Environmental Valorization of Industrial Waste: Alkali-Activated Geopolymers for Potential Bone Tissue Engineering

This study presents a sustainable approach to industrial waste valorization by investigating the effect of sodium hydroxide (NaOH) molarity on the bioactive characteristics of dual-source geopolymers synthesized from coal fly ash and metakaolin waste streams. As a circular economy solution, geopolymer samples were synthesized using NaOH solutions at three concentrations (1 M, 3 M, and 5 M) to determine sustainable processing conditions that maximize waste utilization efficiency and material bioactivity. The waste-derived materials underwent comprehensive characterization using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), real-time pH monitoring, and compressive strength testing. The findings demonstrated that NaOH concentration significantly influences both environmental sustainability and the material's ability to form hydroxyapatite (HA). Geopolymers activated with 1 M NaOH exhibited optimal bioactive performance while requiring lower chemical consumption, representing the most environmentally favorable processing condition. This low-molarity activation resulted in uniform hydroxyapatite layer formation through moderate pH conditions and controlled crystallinity. Conversely, 5 M NaOH activation increased environmental burden through higher chemical consumption while producing unfavorable surface conditions for HA nucleation due to excessive alkalinity and crystallinity. This research establishes that environmentally-conscious geopolymer processing using lower NaOH concentrations reduces chemical consumption and environmental impact while enhancing bioactive performance, demonstrating synergy between sustainable processing and functional properties. These findings contribute to eco-efficient waste valorization strategies and provide a framework for converting industrial by-products into high-value sustainable materials for biomedical and environmental applications.