Sodium gluconate as a retarder modified sewage sludge ash-based geopolymers: Mechanism and environmental assessment

In this study, sewage sludge ash (SSA) was utilized to prepare geopolymers at room temperature, with the aim of achieving the utilization of sewage sludge resources. The study primarily investigates the effects of alkali activators and the sodium gluconate on the setting properties and mechanical properties of geopolymers prepared using a 2:3 wt ratio of SSA and ground granulated blast-furnace slag (GGBS). Results indicated that the addition of sodium gluconate significantly inhibited the reaction of geopolymers paste setting time of the geopolymer compared to changing the alkali activators parameters. An increase in sodium gluconate dosage from 0% to 3% significantly delays the initial setting time of geopolymer pastes from 31 min to 183 min. Microstructural analyses demonstrate that sodium gluconate interferes with the dissolution of precursor materials and the growth of the hydration gel in geopolymer, leading to an inhibitory effect on the early hardening process. The introduction of 3% sodium gluconate increases the area fraction of unhydrated SSA by over 45% and unhydrated GGBS by 13%, as quantified by BSE analysis. Importantly, environmental and economic analyses suggest that delaying the setting of geopolymer through the inclusion of sodium gluconate presents a more environmentally favorable option than altering the alkali activators.