Non-traditional additives of soil stabilization such as resins, polymers and geopolymers, acids, ions and nanomaterials (Ahmadi et al., 2020; Ghadir and Ranjbar, 2018; H. Ghasemzadeh et al., 2020; Hosseini et al., 2019; Latifi et al., 2016; Mehrpajouh et al., 2021; Mirzababaei et al., 2017) have been developed to eliminate detrimental effects of conventional ones like bituminous material, cement, gypsum, lime, fly ash and blast furnace slag on the environment (Khemissa and Mahamedi 2014; Sharma and Sivapullaiah 2016; Ghasemzadeh and Tabaiyan 2017; Baldovino et al. 2019; James 2020) including energy and natural resources consumption, high greenhouse gas emission in their production procedure, increase of soil pH after stabilization and destruction of plant cover and groundwater quality. Although these non-traditional additives had effective role in air emission control, energy conservation and environmental protection, they were not completely bio compatible and renewable. With the increasing attention to the environmental issues and in order to eliminate any stress on the environment, some environmentally friendly additives like microbial induced calcite precipitation, fungus, enzymes, and biopolymers such as lignin, natural gums, chitosan, casein and sodium caseinate (Cabalar et al. 2017; Fatehi et al. 2018; Kushwaha et al. 2018; Sharaky et al. 2018; Wen et al. 2018; Bu et al. 2019; Lim et al. 2020) have been introduced to soil stabilization. Natural gums, also called Hydrocolloids, are long chain polymers with high amounts of hydroxyl groups in their structure that cause strong affinity to water molecules and make them hydrophilic colloids or “hydrocolloids”. Considerable thickening, film forming, viscosity enhancement and adhesive properties of natural gums have made them as applicable materials in different industries such as food, tissue and medicine (Pereira et al. 2018; Mohammadinejad et al. 2019). Such properties also put them in the center of attention for soil investigations. Xanthan with microbial source, guar, taragacanth and ghatti with plant origin, sodium alginate and agar as sea weed gums are examples of natural gums' application in soil environment with the purpose of erosion reduction and mechanical behavior improvement (Ayeldeen et al. 2017; Cabalar et al. 2018; Arab et al. 2019; Cheng and Shahin 2019; Dehghan et al. 2019; Biju and Arnepalli 2020; Anandha Kumar et al. 2021). The enhanced mechanical and physical properties of the soil caused by hydrocolloids on one hand and the increasing global need for renewable and green materials on the other hand have encouraged investigators to seek new sources of hydrocolloids as soil stabilizers.
Persian gum is a novel source of hydrocolloid biopolymers categorized as plant exudates gums and obtained from branches of wild almond (Amygdalus scoparia) plants in arid and semi-arid regions of Zagros forest in Fars province, Iran (Dabestani and Yeganehzad 2019). This polysaccharide has found wide applications in various industries such as food, pharmacy and textile (Abbasi and Rahimi 2015; Chahibakhsh et al. 2019; Raeisi et al. 2019) due to its comparable effects with commonly used hydrocolloids (Amini Rastabi and Nasirpour 2017; Dabestani et al. 2018; Raoufi et al. 2019). In a recent paper, Ghasemzadeh and Modiri (Ghasemzadeh and Modiri 2020) introduced Persian gum as a novel kind of hydrocolloid soil stabilizers. The study proved comparable soil strengthening capabilities of this gum to well-known xanthan and guar gums at their optimum contents. However, improving properties of Persian gum through modification methods can be a major step forward to make it a more enhanced soil stabilizer.
Modification of polysaccharides is changing their molecular structure by strengthening bonds and interactions between polymer chains to obtain networks with more enhanced viscosity, solubility and rheological properties (Akhtar and Ding 2017). The obtained hydrogels have rigid structure that trap water within them and resist against flow and shear forces (Saha and Bhattacharya 2010). Typically, the modification methods are categorized into chemical and physical crosslinking (Patil and Jadge 2008). In physical crosslinking, physical interactions between polymer chains are formed. However, in chemical crosslinking, covalent bonds between different polymer chains are induced. Among modification methods, physical ones with less chemical contamination risk (due to the absence of crosslinking agent that makes obtained hydrogels non-toxic and biocompatible) have been preferred by many researchers (Ullah and Chen 2020). The existing methods for physical crosslinking include ionic interaction, crystallization, hydrogen bonds and hydrophobic interactions (Hu et al. 2019). Ionic crosslinking as a way of physical modification methods involves entanglement of ionic moiety with polymer chains through non-covalent interactions to obtain a more tightly bonded network. There are conducted studies on ionic crosslinking of natural gums hydrogels such as xanthan, alginate and Arabic gums that demonstrate the positive effect of ionic crosslinking on physiochemical and rheological properties of these gums (Mbah et al. 2012; Yang et al. 2013; Petri 2015). However, application of crosslinked hydrogels of natural gums in soil environment has been mostly restricted to water maintenance and purification purposes (Masoumi and Ghaemy 2014; Zonatto et al. 2017). To the best of the authors’ knowledge, the only study on ionic crosslinking of polysaccharides for mechanical improvement of soil is carried out on sodium alginate which showed the effectiveness of Ca-alginate as an ionic crosslinked polysaccharide (Wen et al. 2019). Persian gum with numerous functional groups (Dabestani and Yeganehzad 2019), can easily interact and crosslink with other materials (Mohammadi et al. 2016; Samari-Khalaj and Abbasi 2017). On the other hand, calcium chloride is a practical and inexpensive ionic crosslinker that has the ability to connect polymer chains via Ca2+ ionic moiety and form three-dimensional molecular networks (Khalesi et al. 2012; Yang et al. 2013). Therefore, it can be an appropriate candidate for crosslinking Persian gum hydrogels.
As an extension of the previous study (Ghasemzadeh and Modiri 2020), this paper is an attempt to modify Persian gum stabilizing capabilities by physical crosslinking. For this reason, the effect of crosslinked hydrogel of Persian gum using calcium chloride on low-plasticity clay, as a kind of problematic soil, has been investigated. To understand how enhanced viscosity and rheological properties of modified hydrogel of Persian gum affect stabilization goals, mechanical strength and freeze thaw durability were used as soil improvement indicators. The influential factors including PG content, crosslinker concentration, moisture content and curing time were studied. Also, some micro-scale tests including scanning electron microscopy (SEM), stereo zoom microscopy (SZM), N2-based Brunauer, Emmett, and Teller (N2-BET) test and X-ray diffraction (XRD) analysis were conducted to provide insight into underlying mechanism of soil strengthening before and after stabilization at micro level.