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.