## 2 Sample Preparation  
The coal gangue used in this study originates from the Ping'an Mine in Fuxin City, Liaoning Province. This material is characterized by a ceramic red or brick-red colour after cleaning, and it possesses a loose structure. The mineral compositions and micromorphologies are depicted in Figure 1.  
X-ray fluorescence spectrometry analysis of the primary chemical composition of coarse coal-gangue aggregate reveals the complex nature of the coal-gangue composition of the Ping'an Mine in Fuxin City, which comprises a variety of inorganic substances. The inorganic components are primarily composed of minerals and water. SiO\({}_{2}\) is the predominant mineral, constituting 62.23% of the composition, followed by Al\({}_{2}\)O\({}_{3}\) at 17.51%. The composition proportion of Fe\({}_{2}\)O\({}_{3}\), CaO, MgO, TiO\({}_{2}\), K\({}_{2}\)O, and Na\({}_{2}\)O is 3.26%, 1.82%, 1.91%, 0.81%, and 3.33%, respectively. In order to gain a comprehensive understanding of the physical properties of gangue coarse aggregates, we conducted a meso-structural analysis of fractured gangue using  
Figure 1: Coarse aggregate of coal gangue and its energy spectrum. (**a**) Spontaneous combustion coal gangue from Haizhou Ping’an Mine in Fuxin; (**b**) energy spectrum of coal gangue. field emission scanning electron microscopy (MIRALMS, Czech TESCAN, Brno, Czech Republic). In this experiment, scanning electron microscopy (SEM) is employed, featuring a secondary electron image resolution of 0.9 nm at 15 kV and a backscattered electron image resolution of 2.0 nm at 30 kV. The instrument magnification ranges from 8\(\times\) to 100,000\(\times\). Figure 2 displays scanning electron microscopy images of fractured gangue samples at different resolutions. Figure 2: Scanning electron microscope pictures of crushed gangue; (**a**) 500\(\times\) resolution; (**b**) 1000\(\times\) resolution; (**c**) 2000\(\times\) resolution; (**d**) 5000\(\times\) resolution; (**e**) 10,000\(\times\) resolution; (**f**) 20,000\(\times\) resolution. Figure 2 reveals that the structures of fractured gangue particles are notably compact, featuring surfaces with concavities, convexities, and partially developed pores and cracks. Upon further magnification, local SEM images clearly exhibit rough, unstable, and fine surfaces on the fractured gangue particles. At resolutions exceeding 2000\(\times\), microcracks become more prevalent on the surfaces of these particles, exhibiting layered morphologies that enhance the specific surface area of the gangue aggregate and increase the stability of the gangue concrete structure. After the sampling process, coal-gangue coarse aggregates are manually crushed and undergo two rounds of crushing using a jaw crusher. In accordance with the guidelines set by Pebble and Macadam for Construction (GB/T14685-2011) [41], coal-gangue aggregates within the particle size range of 5-20 mm are chosen as a substitute for natural macadam. The particle size of the coarse aggregates for cylindrical specimens falls within the 5-10 mm range. The particle gradation curves for these coarse aggregates are depicted in Figure 3.  
The raw materials needed for the test are prepared in accordance with the Standard for Sand and Stone Quality and Test Methods for Ordinary Concrete (JGJ52-2006) [42] and the Code for the Mix Proportion Design of Ordinary Concrete (JGJ55-2011) [43]:  
1. The cement used is Fuxin Yingshan ordinary Portland cement; the relevant parameters are shown in Table 2;
2. The fine aggregate is river sand, the maximum particle size is less than 4.75 mm, and the fineness modulus is 2.69, the related parameters are shown in Table 3;
3. The coarse aggregate used is mechanically broken coal gangue that meets the relevant standards;
4. The superplasticizer used is model CQJ-JSS powder polycarboxylic acid superplasticizer;
5. The water used is ordinary tap water;
6. The mineral admixtures mainly include silica fume, fly ash and slag powder, and their chemical compositions and physical properties are shown in Table 4;
7.