared with that at 20°C. At 1,200°C, the holes and channels left by the complete melting of normal steel fibers (NSF) aggravate the high-temperature damage, and the strength decreases with the increase of NSF content, while refractory steel fibers (RSF) exhibit better compatibility with AUHPC matrix.

Strengthening Mechanisms and Microstructure Evolution of Magnesium Potassium Phosphate Cement Modified by Nano-Fe2O3 and Nano-SiO2

Magnesium potassium phosphate cement (MKPC) exhibits rapid setting and high early strength, but its long-term performance is limited by microstructural heterogeneity and pore structure defects. These microstructural defects can increase pore connectivity and facilitate the ingress of aggressive agents, thereby limiting the long-term durability and service reliability of MKPC-based repair and protective materials. Nanomaterials have been applied to improve MKPC performance; however, the differences between conventional nano-SiO2 (NS) and nano-Fe2O3 (NF), particularly their effects on hydration regulation and microstructure evolution, remain insufficiently understood. In this study, the effects of NF and NS incorporation on the hydration behavior, phase evolution, pore structure, and mechanical properties of MKPC were comparatively investigated. Orthogonal experiments, mechanical testing, calorimetry, XRD, FTIR, Raman mapping, SEM/EDS, MIP, and nanoindentation were employed to establish the relationship between nano-modification, microstructural evolution, and mechanical performance. The results provide a basis for selecting suitable nanomodifiers for MKPC-based materials used in rapid repair, protective applications, and other construction scenarios requiring rapid strength development and improved microstructural compactness. Compared with pure MKPC and previously reported NS-MKPC results, NF-MKPC showed higher strength development, refined pore structure, and improved micromechanical uniformity. The observed performance enhancement of NF-MKPC is consistent with accelerated early hydration, possible heterogeneous nucleation, pore refinement, and matrix densification. In comparison, NS-MKPC exhibited a different hydration and pore-evolution behavior under the investigated conditions. These findings indicate that NF and NS may regulate hydration and microstructure development differently in MKPC and provide guidance for selecting suitable nano-modifiers for high-performance phosphate cement materials. Under the investigated conditions, NF modification shows potential for MKPC applications requiring rapid strength development and improved microstructural compactness, such as rapid pavement repair, concrete surface repair, and protective coating applications.

An Experimental Study on GGBS-Fly Ash Geopolymer Concrete: Mechanical Properties and Structural Performance of Reinforced Deep Beams

The shift from Ordinary Portland Cement (OPC) to geopolymer concrete offers significant environmental advantages. However, its application to shear-critical structural elements, such as reinforced deep beams, remains insufficiently researched. This experimental study investigates the mechanical properties and structural performance of Ground Granulated Blast-furnace Slag (GGBS)-fly ash geopolymer concrete with alkaline liquid-to-geopolymer (AL/GS) ratios of 0.7, 0.8, and 0.9. The compressive strength, indirect tensile strength, and behavior of reinforced deep beams were evaluated under ambient and heat-cured conditions. The results showed that an AL/GS ratio of 0.7 yielded the highest compressive strength, 65.7 MPa at 90 days, under ambient curing. The deep beam tests revealed that a reduction in the 28-day compressive strength from 59 MPa (AL/GS = 0.7) to 42 MPa (AL/GS = 0.9) caused a dramatic 68% drop in the ultimate load-carrying capacity (from 325 to 108 kN). This pronounced sensitivity is attributed to the primary strut-and-tie mechanism, where higher compressive strength significantly enhances the efficiency of the concrete strut. The geopolymer concrete also exhibited more brittle tensile behavior compared to conventional predictive models. These findings highlight the importance of optimizing the AL/GS ratio to achieve high and uniform compressive strength for the safe structural application of GGBS-fly ash geopolymer concrete in shear-critical structural members.