Soil Nanoparticles and their Influence on Engineering Properties of Soils

Soil is a particulate material and its constituent particles have a wide range of sizes. Nanoparticles of usually 1 to 100 nm are the smallest particles in soil environments and exist in one of the three different forms: nanoplatelets, nanowires or nanotubes, and nanodots. Due to their tiny size, soil nanoparticles usually exhibit special enhanced surface properties and hence interact more actively with other soil particles and solution. In this paper, some commonly occurring nanoparticles and their properties are briefly summarized, including specific surface area, particle morphology, nanoporosity, and surface charges. Owing to the extremely large specific surface area, surface charges, and sometimes nanoporosity, these particles, even preset at a small fraction, may significantly affect soil’s physico-chemical behavior and engineering properties. Soils containing nanoparticles with nanoscale intraparticle voids usually exhibit much higher liquid and plastic limits that usually show irreversible changes upon drying, while the presence of fibrous nanoparticles usually makes the soil more thixotropic and enhances its shear strength. To reflect the difference between nanoparticles and classical clay sized particles, a new type of soil particle “nanosol” is proposed to define soil nanoparticles. INTRODUCTION Soil is inherently a particulate material that may encompass a wide variety of particles with sizes ranging from as small as 1 nm to as large as 75 mm (i.e., the upper limit of the size of gravel particles) (Lambe and Whitman 1969; ASTM 2006). Such a wide range (i.e., more than 6 orders of magnitude) of particle sizes renders soil one of the most complicate materials to study, understand, model and use. In general, as the size of particles is reduced to the nanometer scale, they usually exhibit dramatically different or enhanced properties, compared with their counterparts at a larger size. This change in behavior is caused by two main reasons: much increased surface area and the dominance of quantum effects (TRS 2004). As a particle decreases in size, a greater percentage of atoms and molecules become exposed at the surface. Therefore, their surface properties (e.g., physical, chemical, electrical, and reactivity) become more important and even dominant, while their mass property 1 Assistant Professor, Louisiana State University, Department of Civil and Environmental Engineering, Baton Rouge, LA 70803; Email: gzhang@lsu.edu GSP 173 Advances in Measurement and Modeling of Soil Behavior Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 2 becomes much less important. A typical example is the significantly different properties exhibited by a cohesive soil consisting of dominantly clay particles and by a coarse-grained sandy soil. A nanoparticle is defined as a particle that has at least one dimension at the nanometer scale (i.e., 1 nm to 100 nm). According to the number of nanoscale dimensions, soil nanoparticles in general have three different forms: nanoplatelets (e.g., some platy clay mineral particles with one dimension at nanoscale), nanowires or nanotubes (i.e., fibrous particles with two dimensions at nanoscale), and nanodots (i.e., nanoscale at all three dimensions) (TRS 2004). In the past, nanoparticles have not been specifically distinguished from clay sized particles and thus are classified as clay size particles, probably due to the lack of advanced facilities for nano characterization and imaging. Recently, advances in nanosciences and nanotechnologies make possible the study of nanoparticles in detail. Moreover, as more and more applications of soil nanoparticles have been found in materials, increasing attention is being paid to the properties of those nanoparticles, including in the natural environments of soils. For example, smectites have been used to make an innovative class of materials – clay polymer nanocomposites that possesses significantly improved properties than pure polymers. Therefore, some studies on the novel applications of soil nanoparticles have been conducted, which require deeper understanding of soil nanoparticles and their properties. This paper first presents a general overview of some commonly (but not necessarily widely) occurring soil nanoparticles and their specific properties, such as particle morphology, specific surface area (SSA), nanoporosity, and surface charges. This is followed by a summary on the influences of these nanoparticles on the engineering properties of soils, including microstructure, index properties, consolidation behavior, and shear strength. Based on the comparison between the properties of nanoparticles and those of classical clay size particles, the paper proposes that, in addition to gravel, sand, silt, and clay, a new type of soil particles – “nanosol” should be used to describe soil nanoparticles in order to distinguish them from clay sized particles that usually show less dramatic changes in properties caused by size reduction and to understand their important role in controlling soil properties. OVERVIEW OF SOIL NANOPARTICLES AND THEIR PROPERTIES According to the definition, nanoparticles are characterized by their tiny size. Common to all nanoparticles, the tiny size results in extremely large specific surface area (SSA) (i.e., the total surface area per unit mass). In addition, soil nanoparticles are usually formed in an uncontrolled natural environment through very slow physical, chemical, or biological processes, and thus may have poorly crystalline structure with variable or permanent surface charges or both (Guven 1992). The former results from broken bonds on surfaces and its charge characteristics depend on the solution chemistry, while the latter is due to isomorphous substitution within the crystal structure and is not affected by solution chemistry. The surface charge is conveniently described by cation exchange capacity (CEC). Moreover, some nanoparticles are porous and contain intraparticle nanoscale voids. The presence of nanoporosity not only significantly increases the SSA (i.e., due to the increased internal surface area within a particle) and hence absorption capacity (e.g., water GSP 173 Advances in Measurement and Modeling of Soil Behavior Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 3 retention, organic absorption), but also reduces bulk density. Therefore, those porous nanoparticles can influence soil properties more dramatically, even present at a small fraction, sometimes as low as a few percent (Zhang et al. 2004a; 2004b). Understanding the influences of soil nanoparticles on soil’s engineering properties requires appreciation of the fundamental aspects of nanoparticles themselves. This section presents a brief overview of those commonly occurring nanoparticles in soil environments, including smectite, imogolite, halloysite, palygorskite, sepiolite, allophane, hematite and goethite. Table 1 summarizes their fundamental properties. These nanoparticles are grouped based on particle morphology into three categories: nanoplatelets, nanowires or nanotubes, and nanodots. It is worth mentioning two other special soil substances: microscopic organic matter (usually humus) and biogenic diatom. Humus may exist as either porous nanodots or nanowires, while diatom with sizes ranging from 5-20 μm usually contains nano pores (Drees et al. 1989). However, due to their complex morphology, chemical structure, and composition, they are not discussed here. Table 1. Fundamental Properties of Some Common Soil Nanoparticles