Evaluation of slag and tannery sludge based geopolymer after thermal exposure: Properties, phase transformation and Cr leaching behavior

It is anticipated that this would address the underlying reasons of the disposal challenges and associated safety hazards if Cr-containing tannery waste from the tannery industry could be produced into stabilized sustainable construction materials. In this paper, the geopolymer was prepared employing solid waste furnace slag (FS) and tannery sludge powder (TSP) and evaluated for mass loss rate, appearance, residual compressive strength, pore properties, phase transformation and Cr leaching behavior after different thermal exposure (200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C) for 2 h. The results revealed that the compressive strength increased and then dropped with increasing thermal exposure temperature, with the residual compressive strength of geopolymer being the maximum (63.00 MPa) at 200 °C. Dehydroxylation and phase transition disrupted the pore structure, and the hydration products, mostly C-(A)-S-H, underwent depolymerization and repolymerization to create new crystalline phases including akermanite and gehlenite. Even at 600 °C, which has the greatest impact on the safety of geopolymers, the leaching concentrations of total Cr and Cr (Ⅵ) were only 1.39 mg/L and 0.074 mg/L, respectively, which were still far below the standard requirements. The possible Stabilization-Migration-Restabilization (SMR) route of Cr was identified as follows: Cr was physically encapsulated by C-(A)-S-H at 20 °C–400 °C; C-(A)-S-H was destroyed at 600 °C–800 °C, and Cr migrated and was oxidized to Cr (Ⅵ) by CaO; it was then probably reduced to Cr(V) by heat and Mg-O at 1000 °C and finally immobilized in akermanite by chemical bonding.