# Influence of Silane-Modified Coal Gangue Ceramsite on Properties of Ultra-High-Performance Concrete
## 3 Results and Discussion
### Mechanical Properties  
To evaluate the impact of modified aggregate replacement rate on the mechanical properties of the UHPC, the flexural and compressive strengths of samples cured for 28 days were measured. The changes in flexural strength are illustrated in Figure 9(a), while the compressive strength variations are shown in Figure 9(b). The flexural and compressive strength of the M0 group without modified aggregate reached 20.6 MPa and 139.64 MPa.  
The strength test data diagram reveals that as the mass substitution rate of modified aggregate increased, the 28-day mechanical properties of the UHPC initially improved and then decreased. The optimal mechanical properties were achieved at a 20% substitution rate, with flexural and compressive strengths reaching 23.9 MPa and 150.1 MPa, respectively, which are 16% and 7% higher than M0. Compared to the 28-day compressive strength of 143 MPa reported by [PERSON] et al. [43] using calcined coal gauge aggregate, this study further substantiates the significant effectiveness and feasibility of silane-modified aggregates in enhancing UHPC's mechanical performance. At a 40% substitution rate, flexural and compressive strengths still improved by 6% and 4%, respectively. However, when the substitution rate reached 60%, the mechanical properties started to fall below those of M0 and continued to decrease with increasing content. When the substitution rate reached 100%, flexural and compressive strengths dropped to only 14.6 MPa and 108 MPa, representing decreases of 29% and 23%, respectively.  
Figure 10: Effect of modified aggregate content on mechanical properties of UHPC.  
Drawing on the relevant literature and in light of the experimental observations regarding mechanical properties, it can be deduced that as the reaction progresses, when the substitution rate of modified aggregate remains below 40%, two key factors contribute to the enhancement of UHPC's mechanical properties. On the one hand, the bond strength between the aggregate and the cement matrix gradually improves. The SCA not only boosts the aggregate's hydrophobicity but also reacts with hydroxyl groups in the cement paste to produce new structured compounds [22], thus reducing the interfacial porosity and improving the mechanical properties of UHPC, which is consistent with [PERSON]'s conclusion [44]. On the other hand, the hydration reaction of cement-based materials is also enhanced [45], and after silane modification, the hydrophobic film on the aggregate surface may mitigate the early-stage \"rapid consumption\" of water, preserving more for sustained hydration later. This optimizes the hydration environment in concrete, ensuring a uniform release of water, better internal humidity control, and a more stable secondary hydration process that extends the reaction time. The combined effect of these factors strengthens UHPC's mechanical properties. However, when the modified aggregate content exceeds 40%, excessive aggregates may lead to poor interfacial bonding, reducing interface contact performance [46]. At the same time, too much free water does not react completely, which leads to a decrease in strength.