Mechanical properties and water absorption behavior of silane-hydrophobized fly ash used as a hydrophobic barrier in fly ash subgrades

Hydrophobic barrier structures are an effective measure for reducing groundwater and rainwater infiltration and maintaining the stability of subgrades filled with untreated fly ash. At present, studies on the application of hydrophobic fly ash in subgrades remain limited, and the effects of silane modifiers on the mechanical properties and hydrophobic performance of fly ash have not been fully clarified. In this study, octyltriethoxysilane (OTES) was used as the hydrophobic modifier, and fly ash was employed as the filling material for the hydrophobic barrier structure. The effects of OTES content on the water contact angle, mechanical properties, and liquid absorption behavior of fly ash were systematically investigated. The capillary water absorption capacity of fly ash was evaluated through water absorption tests. Since isopropanol has a lower surface tension and can more readily wet hydrophobic surfaces, isopropanol absorption tests were introduced as a comparative test to further examine the influence of surface hydrophobicity on capillary suction. The results showed that the water contact angle first increased and then decreased with increasing OTES content. OTES had a more significant influence on the early strength of fly ash, while its effect on long-term mechanical strength was relatively limited. At an OTES content of 1%, the loss of mechanical properties was the lowest, and the cohesion slightly increased. This may be attributed to the formation of a siloxane network within the specimen at 1% OTES content, which produced a bridging effect. Under water immersion conditions, the mechanical strength loss rates of the 1% OTES-modified fly ash were all below 5%, confirming that silane modification provided good stability and weakened the adverse effect of water immersion on the structure. The 1% OTES content significantly reduced the water absorption capacity of fly ash while increasing the absorption of isopropanol, indicating that the capillary pore walls in the fly ash specimen exhibited better hydrophobicity at this dosage. With further increases in OTES content, excessive OTES tended to undergo polycondensation, which reduced the effective utilization of OTES and instead led to an increase in water absorption. Overall, fly ash modified with 1% OTES exhibited favorable mechanical properties and hydrophobic performance, which is beneficial for reducing water intrusion and can be applied in waterproofing and water-separation systems for fly ash subgrades.