Correlation of Critical State Strength Properties with Particle Shape and Surface Fractal Dimension of Clinker Ash

Clinker ash is a byproduct obtained from coal-fired electrical generators. It is a type of recycled granular material with a complex particle shape and large surface roughness. Particle shape descriptors and surface fractal dimensions are effective parameters to quantify sand grain morphologies. A simple image analysis approach is employed to determine the aspect ratio, sphericity, roundness, and convexity of 12 types of clinker ash collected from thermal power plants distributed on the western island of Japan. Compared to the majority of sands with other mineralogical components, clinker ash particles have a smaller aspect ratio, sphericity, and roundness. Under similar test conditions, clinker ash exhibits higher shear resistance than natural sands at the critical state. Critical state friction angles tend to decrease with increasing aspect ratio, sphericity, roundness, and convexity. A decreased aspect ratio, roundness, and convexity enhance the slope and intercept of critical state lines in the void ratio and mean stress plane. The mean regularity, which is the average of four particle shape descriptors, is regarded as an effective index to characterize the shear strength of clinker ash at the critical state. The surface fractal dimension could also be decided from digital image analysis. The results also show that the critical state friction angle is well correlated with the surface fractal dimension for different types of clinker ash. The implication is that a rougher surface texture hinders the rotation, frustration, and interlocking of particles at the critical state, resulting in higher shear resistance.