Dual-function valorisation of low-grade fly ash via zeolitization for environmental remediation and concrete enhancement

The accelerated advancement of Indonesia’s forthcoming capital, Ibu Kota Nusantara (IKN), intensifies environmental challenges related to acid mine drainage (AMD) and the accumulation of circulating fluidized bed combustion fly ash (CFA) alongside palm oil fuel ash (POFA). This study proposes a circular dual-valorisation strategy by synthesizing Na-P1 (gismondine-type) zeolite from low-grade CFA and high-silica POFA through alkali activation, followed by steam curing and calcination, to enable simultaneous AMD remediation and material reuse in concrete applications. Mineralogical analyses confirmed the formation of crystalline aluminosilicate frameworks. The synthesized zeolite demonstrated high remediation efficiency, increasing AMD pH from 2.1 to 8.5 and achieving near-complete removal of Fe, Mn, and sulfate ions, along with significant reduction of total suspended solids within 24 h. Post-treatment characterization revealed partial structural transformation of the zeolite; however, its functionality as an artificial aggregate was retained, contributing to internal curing and achieving compressive strengths of 18.4–18.9 MPa at 10% replacement in conventional concrete, and up to 44.4 MPa in self-compacting concrete. The results further indicate that alkali molarity critically governs the competition between zeolitization and geopolymeric gel formation, thereby controlling both adsorption capacity and mechanical performance. Unlike conventional studies that treat adsorption performance and structural reuse separately, this work demonstrates a coupled mechanism in which AMD-reacted zeolite undergoes controlled physicochemical evolution while remaining functionally viable as a construction material. This work presents a scalable circular solution for integrated waste valorisation, AMD remediation, and sustainable construction material development, addressing both environmental pollution and resource scarcity in emerging urban regions.