# Influence of Silane-Modified Coal Gangue Ceramsite on Properties of Ultra-High-Performance Concrete
## 3 Results and Discussion
### Micro-Mechanism Analysis
#### 3.5.3 Pore Structure Analysis  
MIP was employed to analyze the pore evolution in UHPC with different ratios of modified aggregate, namely M0, M20, and M100, as illustrated in the following figure. Figure 13a depicts the variation in porosity; the pore content and pore size distribution of each classification are demonstrated in Figure 13b-d which show the changes in cumulative pore volume.  
Figure 12: Thermogravimetric analysis of different UHPC formulations.  
As evident from Figure 13a, with an increasing mass substitution rate of modified aggregate, the porosity of UHPC initially decreases and then rises. Specifically, the porosity of M0, M20, and M100 are 15.49%, 12.18%, and 18.92%, respectively. UHPC with 20% modified aggregate exhibits the lowest porosity. From a porosity perspective, 20% modified aggregate is the best mass substitution ratio in UHPC.  
Pores in concrete can be broadly categorized into four categories: gel pores (<10 nm), less harmful pores (10-50 nm), capillary pores (50-200 nm), and macropores (>200 nm) [48]. As depicted in Figure 13b,c, the gel pores in UHPC decrease as the content of modified aggregate increases, and the pore distribution in M0 and M20 is primarily concentrated in gel pores and less-harmful pores. The addition of an appropriate amount of modified aggregate also optimizes the content of less-harmful pores in M20 to some extent. However, the pore distribution in M100 is mainly centered around less-harmful pores and macropores, and macropores are the key pores that negatively impact the mechanical properties of cement-based materials. When the substitution rate of modified aggregate reaches 100%, the macroporous content in UHPC rises, resulting in the lower mechanical properties of M100. From the change in cumulative pore volume depicted in Figure 13d, it is clear that adding a suitable quantity of modified aggregate helps refine the pores and enhance the mechanical properties of UHPC, as evidenced by the left-shift of the cumulative pore volume curve for M20. However, an excessive amount of modified aggregate leads to more harmful pores, causing the cumulative pore volume curve for M100 to shift to the right, so the mechanical properties of UHPC will decrease.  
The change in pore structure reveals that the incorporation of hydrophobic-modified aggregate exerts a significant effect on the internal pore characteristics of UHPC. With the  
Figure 13: Pore size analysis of different UHPC formulations.  
overall water-binder ratio held constant, when the modified aggregate content reaches 20%, the UHPC system exhibits a more favorable pore distribution. On the one hand, the surface of the modified aggregate demonstrates an excellent combination of affinity and hydrophobicity, which enhances the interfacial bonding strength between aggregate and cement matrix [49]. On the other hand, the structure of the interface transition zone is optimized, facilitating the further hydration of cement particles [45], thus reducing the number of harmful pores in the UHPC, further refining the gel pores, and lowering the overall porosity. However, when the replacement rate of modified aggregate is increased to 100%, several issues arise. First of all, the interface bonding between the modified aggregates is relatively weak, making it prone to the formation of micro-cracks and gaps, which subsequently increases interface defects. Secondly, due to the high proportion of aggregate replacement, some areas become enriched with cement paste, resulting in a local retention of free water, which gradually evaporates during the later curing process, thus forming new pores in the material. Under the combined effect of these factors, the porosity of the UHPC increases, and the rise in macropores is also a contributing factor [47].