Deamination mechanism of modified magnesium slag using wet aging and its performance evolution as backfill material.

The purpose of this study is to solve the problem of ammonia (NH) release when modified magnesium slag (MMS) is used as coal mine backfill cementitious material, and to explore its chemical mechanism and put forward effective solutions. Uniaxial compressive strengths (UCS) hydration kinetics, scanning electron microscope (SEM), and thermogravimetric analysis-derivative thermogravimetry (TG-DTG), X-ray diffractometer (XRD) and other testing methods were used to study the evolution of the properties of MMS-based backfill material, which provided a scientific basis for the safe utilization of MMS. First, the chemical mechanism underlying the release of NH from MMS was identified, and it was confirmed that MgN and LiN are the main nitrogen sources. This conclusion is based on thermochemical theory and the Pidgeon method. Secondly, the wet aging method was proposed to reduce the NH release from MMS-based backfill material. In addition, a continuous on-line gas detection system was used to determine the NH release pattern of MMS-based cementitious material under various aging water contents. The experiment showed a significant reduction in NH release with a lower aging water content of 3% and a reduction ratio as high as 81.55%. As the aging water content increased, the 28-day UCS gradually decreased. However, compared to the dry aging group, it remained above 3 MPa when the aging water content was below 6%. The wet aging method can significantly shorten the pre-induction, induction, and acceleration periods of MMS-based cementitious material. The microstructure of the wet aging group exhibited fewer needle-like and rod-like ettringite (AFt) crystals as well as less flocculent hydrated calcium silicate (C-S-H) gel, whereas the unhydrated group had more fly ash (FA) and pores. Considering the UCS and NH release performance of MMS-based cementitious material, it is recommended to adopt a 3% aging water content and control the aging period to 7 days to effectively reduce NH release while ensuring the material's strength, thereby meeting the application requirements for coal mine backfill cementitious material.