Optimization via Response Surface Methodology of Palm Kernel Shell Biochar for Cementitious Replacement: A Comprehensive Review

Abstract

The cement industry's high CO₂ footprint (~8% of global emissions) has led to intensive research on supplementary cementitious materials (SCMs) from waste. Palm kernel shell (PKS) biochar — a carbon-rich product of biomass pyrolysis — is emerging as a low-cost carbonaceous filler and multifunctional cementitious additive that can partially replace cement and sequester CO₂. This paper presents a full A–Z review of PKS biochar (PKSB) as a cement additive, with emphasis on optimization via Response Surface Methodology (RSM). Conventional SCMs are reviewed first, alongside the motivation for using biochar from palm waste. PKSB production (pyrolysis parameters, yield, carbon content) and its physicochemical properties (chemical composition, surface area, reactivity) are then summarized, clarifying why PKSB functions primarily as a carbonaceous filler and nucleation agent rather than a true pozzolan. PKSB applications in cementitious materials are subsequently discussed: effects on fresh properties (e.g., reduced slump due to water absorption), mechanical performance (compressive and flexural strength, optimal replacement levels), durability (porosity, permeability), and microstructure (SEM and hydration phases). The core of the review covers RSM fundamentals (designs such as Central Composite and Box–Behnken, statistical analysis including ANOVA, R², p values, lack-of-fit and adequate precision) and their application to optimizing PKSB production and mortar mix design. A comprehensive table summarizes all published RSM studies involving PKSB (with variables, design, responses, optimum conditions, key findings). Environmental and economic aspects are analyzed: carbon footprint benefits (LCA studies report up to ~38% emission reduction relative to control), CO₂ sequestration (14.4 kg CO₂/kg biochar at optimal pyrolysis), and cost considerations. Finally, challenges and research gaps are identified (e.g., standardizing PKSB quality, long-term durability, scale-up costs), and future research directions are proposed.