# Influence of Silane-Modified Coal Gangue Ceramsite on Properties of Ultra-High-Performance Concrete
## 4 Conclusions  
In this study, the application of coal gangue ceramsite in UHPC is successfully designed through an MAA model. At the same time, the influence of silane coupling agent KH560 solution on the working properties, mechanical properties, and microstructure of CGC after it was hydrophobically modified by UHPC was studied. According to the experimental results in this paper, the following conclusions can be drawn.  
1. A 5% concentration of SCA solution can make CGC obviously hydrophobic. It is found that the water absorption of aggregate decreases by 27.9% and the contact angle reaches 111.32\({}^{\circ}\). The changes of functional groups and morphology of the aggregate surface can be obviously observed by infrared analysis and scanning electron microscope.
2. The working performance of UHPC was significantly improved with the increase of modified CGC content. When the substitution rate reached 100%, the fluidity  
Figure 15: (**a**,**b**) interface morphology of modified aggregate; (**c**,**d**) adding the hydration product of the modified aggregate.  
increased by 38.51% compared with the reference group, which could effectively solve the influence of CGC super water absorption on the working performance of UHPC. However, the improvement of working performance is limited at low dosage, which shows that the effect of modified CGC on improving the fluidity of UHPC is more significant at a high substitution rate.
3. When the substitution rate of modified CGC is less than 40%, the mechanical properties of UHPC 28d are improved. When the dosage is 20%, the flexural strength and compressive strength of UHPC are improved by 16% and 7%, respectively, compared with the M0 group without modified aggregate.
4. Through XRD and TG-DTG experiments, it is found that the addition of modified aggregate is beneficial to improve the hydration degree of UHPC. The addition of hydrophobic aggregate can prevent too much free water from being absorbed by the aggregate and thus participate in the reaction of cement slurry to improve the degree of hydration. At the same time, MIP and SEM experiments show that adding 20% modified aggregate is beneficial in reducing the porosity of UHPC, optimizing the interfacial bonding performance, and improving its mechanical properties. However, when the content is further increased by more than 40%, the internal porosity of UHPC increases and the mechanical properties decrease. Although MIP and TG-DTG did not analyze the intermediate group, the trend showed that the porosity gradually increased from M20 to M100, and the interfacial bonding strength gradually decreased, which was consistent with the mechanical strength results.  
Conceptualization, S.H.; methodology, Y.Q.; software, H.Y.; validation, Y.Q., D.Z. and H.Y.; formal analysis, Y.Q.; investigation, Y.Q.; resources, S.H.; data curation, Y.Q.; writing--original draft preparation, Y.Q.; writing--review and editing, Y.Q.; visualization, D.Z.; supervision, Y.Q.; project administration, D.Z.; funding acquisition, S.H. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge funding from the Jiangsu Province Carbon Peak Carbon Neutral Science and Technology Innovation Special Funds Project (BE2022608), the Research and development of key technologies for the classification and utilization of steel slag resources, a key core technology project of Suzhou City facing the world (SZJB202505), China Construction Group Co., Ltd. Construction Waste Recycled Aggregate 3D Printing Concrete Material Research and Development and Durability Research Project (202410191).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The authors declare no conflict of interest.