Experimental investigation on the mechanical response of calcium alginate hydrogel-treated sand

• This research uses calcium alginate hydrogel as an eco-friendly sand stabilizer. • The optimal formulation used in gel preparation enhanced water retention capacity. • Higher calcium alginate dosage boosts soil strength and shifts to cohesion control. • The gel network showed greater strength and thermal stability than earlier studies. • SEM images show hydrogel bridges at sand contact points, keeping the pores unfilled. Biopolymer-based soil treatment (BPST) has gained considerable attention in recent years as a sustainable and eco-friendly approach in geotechnical engineering. Among various biopolymers, sodium alginate (SA) has been recognized as an effective soil stabilizer due to its unique gel-forming capability. However, experimental evaluation of the mechanical response of hydrogel-treated granular soils remains challenging due to their inherent instability. This study introduces a modified method for preparing calcium-alginate (CA) hydrogel aimed at improving the strength and stiffness of treated sands. For this purpose, a series of Unconfined Compressive Strength (UCS) and Direct Shear (DS) tests was conducted on Firoozkuh sand No. 161 stabilized with hydrogel, supported by Scanning Electron Microscopy (SEM) analyses. The results showed that increasing the SA concentration from 0.4% to 0.8% significantly enhanced the UCS across all moisture levels, with the maximum value occurring after 7 days of curing. Freeze-thaw cycles up to three improved the UCS, whereas additional cycles and heat treatment at 100°C led to gel network degradation. Moreover, with increasing polymer concentration, the shear strength of the treated samples increased by more than 224% relative to clean sand, primarily due to increased interparticle cohesion, whereas the internal friction angle showed a decreasing trend. SEM observations revealed the formation of hydrogel bridges at particle contacts, without complete pore filling, indicating that strength increased while maintaining permeability. Overall, the findings indicate that CA hydrogel stabilization can serve as a promising bio-based approach for sustainable ground improvement.