Effect of lime neutralization wastewater on the performance of alkali-activated slag/fly ash mortars

This paper was conducted to upcycle lime-neutralize etching wastewater (LNW), produced from the etching of cenospheres for producing perforated cenospheres, into alkali-activated slag/fly ash, considering the high contents of Ca²⁺ (Calcium ion), Cl⁻ (Chloride ion), and OH⁻ (Hydroxyl). The aim of this study is to limit the discharge of calcium-rich wastewater and minimize the pollution of water and soil resources. To this end, the effects of different LNW content on the fresh properties, hydration products, compressive strength, microstructure and nanomechanical properties of AASF were investigated. Results revealed that the addition of LNW extended the setting times and inhibited the early hydration of AASF due to the existence of Ca²⁺ and NH⁴⁺ (Ammonium) in LNW. In addition, the existence of Ca²⁺ in LNW reacts with NaOH to form Ca(OH)₂, which can work as an auxiliary activator for AASF. Consequently, the LNW-added AASF mortars exhibited denser pore structures and higher compressive strengths. By using 25–100% LNW, the compressive strengths of AASF mortars can be improved by 2.7–18.9% (3d), 4.8–19.5% (7d) and 5.6–19.7% (28d), respectively.