FLY-ASH BASED GEOPOLYMER MORTAR FOR PAVING BLOCKS: MECHANICAL PERFORMANCE AND MICROSTRUCTURE ANALYSIS

Abstract

This study investigates low-carbon alternatives to cement-based paving blocks by assessing ambient-cured, fly-ash-based geopolymer mortars with varying fly ash (FA) proportions (100%, 85%, 80%, and 75% of binder) and minimal cement content (0%, 15%, 20%, and 25% of binder). To mitigate the approximately 8% contribution of global CO₂ emissions from cement production, mixes were activated using an alkaline system consisting of 10 M sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) at a Na₂SiO₃/NaOH ratio of 2.5, generated at a solid-to-liquid (binder-to-activator) ratio of 2.0. Specimens were cast into 5×5×5 cm cubes and evaluated for compressive strength (ASTM C109) at 7 and 28 days; microstructure was examined using scanning electron microscopy (SEM). The ideal formulation (80% FA + 20% cement) attained 14.0 MPa at 7 days and 20.7 MPa at 28 days, conforming to Indonesia’s paving block standard (SNI 03-0691-1996 Class B). SEM demonstrated a compact, densely arranged matrix with minimal apparent porosity and insignificant cracking at the optimal level, while elevated or reduced FA amounts exhibited microcracking, voids, or unreacted particles. Restricting cement to 20% of the binder significantly decreases binder-associated CO₂ emissions while fulfilling performance standards, thereby substantiating fly-ash geopolymers as viable, sustainable materials for paving blocks and elucidating the microstructural mechanisms that support the appropriate fly ash concentration.