## 5 Conclusions  
The results of this study revealed distinct moisture and mechanical responses of expansive soils treated with cement and guar gum under cyclic wetting-drying (WD) conditions. Untreated soils exhibited high initial swelling (\(\sim\)14%) and shrinkage (\(-\)2% to \(-\)7%), which diminished over cycles due to structural degradation. Cement-treated soilsshowed increasing swelling (22% to 30%) with higher saturation moisture contents (up to 43%), attributed to microcracking and expansive hydration products. Despite strength gains, post-drying residual swelling plateaued at ~15%. Guar gum-treated samples had similar volumetric strains to untreated soils but demonstrated improved crack resistance and delayed saturation due to reduced water infiltration and better structural cohesion.  
Mechanically, all soils showed strain-softening at \(-\)1500 kPa suction, with cement-treated soils achieving the highest undrained shear strength (\(S_{\text{u}}\)), though degrading from 579 to 288 kPa over 15 cycles. Guar gum-treated soils exhibited lower \(S_{\text{u}}\) but retained higher toughness, reflecting superior energy absorption. At \(-\)33 kPa suction, cement again provided the highest \(S_{\text{u}}\), but guar gum-treated soils retained strength more effectively (26% reduction vs. 34% for cement) and achieved the highest toughness, highlighting their ductility. Under saturation, all soils showed strain-hardening, with \(S_{\text{u}}\) for cement and guar gum-treated samples being comparable, yet guar gum-treated soils maintained the highest toughness. These benefits were attributed to the biopolymer's pore-blocking effects and cohesive gel network.  
A preliminary cost-carbon assessment demonstrated that guar gum, despite its higher unit cost (GBP 24/m\({}^{3}\) compared to GBP 15/m\({}^{3}\) for cement), offers a markedly lower carbon footprint, with CO\({}_{2}\) emissions of only 1.6 kg/m\({}^{3}\) versus 86.4 kg/m\({}^{3}\) for cement. This substantial reduction in embodied carbon underscores the environmental advantages of guar gum as a sustainable binder. Beyond its ecological benefits, guar gum-treated soils exhibited improved ductility, superior crack resistance, and greater long-term performance under cyclic environmental loading--characteristics essential for stabilising expansive soils. These results position guar gum as a technically viable and environmentally favourable alternative to conventional cementitious stabilisers. However, further investigations, particularly extended in situ studies, are warranted to establish its field applicability and long-term durability fully. Such research should focus on the influence of varying geochemical and hydrological conditions on the biodegradation behaviour of guar gum, thereby bridging the gap between laboratory observations and real-world performance.  
Conceptualisation, M.N. and K.S.T.; methodology, M.N., S.S.A.S. and K.S.T.; formal analysis, M.N., S.S.A.S. and K.S.T.; investigation, S.S.A.S. and K.S.T.; writing--original draft preparation, M.N. and K.S.T.; writing--review and editing, M.N., S.S.A.S. and K.S.T.; visualisation, K.S.T.; supervision, M.N.; project administration, M.N.; funding acquisition, M.N. and K.S.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the PhD Vice Chancellor Scholarship for [PERSON] at the University of West London.  
Not applicable.  
Due to privacy, the data presented in this study are available upon request from the corresponding author.  
We thank the anonymous reviewers for their time and comments. The National Research Facility for Lab X-Ray CT(NXCT)at the \(\upmu\)-VIS X-ray Imaging Centre, University of Southampton, supported the X-ray computed tomography scanning through EPSRC grant EP/T02593X/1.  
The authors declare no conflicts of interest.