Microstructural properties of alkali-activated mortars prepared from Fenton oxidation and heat-treated dyeing sludge as the substitutions of slag

This study investigates three treatment methods for removing organic residues and enhancing dyeing sludge (DS) activity: high-temperature calcination, Fenton oxidation, and mechanical grinding. The treated DS was used as a substitute for ground granulated blast slag (GGBS) in producing alkali-activated mortars. A comprehensive characterization of the chemical composition and microstructure of DS was conducted to evaluate the effectiveness of these treatment methods. Results showed that DS contains phosphorus oxides, which, upon dissolution, react with calcium and alkaline ions to form insoluble calcium phosphate and fluorine hydroxyapatite. Subsequently, the mechanical properties, micromorphology, and microstructure of alkali-activated mortars with varying DS contents were analyzed to assess the potential of treated DS as a precursor. Test results revealed that mechanical/heat-activated DS (HDS) experienced minimal strength reduction at 10 % doping, achieving a compressive strength of 99 % compared to the control group. However, controlling the amount of alkali used was crucial, as an increase in alkali resulted in decreased compressive strength. DS treated through mechanical/chemical activation (CDS) also showed promising results, with CDS-9 achieving a compressive strength of 78.4 % of the control group after 56 days of curing. Notably, although the early strength of specimens with CDS was lower, a more significant increase in strength was observed at later stages. Mechanical activation (MDS) had the most adverse effect on strength, independent of the curing duration. The pore size distribution of alkali-activated mortars can be optimized by refining the macro-pores in samples and transforming them into harmless gel pores through HDS and CDS integration. This study offers a feasible strategy for DS activation and demonstrates the highly promising utilization of DS as a precursor for alkali-activated materials. •Modified dyeing sludge via Fenton oxidation process.•Preparation of alkali-activated mortar incorporating treated dyeing sludge.•Phosphates in sludge inhibit silicate hydration and setting.•Treated sludge promotes slag hydration within the system.•Treated sludge prolongs setting time, refining pore structure.