## 4 Results and Discussion
### Particle Breakage Evolution  
Figure 6 shows the accumulated mass proportions of the particles in different size grades and the mass change of each grading particle before and after compressive loading. Figure 6: Mass percentage change in particles in different size grades before and after compression graph. CPPAC is the change of particle percentage after compression. If n is the initial mass of the particles in a particular size grade before compression, and m is the mass of the particles in such size grade after compression, then CPPAC = (m - n)/n \(\times\) 100%. (**a**) is a graph for specimens i-1 to i-4; (**b**) is a graph for specimens i-5 to i-8. after compression tends to gradually become smooth, indicating that the particle content of each specimen becomes more uniform and continuous and that there is a better effect of particle breakage in the specimen. In the process of axial compression, when the loading stress increases, particles with a size of 16\(\sim\)20 mm are first crushed in large quantities (at the stress level of 5 MPa), and their CPPAC reaches \(-65.29\%\). During this period, particles with a size of more than 20 mm have a stronger bearing capacity than those of 16-20 mm. A large number of gangue particles with a particle size larger than 20 mm are crushed when the loading stress is equal to or greater than 10 MPa. Under the loading stress of 20 MPa, lots of particles over 20 mm are crushed, which compensates for the consumption of particles with a size of 16\(\sim\)20 mm and causes the particles with a size of 0\(\sim\)1.25 mm to have the maximal CPPAC (174.74%). Figure 6b shows that the loading velocity has a limited influence on the degree of particle breakage, but it regulates the particle size distribution of the specimen after compression to a certain degree. Holding the maximum loading stress at 20 MPa, the cumulative content curves of the specimens under different loading velocities are smooth, and the distribution tendency of the CPPAC is the same for different specimens, that is, the CPPAC is always largest for the particles with a size of 0\(\sim\)1.25 mm and smallest for particles with a size of 25-30 mm. However, as loading velocity increases, the gangue particles are subject to a higher impact effect; the contact structures among the particles are damaged and the amount of crushed particles also increases. Taking the particles of 0\(\sim\)1.25 mm as an example, when the loading velocity increases from 1 kN/s to 15 kN/s, the CPPAC grows from 175.14% to 187.47%, with an increment of 7.04%. On the contrary, regarding the particles with the size of 16-20 mm, the CPPAC declines from \(-16.16\%\) to \(-44.86\%\) and the change amplitude is 177.59%. The breakage ratio (\(B_{\text{M}}\)) is used to quantify the breakage degree of the gangue particles. With the increase in loading stress and loading velocity, the \(B_{\text{M}}\) of gangue filling materials rapidly increases at first and then stabilizes. Therefore, a better fit can be obtained when logarithmic functions are used to describe the relationships between the breakage ratio and loading stress and between the breakage ratio and loading velocity. The fitting equations are \(y=5.07\) ln(1.88\(x+0.99\)) and \(y=41.76\) ln(0.013\(x+1.52\)), respectively, as depicted in Figure 7.  
In general, the loading stress greatly affects the particle breakage, and the degree of breakage will continue to increase with an increased loading velocity when the maximum loading stress remains unchanged. Figure 7: Trend of breakage ratio (\(B_{\text{M}}\)) with main control factors: (**a**) is the trend of \(B_{\text{M}}\) with loading stress for specimens i-1 to i-4; (**b**) is the trend of \(B_{\text{M}}\) with loading velocity for specimens i-5 to i-8.