# Influence of Silane-Modified Coal Gangue Ceramsite on Properties of Ultra-High-Performance Concrete
## 1 Introduction  
Ultra-high-performance concrete (UHPC) is the most groundbreaking cement-based engineering material to emerge over the past thirty years. Thanks to its remarkably high compressive strength (>120 MPa) and flexural strength (>15 MPa) [1], UHPC is particularly well suited for specialized projects, including long-span bridges, super high-rise buildings, and militized protective structures [2, 3]. Typically, common UHPC utilizes natural sand, river sand, or quartz sand as aggregates, resulting in a density ranging from 2500 to 2700 kg/m\({}^{3}\), which is about 10% higher than that of traditional concrete [4]. However, with the decreasing of natural sand and gravel resources and a growing demand for various lightweight functional UHPC materials, the preparation of UHPC using various artificial lightweight aggregates has emerged as a new trend in recent years [5]. Currently, the lightweight aggregates frequently employed in UHPC preparation include expanded clay, shale ceramicsite, and pumice. Prior research has demonstrated that incorporating lightweight aggregates helps lower the apparent density of UHPC [6, 7]. However, these aggregates typically exhibit high water absorption and porosity. Some studies [8] reveal that even after soaking for 50 days, lightweight aggregates can still take in a certain volume of water. When used in UHPC with a low water-cement ratio, they are prone to disrupting the water distribution and hydration reaction within the slurry, thus potentially weakening the working performance and mechanical properties of UHPC [9]. Therefore,further investigation is warranted to address the challenges posed by the low mechanical properties and high water absorption of lightweight aggregates on UHPC performance. Coal gangue is a black waste rock generated during coal mining, with the global coal industry producing about 350 million tons of inactive coal gangue annually. Over the past several decades, Europe has amassed 1.75 billion tons of coal gangue waste in landfills [10; 11]. Currently, China's coal gangue reserves have surpassed 7 billion tons and are growing at a rate of 300-350 million tons each year. There are over 2600 large-scale coal gangue piles in China, covering a total area of approximately 15,000 hectares [12]. The chemical makeup of coal gangue is similar to that of clay, being rich in carbon and sulfur and having a high ignition loss. Only within a certain temperature range can it generate a sufficient amount of molten substances with suitable viscosity and expansibility. Based on this feature, China has developed CGC through sintering and applied it to concrete. CGC boasts low apparent density, a round shape, and high mechanical properties [13; 14; 15; 16]. Meanwhile, CGC exhibits corrosion resistance, freeze-thaw resistance, and other favorable properties [17; 18], making it suitable for thermal wall insulation in a variety of buildings [18]. At present, while some studies [19] have found that the brittleness of CGC can be significantly reduced when the volume fraction of steel fiber does not exceed 1.5%, the mechanical properties of concrete mixed with CGC are usually worse than those of conventional concrete containing natural aggregate due to the porosity and low strength of CGC aggregate [20]. At the same time, research on the application of CGC in UHPC remains scarce. Only [PERSON] et al. [21] have successfully prepared UHPC with excellent performance using saturated pre-wetted CGC. Moreover, there has been no reported research on how to improve the influence of high water absorption of CGC on concrete performance. Silane coupling agent (SCA) is a type of silicone compound featuring two chemical groups within its molecule, and after hydrolysis, these groups can undergo chemical reactions with both inorganic substances and organic polymers, enabling SCA to serve as a \"bridge\" connecting organic and inorganic materials [22]. For example, [PERSON] et al.