Mechanical Performance, Durability, and Microstructure of Dune-Sand Concrete Incorporating Rice Husk Ash and Dried Oil Sludge

This study evaluates sustainable dune-sand concrete produced using local dune sand, rice husk ash (RHA), and dried oil sludge. The experimental program included a control mixture (C0) and three modified mixtures (M1–M3). RHA was used as a partial replacement for Portland cement at contents of 20, 40, and 60 kg/m3, while dried oil sludge was introduced as a waste-derived additive at 10, 15, and 20 kg/m3, respectively. The total binder content, defined as Portland cement plus RHA, was maintained at 350 kg/m3, while the mixing-water content and water-to-binder ratio were kept constant at 185 kg/m3 and 0.53, respectively. The RHA was heat-treated at 800 °C for 60 min, immediately quenched in water, and subsequently dried at 105 ± 5 °C until a constant mass was achieved. Among the investigated compositions, M2, containing 310 kg/m3 Portland cement, 690 kg/m3 dune sand, 1110 kg/m3 crushed stone, 40 kg/m3 RHA, and 15 kg/m3 dried oil sludge, exhibited the most balanced overall performance. Its compressive strength reached 27.8 MPa at 7 days, 42.5 MPa at 28 days, and 47.7 MPa at 90 days, while its static modulus of elasticity was 32.7 GPa. M2 also showed the lowest water absorption (3.63%), the highest watertightness grade within the investigated series (W10), and the greatest measured freeze–thaw resistance, completing 223 cycles. Optical microscopy and X-ray diffraction observations indicated a visually more continuous matrix and lower relative intensities of several portlandite peaks in the modified concrete. For the replicate-based properties, mixture composition had a statistically significant effect (p < 0.001). Because RHA and dried oil sludge were varied simultaneously, the results represent their combined effect. M2 is therefore identified as the most balanced composition within the present limited test series, rather than as a universal optimum.