Polymer soil stabilization refers to the addition of polymers to improve the physical properties of soils, most often for geotechnical engineering, construction, or agricultural projects. Even at very small concentrations within soils, various polymers have been shown to increase water retention and reduce erosion, increase soil shear strength, and support soil structure. A wide range of polymers have been used to address problems ranging from the prevention of desertification to the reinforcement of roadbeds. Polymers that have been tested for soil stabilization effects include a range of synthetic polymers and biopolymers. Biopolymers in particular offer a more eco-friendly alternative to traditional chemical additives, such as ordinary cement, which may generate a large amount of carbon dioxide during production or cause lasting environmental damage. Polymers mainly affect the aggregation and strength of soils through their interactions with fine clay particles. Coatings of adsorbed polymers on clays can increase their steric stabilization by preventing clay particles from approaching each other as closely. Alternatively, polymer molecules that bond with multiple clay particles promote flocculation. Hydrogel networks can result in more indirect strengthening within soils by creating a scaffolding for soil particles. Additional strength can be imparted to polymer networks within soils through chemical cross-linking and curing.
Overview Synthetic polymers began replacing other chemical binders for soil stabilization in agriculture in the late 20th century. Compared to traditional chemical binders, polymer soil additives can achieve the same amount of strengthening at much lower concentrations – for example, mixtures of 0.5-1% of various biopolymers have strength levels that match or exceed those of 10% cement mixtures in soils. Synthetic polymers, including geopolymers, and biopolymers, have been tested for their beneficial interactions with soils. Methods for introducing polymers into soils include mixing, injecting, spraying, and grouting. Liquid polymers, sold as concentrated solutions, can be applied deep within the soil through pressure injection or applied directly to uncompacted soil.
Synthetic polymers Alumino-silicate based, synthetic geopolymers provide many of the same binding properties as Portland cement. Compared to other polymer additives, many geopolymers are quite durable, with high mechanical strength and thermal stability. They react readily with calcium hydroxide in water, which allows them to act as cementitious binders. Geopolymers offer the advantage of being more environmentally friendly and energy-efficient to produce than traditional chemical additives, and can be synthesized from waste products such as mine tailings or fly ash. When these waste products are treated with an alkaline reagent, the aluminosilicate rapidly depolymerizes and polycondenses into a rigid three dimensional polymeric structure that coats and strengthens soil pores. Geopolymers have been applied to stabilize gypseous soils because of their resistance to sulfur and other chemical attacks, which weaken traditional cement.
Biopolymers Biopolymers are synthesized as a result of biological processes, and are often less harmful to the landscape and its biota because of their natural origins. Of the three types of biopolymers, polysaccharides have proven more useful as soil binders than polynucleotides or polypeptides. Biopolymers that have been tested for use in soil stabilization include cellulose, starch, chitosan, xanthan, curdlan, and beta-glucan. Some biopolymers are sensitive to water, and wetter soils exhibit weaker biopolymer-clay cohesion. Because of this, when wetted, gel-type biopolymers form hydrogels which have decreased tensile strength but significantly higher compressive strength compared to the original soil. Protein-based biopolymers, though less common, have been used as an alternative to polysaccharides for projects requiring greater water resistance. Biopolymers may increasingly replace synthetic polymers for soil stabilization projects. They are more environmentally friendly than many other chemical soil additives, and can achieve the same amount of strengthening at much lower concentrations. Increasing use of biopolymers could offset the carbon dioxide emissions associated with cement production, which can be as high as 1.25 tons of carbon dioxide per ton of cement.
Polymer-soil chemistry Polymer treatments modify the size, shape, and cohesion of soil aggregates by changing the interactions between soil particles. Because polymer-soil interactions occur on the surfaces of soil particles, the amount of surface area in the soil (in other words, its dominant particle size) is of great importance. Polymers have only weak interactions with the large sand- and silt-sized particles of soil, while they bond directly to finer clays. Although polymers mainly interact with the clay fraction of soils, they do change the properties of sandy soils to a lesser degree. Polymer structure dictates how they will interact with clay particles. For example, block copolymers result in very different soil properties than homopolymers, as do ionic and nonionic polymers. Additionally, the mechanisms by which different polymers adsorb onto clay particle surfaces result in different soil properties and responses.
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