Performance regulation of sulfates, alkalis and triethanolamine for sintered sludge-based synergistic cementitious materials: Macro-micro experiments and density functional theory calculations

The sintered sludge-based synergistic cementitious materials developed by utilizing mineral-phase differences among waste residues features environmental and construction benefits, yet insufficient reactivity frequently causes low early-age strength. Herein, the effects of sulfates, alkalis and triethanolamine (TEA) on mechanical performance, hydration kinetics, reactive-ion leaching and microstructure of limestone-blast furnace slag-sintered sludge-cement (LBSC) paste are systematically investigated. Simultaneously, first-principles calculations based on density functional theory (DFT) are adopted to characterize interfacial interactions between the above admixtures and C-A-S-H gel, unraveling the synergistic regulation mechanism for performance. Results show that NaOH and KOH trigger rapid cement hydration and immediately drive the paste into the diffusion-controlled stage, which yields inadequate reaction of SCMs. TEA and Na 2 SO 4 alone exhibit limited early-strength improvement and severe later-strength reduction, respectively. But their co-addition increases the 3d-compressive and flexural strengths by 52.2% and 102.3%, respectively, and the paste stably meets the grade of P·O 42.5 after 28 days. This accelerates Si 4+ and Al 3+ dissolution through the Na 2 SO 4 -enhanced alkalinity environment and the complexation of TEA, continuously stimulating early pozzolanic and aluminate reactions to rise chemically bound water by 15.4% and cut harmful porosity by 79.2%. DFT calculations demonstrate that appropriate SO 4 2− induces hydroxyl‑hydrogen dissociation of TEA via a synergistic polarization effect and facilitates deeper chemical anchoring, enabling functional coexistence of inorganic activation and organic modulation on the C-A-S-H surface. However, excessive Na 2 SO 4 preferentially generates U phase, prone to later-age destabilization and transformation into AFt. The resultant delayed expansion, coupled with accelerated drying shrinkage induced by surplus Na + , jointly deteriorates the microstructure.