From Variable Palm Residues to Certifiable Low-Carbon Concrete: A Critical Review of POFA and Biochar Processing, Microstructure, Durability, and Performance-Based Standardisation

Palm oil fuel ash (POFA) and palm-biomass biochar have attracted interest as lower-carbon concrete constituents, yet material variability continues to impede confident engineering adoption. This critical qualitative literature review shifts attention from cement-replacement feasibility to the causal relationships among residue origin, processing, cement microstructure, exposure-dependent durability and qualification. Evidence from 2020–2025 journal research is interpreted across mineral-ash treatment, biochar pyrolysis, hydration kinetics, strength and deformation, chloride transport, carbonation, reinforcement corrosion and life-cycle performance. POFA primarily offers potentially reactive siliceous and packing functions; biochar introduces distinct porosity, moisture-management and carbon-retention functions. Neither has universally beneficial effects, and results from non-palm biochars should not be interpreted as direct validation of palm-residue formulations. The review proposes five qualification gates spanning source traceability and environmental safety, processing control, role-specific screening, exposure-based concrete verification and independently monitored pilot deployment. It also outlines an Indonesian tropical/coastal validation roadmap and recommends service-life-equivalent carbon assessment. The principal conclusion is that reproducibility, durability and transparent climate accounting—not impressive isolated strength gains—must determine whether variable palm-derived residues can become certifiable engineering constituents.