关键词: 胶凝砂砾石, 含泥量, 正交试验设计, 力学性能, 微观结构

Abstract:

[Objectives] This study conducts a systematic investigation into the influence of mud content on the mechanical properties and microstructure of Cemented Sand and Gravel (CSG), focusing on the low mud content range (<5%) that has not been fully addressed in previous research. The objectives include: identifying key factors affecting CSG strength through orthogonal experimental design; determining the optimal mix proportion balancing technical performance and economy; and revealing the micro-mechanism by which mud content affects CSG properties. [Methods] A four-factor (mud content, cement content, fly ash content, water-binder ratio) and four-level orthogonal experimental design (L₁₆(4⁴)) was used. Compressive strength, splitting tensile strength, and elastic modulus of CSG specimens were tested for 16 mix proportions at 7 days, 28 days, and 90 days. By graded washing of natural aggregates, the mud content was controlled at 0.39%, 1.28%, 2.05%, and 6.97%. Techniques such as X-ray diffraction (XRD), scanning electron microscope with energy dispersive spectrometer (SEM-EDS), and back scattered electron-image analysis (BSE-IA) were used to analyze hydration products, pore structure, and interface bonding characteristics. [Results] 1. Mechanical properties: Mud content was the most influential factor on compressive and splitting tensile strengths, with a significance ranking of: mud content > fly ash > cement > water-binder ratio. The optimal mix proportion—cement 60 kg/m³, fly ash 60 kg/m³, water-binder ratio 1.1, and mud content 2.05%—achieved a 28-day compressive strength of 7.68 MPa and an elastic modulus of 20.3 GPa. When the mud content increased to 6.97%, the elastic modulus decreased by 46.3% compared to the optimal group. Strength was age-dependent: compressive strength increased continuously (with an increase of >20% in each stage), while the growth rate of splitting tensile strength slowed after 28 days, stabilizing at 8%-11% of the compressive strength. 2. Microstructural Mechanism: In the low mud content (2.05%) group, the hydration process proceeded smoothly, promoting the formation of calcium silicate hydrate (C-S-H) gel, which effectively filled pores and cemented aggregates to form a dense structure. In contrast, high mud content (6.97%) caused unreacted mud powder to accumulate, which interfered with hydration and created interfacial cracks and large pores. XRD and EDS analyses further showed that excessive mud powder adsorbed free water, inhibited the secondary hydration of fly ash, and retained flaky calcium hydroxide (CH) crystals, ultimately reducing the overall integrity of the material. [Conclusions] This study innovatively fills the research gap on the influence of low mud content (<5%) on CSG performance. The proposed optimal mix proportion offers both economic and performance advantages, providing a practical solution for the direct use of natural aggregates with mud content in engineering (thus avoiding excessive washing). Microstructural evidence shows that appropriate mud content can improve material density through hydration products, while excessive mud content disrupts the hydration process and interfacial bonding between CSG components.

Key words: Cemented Sand and Gravel (CSG), mud content, orthogonal test, mechanical properties, microstructure