Polymer soil stabilization

[1][5] 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.

[1][5] Synthetic polymers began replacing other chemical binders for soil stabilization in agriculture in the late 20th century.

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.

[7] 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.

[8] Geopolymers have been applied to stabilize gypseous soils because of their resistance to sulfur and other chemical attacks, which weaken traditional cement.

Biopolymers that have been tested for use in soil stabilization include cellulose, starch, chitosan, xanthan, curdlan, and beta-glucan.

[5] Polymers have only weak interactions with the large sand- and silt-sized particles of soil, while they bond directly to finer clays.

For example, block copolymers result in very different soil properties than homopolymers, as do ionic and nonionic polymers.

[2] Polymers on the surfaces of the colloidal fraction of soils promote steric stabilization of those particles by preventing them from approaching each other and aggregating.

This is unfavorable because it increases the Gibbs free energy of the system, and it will be more energetically favorable for the colloid particles to remain farther apart.

Such interactions are called bridging flocculation because a single polymer chain is linked to multiple soil particles.

In one study, PAM was found to increase the size of kaolinite flocs in suspension experiments from 10 μm to several millimeters.

[1] When ionized polymers (such as many biopolymers) with the same charge as clay particles adsorb to their surface, they increase the electrical double layer repulsion.

[5] Soil characteristics that have been altered by addition of polymers include compressive strength, volume stability, hydraulic durability, and conductivity.

[1] This is particularly applicable in arid regions like deserts where droughts leave soils susceptible to high rates of erosion during precipitation events.

Polymers stabilize soils through interactions with soil particles. Above is a schematic representing different configurations which polymer molecules (such as alkylammonium cations ) can adopt when adsorbed between clay layers, in response to the magnitude of charge density on the clay surface. Greater charge densities here result in righter packing and greater clay- clay distances.
The synthesis of chitosan, an example of a biopolymer which has been employed as a soil additive for its stabilizing properties.
The morphology of polymer-clay aggregates is dictated by the entropic and enthalpic contributions to the energy of their interactions. Three possible morphologies depicted above include (1) intercalated, in which polymer molecules alternate with clay layers, (2) flocculated, in which the alternating clay/polymer layers begin to aggregate, and (3) exfoliated, in which a polymer matrix supports individual, separated clay layers.
The structure of montmorillonite clay, a 2:1 clay made up of tetrahedral sheets surrounding an octahedral sheet. The clay particles are only weakly bound to each other, so various cations or polymers can be bound within the interlayer space.
The structure of polyacrylamide (PAM), a common synthetic polymer flocculating agent used to increase aggregate sizes in clay-rich soils.