Improvement of Rock Grouts Using Eco-Friendly Lightweight Geopolymer Mortar Modified with Castor Oil-Based Rigid Polyurethane Foam

Conventional cement-based grouts used to stabilize fractured rock slopes and fill large cavities are heavy, carbon-intensive, and can impose excessive dead load on weakened rock masses. This study addresses these limitations by integrating bio-based rigid polyurethane—particularly castor oil-based polyurethane—into a slag-based geopolymer mortar, producing a lightweight, low-permeability, rock-adherent alternative grouting system. Three formulations were evaluated: unmodified geopolymer grout (GG), geopolymer grout modified with petroleum-based polyurethane (G-PUG), and geopolymer grout modified with castor oil-based polyurethane (GCO-PUG). Materials were characterized using infrared spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray spectroscopy, and tested for apparent density, permeability, unconfined compressive strength, and direct shear behavior—including at the grout–limestone interface using rock sourced from El-Mokattam plateau, Cairo. Polyurethane incorporation reduced apparent density by up to 24% (from 22.15 to 16.89 kN/m3) and permeability to as low as 2.3 × 10−8 m/s, at the cost of a substantial reduction in compressive strength (from 5564 to 139 kN/m2). The castor oil-based grout also showed improved rock adhesion, with interfacial cohesion increasing by 67% relative to its standalone state. Accordingly, unmodified GG is recommended for high-load structural applications, whereas GCO-PUG provides a lightweight, low-permeability, and strongly rock-adherent alternative for filling large cavities and stabilizing slopes under moisture-sensitive and weight-critical conditions, where reducing self-weight and limiting water ingress are more important than achieving maximum compressive strength.