From waste to worth: Assessing the feasibility of sodium aluminate as an activator for transforming steel slag modified waste foundry sand into a valuable resource

Abstract

Steel industries and foundries worldwide produce substantial amounts of waste, such as slag and waste foundry sand, which pose significant environmental hazards if not managed properly. This study explores the utilization of basic oxygen furnace slag (BOFS) and green waste foundry sand (GWFS) as raw materials for synthesizing alkali-activated materials (AAMs) using sodium aluminate (NaAlO₂) as an alkaline activator. The research investigates various mix designs of BOFS and GWFS in different proportions 100:0, 85:15, 70:30, 55:45, and 50:50 to determine the optimal combination for maximum strength. The curing conditions, including temperature and time, were varied, with temperatures ranging from 40 °C to 80 °C and curing time from 1 to 6 days. The AAM formulated using 50% BOFS and 50% GWFS, cured at 80 °C with an alkaline activator concentration of 8 M, achieved the highest compressive strength of 14.25 MPa. X-ray diffraction (XRD) analysis revealed the formation of calcium silicate hydrate (C-S-H) and calcium aluminium silicate hydrate (C-A-S-H) phases, which are critical in the strength development of AAMs. Additionally, scanning electron microscopy (SEM) results demonstrated that BOFS-GWFS specimens exhibited enhanced structural densification and compaction as the concentration of sodium aluminate increased. The Toxicity Characteristic Leaching Procedure (TCLP) confirmed that metals were effectively immobilized within the AAM matrix, indicating that these materials pose minimal environmental risks. Moreover, the environmental footprint of the synthesized monolith is sufficiently low, allowing it to be safely used alongside standard masonry bricks in compliance with the specifications outlined in ASTM C34–03 and ASTM C62–10.