Mechanical grinding activation of modified magnesium slag and its use as backfilling cementitious material

The utilization of industrial solid waste with potential cementitious activity to prepare low carbon, economic and environmental backfilling cementitious materials is an urgent issue in cemented paste backfill (CPB). In this work, the mechanical grinding activation of modified magnesium slag (MMS), including grinding kinetics, crystallinity, and the influences on backfilling paste characteristics, have been investigated systematically. The indexes of particle size distribution (PSD), fineness, and sieve residue indicate that the grinding efficiency of MMS is high when grinding time ranges [20, 80] min. The particle coarsening appears when the grinding time increases to 120 min from 100 min. A fitted grinding kinetics function of MMS has been established based on the measured PSDs with six grinding time. The XRD shows that the diffraction peak intensities of CaSO4 and SiO2 phases decreases with the increasing of grinding time, indicating the increasing of lattice defects and amorphization degree. Both of grinding dispersion refinement and amorphization increase of MMS contribute to the activation of cementitious reactivity. Grinding time has a great influence on the properties of modified magnesium slag-based (MMS-based) backfilling pastes. The fresh backfilling pastes conformed to both of the H-B model and revised Bingham model well. The critical shear rate, critical apparent viscosity, and extended degree increase with the increasing of grinding time, indicating that the fluidity decreases. The TG-DTG, SEM tests show that hydration rate of the MMS-based backfilling pastes is slow and there are many interconnected micropores when the grinding time is below 60 min. Besides, the uniaxial compressive strengths (UCS) of MMS-based backfilling mortar samples also show a similar pattern. All tests showed that grinding for 60 min could stimulate most of the potential cementitious reactivity of MMS. Meanwhile, the corresponding backfilling paste behaves good working condition. Taken together, 60 min is suggested as the optimal grinding time for MMS in consideration of energy consumed and technical benefits, and the corresponding specific surface area (SSA) and 45 μm sieve residue (SR) are 503 m2/kg and 3.83%, respectively.