Compressive Strength of Metakaolin Geopolymer Concrete Incorporating Palm Kernel Shell as Lightweight Coarse Aggregate *

Abstract

 The global construction industry faces mounting pressure to reduce its environmental footprint, particularly through the elimination of Portland cement (PC), a major contributor to global CO₂ emissions. This study investigates the compressive strength of metakaolin (MK) geopolymer concrete having palm kernel shell (PKS) incorporated as a lightweight coarse aggregate, with the aim of establishing a PC-free, eco-efficient concrete system. Kankara kaolin from Katsina State, Nigeria, was calcined at 800°C to produce metakaolin, which was activated using a blend of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) at Si/Al molar ratios of 2.0, 2.5 and 3.0. PKS sourced from Ile Oluji, Ondo State, was used as the sole coarse aggregate. Specimens were cured under ambient laboratory conditions (27°C) and tested at 7, 14, 28 and 56 days. Results showed that the Si/Al ratio of 2.5 (Mix C2) consistently yielded the best performance, achieving 15.60 N/mm² at 28 days and 16.80 N/mm² at 56 days, with a 80 mm slump and 90 minutes initial setting time. X-ray fluorescence (XRF) analysis confirmed 92.85 % pozzolanic oxides in metakaolin, exceeding ASTM C618-2022 requirements, while BET analysis revealed an ultrafine particle size (0.169 µm) and specific surface area of 14.2 m²/g up to 47 times greater than PC. The ambient-cured MK-PKS geopolymer concrete outperformed PKS-cement concrete by 9% at 28 days. Despite being 40-42% below the PC granite concrete benchmark (of 26 N/mm² at 28 days), these results establish a viable foundation for sustainable, PC free lightweight concrete using locally sourced Nigerian materials, with a projected potential for 50-55% CO₂ reduction compared to PC-based systems, in reference to established geopolymer emissions benchmarks in the literature, pending formal lifecycle assessment.