Using Fractal Geometry Theory to Quantify Pore Structure Evolution and Particle Morphology of Stabilized Kaolinite

Abstract

Soil improvement via cement-based stabilizers is often necessary to improve the workability and strength of problematic soils. However, understanding the underlying mechanisms of the stabilization process merits further study, particularly concerning changes in the microscale structure that affect macroscale behavior. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) are often paired to characterize the microstructure and pore networks and can be used to quantitatively describe pore structure and surface complexity. Fractal geometry (e.g., fractal dimension and lacunarity) has been shown to provide a quantitative description of structural complexity in nature. Therefore, these fractal geometry fundamentals (fractal dimension and lacunarity) were implemented in the analysis of SEM micrographs and MIP results of a single-mineral kaolinitic soil (SA-1 kaolinite) stabilized with a portland cement stabilizer (portland cement Type I/II) to better understand the evolution of the soil microstructure with curing time. Particle size distributions (PSD) were developed based on image analysis of SEM micrographs collected at curing times of 1, 7, 14, 28, and 90 days. The surface fractal dimension obtained via analysis of MIP results was used to describe changes occurring in the pore network with curing time. The formation of cementitious products was inferred from changes in the PSD as gels first formed and then fused with clay surfaces. Box-counting fractal dimensions and lacunarity showed evidence of particle restructuring with cementation. The transition pore size between intraaggregate and interaggregate pores, obtained via fractal analysis of MIP data, decreased with curing time, indicating the formation of hydration products with stabilization. Using fractal geometry to help analyze the microstructural properties of stabilized clays may lead to better insight into their engineering scale behavior. Problematic soils pose an expensive problem to engineers and are often treated with cement-based stabilizers to improve strength and decrease compressibility or the potential to deform or collapse. However, the underlying mechanism causing problematic behavior, such as low strength or shrinking and swelling, is not well understood and techniques to characterize these soils at the microscopic level are needed to better prevent the damage posed to infrastructure. The current standard of practice utilizes only qualitative measurements of the soil structure and cannot be used in models attempting to predict clay behavior. Therefore, concepts from fractal geometry were used in this study to provide a quantitative, measured value of the soil and pore surface which can be used in future models. Analysis of images at the microscale provided a quantitative measurement of the change in soil structure as stabilization reactions occurred. Moreover, the geometric parameters obtained showed strong correlations with strength values, indicating the utility of the technique for predicting engineering behavior. The results of this study show promise for adapting the box-counting procedure to other, more complex soils. Additionally, because there was a good correlation between the fractal parameters and strength, the results should be correlated with other soil parameters.