Synthesis of high-performance multifunctional proppants from solid waste materials with an unsintered process for hydraulic fracturing

• Core-shell multifunctional proppant was synthesized with up to 75.92% solid waste. • The proppant exhibited excellent crush resistance, high roundness, sphericity, and low density. • The fabricated core exhibited excellent hydrophobicity, permeability, and compressive strength. • It effectively prevented silt intrusion while maintaining high fluid conductivity. To address the issues of poor mechanical strength and limited functionality of conventional natural proppants, a novel multifunctional proppant was prepared using walnut shell as the core material and fly ash as the modifying component. Through novolac phenolic resin (NP) impregnation and fly ash coating, a proppant with core–shell structure was developed, exhibiting improved sphericity, mechanical strength, and floatability. Characterization results showed that the apparent density and crush resistance (at 52 MPa) of the multifunctional proppant were 1.56 g/cm³ and 6.24%, respectively, meeting the industrial requirements. The water absorption rate decreased by nearly 80% compared to raw walnut shell, while the surface exhibited hydrophobic properties after curing. Core consolidation experiments demonstrated that under a sample preparation pressure of 30 MPa at 120°C, the unconfined compressive strength exceeded 29 MPa, with a permeability of 1.17 μm². Sedimentation and resuspension tests indicated that the proppant maintained long-term floating ability, owing to the synergistic effects of air retention in surface grooves, water-induced expansion of NP, and its relatively low bulk density. This work not only provides a multifunctional proppant for hydraulic fracturing but also demonstrates a high-value utilization pathway for agricultural (walnut shell) and industrial (fly ash) wastes.