Performance optimization and heavy metal solidification mechanism of salt-alkali activated all-solid waste filling material

This study prepares a novel green filling material for goaf areas using industrial ground granulated blast-furnace slag (GGBS), fly ash (FA), municipal solid waste incineration fly ash (MSWI FA) as cementitious components, carbide slag (CS), sodium sulfate (Na₂SO₄) as activators, and coal gangue (CG) as aggregate. Firstly, the influence of GGBS content, ratio of CS to sodium sulfate (CS/Na₂SO₄) and activator content on the uniaxial compressive strength (UCS) of cementitious materials was analyzed by single factor method. Subsequently, using bone cement ratio, mass concentration, and GGBS content as influencing factors, the BBD response surface methodology was used to design the experimental mix ratio and analyze the influence of each influencing factor on the mechanical properties and slump of the filling material. Finally, the test results were analyzed by combining X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS), and inductively coupled plasma mass spectrometer (ICP-MS), the study elucidates the hydration products, microscopic morphology, strength formation mechanism, and heavy metal solidification mechanism of the cementitious material. The research results indicate that the optimal proportion of the filling material is as follows: bone glue proportion (3.32:1), mass concentration (79.12 %), GGBS (33.21 %), CS/Na₂SO₄ (1:6), activator content (8 %), with the 28 d UCS of the sample is 2.08 MPa. CS initially reacts with Na₂SO₄, forming anionic derivative products such as calcium sulfate (CaSO₄) and AFt, the early strength of the material is improved. In the Na⁺ environment, NaOH is locally generated, which increases the alkalinity of the medium, thus accelerating the polymerization process of the cementitious material, and promoting the formation of C-(A)-S-H gel and AFt. At the optimal proportion, the leaching concentrations of heavy metal ions from the filling material all meet the Class Ⅲ groundwater pollution standards (GB/T14848-2017).