Development of a sustainable hydrophobic concrete surface treatment using palm oil mill effluent (POME) and calcium carbonate

Moisture penetration in concrete structures accelerates deterioration, necessitating effective surface protection to extend service life. At the same time, untreated raw palm oil mill effluent (POME) disposal poses severe environmental challenges due to high chemical and biological oxygen demands. This study investigates the development of a sustainable hydrophobic coating by repurposing raw POME and calcium carbonate (CaCO₃) to determine its optimal mixture ratio and evaluate its water absorption capacity on concrete. Coating solutions were synthesised using varying POME volumes (25 to 100 mL) and CaCO₃ amounts (1.0 to 3.0 g) in 50 mL of ethanol solvent, then applied to mortar cubes. Surface hydrophobicity and moisture resistance were evaluated via water contact angle (WCA) and 7- and 14-day water absorption tests. The treatment successfully lowered surface energy, rendering the mortar surfaces hydrophobic. A maximum WCA of 130.6° was achieved on the mortar surface using the highest tested concentration of 100 mL POME and 3.0 g CaCO₃. However, the optimum resistance to prolonged water absorption was recorded at an intermediate formulation of 75 mL POME and 2.0 g CaCO₃, yielding minimum absorption rates of 2.0% and 2.3% at 7 and 14 days, respectively. Higher material concentrations reduced moisture resistance, potentially due to excessive coating thickness and particle agglomeration that induced microcracking after drying. The use of low-cost POME as a coating constituent also offers potential material cost savings compared with conventional synthetic waterproofing agents, while improved moisture resistance may reduce long-term maintenance requirements. Overall, the developed coating provides a potentially cost-effective and environmentally sustainable approach to concrete protection while simultaneously valorising POME waste and mitigating associated aquatic pollution risks.