## 2 Literature Review
### Lignin Utilization for Road Subgrades Stabilisation
[PERSON] and [PERSON] [66] compared two different types of soil and observed that the highest values of CBR were measured for the soil with the smaller plasticity index; therefore, they concluded that lignin application is more effective for soils with lower PI. For a low liquid limit silt (A-4/ML), [PERSON] e al. [57] observed that an increase in lignin content produced an increase in both the liquid limit and plastic limit but a reduction in the plasticity index. [PERSON] [55], in his study, investigated the performance of four different soils (A-6 and A-4 types) stabilized by a 12% bio-based energy co-product containing lignin. The results showed that the Atterberg limits were higher for all the treated soil types compared to the untreated soils. However, the change in lignin content may not always show a change in the Atterberg limits, given the dependency on soil gradation and the chemical properties of lignin [61; 66]. [PERSON] et al. [68] indicated that for expansive soils, the decrease in the plastic index is mainly attributed to aggregation between soil particles and the consequent decrease in clay size particles. Figure 1 depicts a comparison between previous works (see Table 1). Liquid limit (LL) and plasticity index (PI) are reported for the untreated soil (US) and optimum lignin-treated soil (OLTS), respectively. In the second case, values refer to lignin-stabilised soil with the optimum additive quantity defined in each study. Data on optimum lignin content is reported in Table 1. It can be noted that in most cases there is not a significant variation in both Atterberg limits between untreated and lignin-treated soil. Several research studies carried out a series of tests in order to investigate the stabilization mechanism and mechanical features at the particle level such as X-ray diffraction  
Figure 1: Comparison between Atterberg Limits. (*) AASHTO soil classification. (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and Fourier transform infrared resonance spectroscopy (FTIR). An important result showed that the addition of lignin to silty soil caused an increase in the particle size after a period of curing [57]. The stronger microstructure in the lignin stabilized soil compared to that of the untreated soil is strictly related to the cementitious feature of lignin which allows the material to occupy the interparticle pores and facilitate the bonding of soil particles [60]. Cementation is the main effect characterizing the liposomal stabilization mechanism [69]. For untreated silty soils, the high number of particles that are originally detached are bonded together; this phenomenon helps to develop a high-density structure; the structure becomes denser as a consequence of the increase in the lignin content [61]. The increase in the percentage of lignin significantly reduces the volume and the mean diameter of the pores thus the structure of the treated soil is more stable [56]. Similar results were reported by [PERSON] and [PERSON] [67] in their study on an expansive clayey soil treated by lignocellulosic. SEM images showed the formation of polymeric chains in the microstructure of the lignocellulosic soil. In a recent study, [PERSON] et al. [54] investigated the microstructure of silt sand treated with lignin. The SEM analysis carried out after a freeze-thaw cycle showed that the soil structure remained compacted at the particle level and almost all the pores were filled with cementitious material which bonded together the soil particles. In addition, the same authors in a before-after analysis observed that the mixture of lignin and silt sand does not create new chemical elements or new mineral components. [PERSON] et al. [63] investigated the moisture susceptibility of a subgrade soil classified as AASHTO A-6(8). They concluded that coproducts containing sulphur-free lignin can be used as an effective soil stabilizing agent; in fact, it has been observed that these products have excellent properties in contrasting moisture degradation.