The use of admixtures in soils to improve their properties has been implemented since ancient times. Diverse conventional admixtures such as lime, fly ash, and cement have been added to soils. Also, petrochemicals and bacteria are increasingly used for soil improvement and soil stabilization both mechanically and chemically. However, the conventional admixtures for soils cause significant environmental problems (e.g., CO₂ emission). Biopolymers can increasingly replace the conventional admixtures for the application of soil improvement and soil stabilization. Numerous studies have been conducted in the past decades to investigate the suitability and efficiency of various biopolymers for soil improvement. This paper focuses on the properties of the most common biopolymers (xanthan gum, gellan gum, agar, polyacrylamide, and guar gum) and gives the mechanism of biopolymer‐treated soils for soil improvements proposed by researchers.

Biopolymers are viewed as effective and eco-friendly agents in soil modification. This study focuses on the wettability analysis of polyacrylamide (PAM) solutions for soil remediation. The contact angle, surface tension, and viscosity of PAM solutions were experimentally evaluated in air- and decane-biopolymer solution systems. Furthermore, a micromodel was used to investigate the pore-scale displacement phenomena during the injection of the PAM solution in decane and or air saturated pores. The contact angle of the PAM solution linearly increases with increasing concentration in air but not in decane. The surface tension between the PAM solution and air decreases at increasing concentration. The viscosity of the PAM solution is highly dependent on the concentration of the solution, shear rate, and temperature. Low flow rate and low concentration result in a low displacement ratio level, which is defined as the volume ratio between the injected and the defended fluids in the pores. The displacement ratio is higher for PAM solutions than distilled water; however, a higher concentration does not necessarily guarantees a higher displacement ratio. Soil remediation could be conducted cost-efficiently at high flow rates but with moderate concentration levels.

Stabilization of sulfate-rich expansive subgrade soils is a persistent cause of concern for transportation infrastructure engineers and practitioners. The application of traditional calcium-based stabilizers is generally not recommended for treating such soils because of the formation of deleterious reaction products such as ettringite. Sulfate-induced heaving causes severe structural damage to pavements and accounts for enormous expenditure from routine maintenance and rehabilitation activities. A research study was undertaken to evaluate the feasibility of using a metakaolin-based geopolymer (GP) for the treatment of sulfate-rich expansive soil. Laboratory studies were conducted on natural soil and artificially sulfate-rich soils, when treated with either lime or GP, to evaluate and compare the improvements in the engineering properties, including unconfined compressive strength, swelling and shrinkage, and resilient moduli characteristics over different curing periods. Microstructural studies, such as field emission scanning electron microscopy and X-ray diffraction, were performed on treated soils to detect the formation of reaction products. The engineering studies indicate that GP treatment enhanced strength and resilient moduli while suppressing ettringite formation and the associated swell–shrink potential of the treated soils. The microstructural studies showed that GP gels contribute to the improvement of these engineering properties through the formation of a uniform geopolymer matrix. In addition, the absence of a calcium source suppressed the formation of ettringite in the GP-treated soils. Overall, the findings indicate that GPs could be used as a potential alternative to existing traditional stabilizers for treating sulfate-rich expansive soils.

The utilization of traditional calcium-based stabilizers, such as lime or cement, for the treatment of sulfate-rich expansive soil, is a cause of concern for geotechnical engineering practitioners. The major problem associated with the application of such stabilizers is that when calcium ions from the stabilizers are exposed to soluble sulfate in the presence of sufficient soil moisture, they form a highly expansive mineral ettringite. This study aims to investigate the feasibility of stabilizing sulfate-rich expansive soils using a novel eco-friendly metakaolin (MK)-based geopolymer (GP) for the long-term durability of transportation infrastructure. Laboratory studies were undertaken to understand the influence of the geopolymer composition and dosage percentage for different curing periods in stabilizing the sulfate-rich expansive soils. Engineering tests such as the linear shrinkage and unconfined compressive strength tests were performed to compare the behavior of natural, lime-stabilized, and geopolymer-stabilized soils over different curing periods. Microstructural characterization using field emission scanning electron microscopy (FESEM) with energy-dispersive x-ray spectroscopy (EDXS) was performed to compare the morphological and chemical changes between geopolymer-treated and untreated natural soil. The results indicated that the geopolymer treatment of sulfate-rich expansive soils could result in significant reductions in shrinkage and subsequently help in a greater strength gain than lime-treated high-sulfate soil.

The durability and performance of pavements and other lightweight structures constructed over expansive soils are persistent causes of concern due to moisture-induced volumetric fluctuations. Typically, traditional calcium (Ca)-based stabilizers, especially lime, effectively mitigate the problems associated with such problematic soils by reducing their swell-shrink potential and improving their strength properties. However, the treatment of sulfate-rich expansive soils using the Ca-based stabilizers results in detrimental heave due to the formation of a highly deleterious mineral, ettringite. This research study was performed to investigate the efficacy of utilizing an eco-friendly metakaolin-based geopolymer (MKG) stabilizer for treating sulfate-rich expansive soils. A comparative study was conducted by performing engineering tests such as free swell tests and unconfined compressive strength (UCS) tests on untreated, lime-treated, and geopolymer-treated soils for different curing periods. The effects of geopolymer dosage, curing period, and moisture intrusion on swelling characteristics and strength properties of treated soils were investigated. In addition, microstructural analyses were performed to study the changes in the soil after chemical treatment with geopolymer and lime. The results of engineering tests and microstructural studies indicate that geopolymer can effectively stabilize sulfate-rich expansive soils and could be used as a sustainable and eco-friendly alternative to traditional Ca-based stabilizers.

The sulfate release from solidified/stabilized fluorogypsum was measured to develop the effective diffusion coefficients (${\mathrm{D}}_{\mathrm{e}}$ ) as a parameter to assess the dissolution potential in aquatic applications. Specimens from 11 compositions consisting of 60–90% pH-adjusted fluorogypsum (pFG), 2–10% Type I/II portland cement (PC), and 0–38% Class C fly ash (FA) were exposed to solutions of $30{\mathrm{g}\mathrm{L}}^{-1}$ (saltwater), $15{\mathrm{g}\mathrm{L}}^{-1}$ (brackish water), and $0.5{\mathrm{g}\mathrm{L}}^{-1}$ (freshwater) total dissolved solids (TDS). The effects of composition and salinity, measured as the TDS, on the sulfate ${\mathrm{D}}_{\mathrm{e}}$ were determined to select the compositions for use in various aquatic environments. The results indicated that the solidified/stabilized pFG had a lower dissolution potential in saltwater and freshwater in comparison with brackish water. On the basis of the low ${\mathrm{D}}_{\mathrm{e}}$ and low critical times (the time when the diffusion out of the specimen equals the precipitation onto the specimen), a composition of 80% pFG, 10% PC, and 10% FA is recommended as an alternative to limestone in saltwater and freshwater applications.