## 2 Literature Review
A substantial difference must be highlighted between two mechanisms that occur in the overall process commonly known as \"stabilization\". The first phase includes the improvement of soil strength achieved in the short term; this mechanism is defined as \"modification\" and is mainly characterised by the reduction in soil plasticity. At this stage, an advantage is an improvement in workability. The stabilization mechanism implies long-term reactions and significant changes in the mechanical behaviour of the treated soil. By comparing the strength of the stabilized soil with the strength of the untreated soil, an increase in strength of 350 kPa can be linked to a process of stabilization [1; 35]. Modification/stabilization of the existing in-situ materials can be achieved in many ways, such as chemical, mechanical, thermal, and electrical methods. Selection of the stabilizing method type is a function of several factors. Among the parameters governing the choice of the stabilizing process, the most important are the type of in-situ soil (e.g., gradation and plastic index), the environment (e.g., the climate), the characteristics of the areas requiring stabilization, availability, and costs of admixture [4; 36]. The most widely used mechanical stabilization technique involves compacting the in-situ soil [35]. The compaction mechanism is based on the basic assumption that a long-lasting condition of stability of the subgrade depends on a higher level of soil compaction. A soil properly and thoroughly compacted is less permeable and less compressible, increases its density, and potentially improves its property to carry a higher load [4; 37]. Chemical stabilization involves mixing the soil with chemicals and emulsions. Traditional chemical stabilizers are cement, lime, fly ash, and bitumen. Non-traditional soil stabilization additives are typically grouped into the following categories: ionic, enzymes, lignosulfonates, salts, petroleum resins, polymers, and tree resins [38]. Chemical agents may result in a higher compaction level of the soil, contribute to dust control, water erosion control, and development of a stronger composite material [35; 39]. As summarised by [PERSON] [39], the introduction of chemicals into subgrades improves their durability, strength, and compaction level. The combination of both chemical and mechanical mechanisms is probably the most common practice of treating soils [4]. The subgrade stabilization process includes the following macro-activities: soil preparation, application of the stabilizing agent, mixing, compaction, and curing [5]. In the field of soil stabilization, the term \"lignosulfonates\" refers to one of the seven categories in which non-traditional stabilizers are typically grouped. This macro-category includes lignosulfonates, lignin, lignin sulphate, and lignin sulphides [35]. Besides the use as a stabilizing agent for road subgrades, previous studies focused on the advantages and disadvantages related to a different application of lignin-based products in the road industry. The use of lignin covers many applications to achieve the following main purposes:  
* dust suppression;
* unpaved roads stabilization;
* erosion resistance improvement (e.g., for road embankments);
* asphalt substitution as an environmentally friendly alternative to asphalt binders. The cementitious features of lignosulfonates allow the loose material to be bonded together; the reduction in particle loss translates into the inclusion of these materials in road surface dust suppression solutions [40]. Lignosulfonates are commonly applied by a sprayed-on method on the dirt road surface previously scarified to allow a quicker and more uniform penetration into the surface material [41]. Usually, the application at low rates concerns an unbound thickness of 25 mm of the top surface [6]. For lignosulfonate-treated roads, the dust could be reduced by more than 50%, according to some researchers [41].