Single-component low-carbon sustainable geopolymer synthesis using calcium carbide residue, gypsum, sewage sludge ash, and ground granulated blast slag

The application of sewage sludge ash-based geopolymers has been hindered by the significant environmental impact associated with traditional alkaline activators, such as (Na₂SiO₃ and NaOH). Calcium carbide residue (CCR) has emerged as a promising alternative solid activator due to its low environmental footprint. However, its limited alkalinity results in slow strength development, posing challenges to its practical application. This study explores a novel synergistic activation strategy that combines gypsum with CCR to enhance the performance of sludge ash-based geopolymers. The effects of varying gypsum contents on the compressive strength and phase composition of the geopolymer paste were systematically investigated, alongside an evaluation of the environmental benefits of this co-activation approach. The results reveal that the synergistic activation significantly improves the compressive strength of the specimens. Notably, the optimal blend—comprising 5 % CCR and 15 % gypsum—achieved a remarkable compressive strength of 53 MPa after 28 days of curing. Microstructure analysis shows that the sulfate ions released by gypsum promote the formation of ettringite in the geopolymer matrix. The presence of ettringite, together with the hydrated C-A-S-H gel, reduced porosity and densified the microstructure, leading to improved mechanical properties. Importantly, the sustainability analysis highlighted the significant advantages of this activation method, with CO₂ emissions and energy consumption reduced by over 75 % and 62 %, respectively, compared to traditional activators. This research provides guidance for the low-carbon disposal and management of substantial amounts of solid waste.