Macro-meso-micro strengthening and damage characteristics of composite excitation paste filling material under dry-wet cycle
In order to explore the performance change law of salt-alkali composite excitation cemented paste backfill materials and the multi-scale damage mechanism of backfill structure under the action of dry-wet cycles,multi-objective optimization of paste filling materials with desulfurization gypsum and cement clinker as composite activators was carried out.The backfill specimens under optimal ratio was subjected to 0-25 dry-wet cycles,and tests on compressive strength,X-ray diffraction(XRD),low-field nuclear magnetic resonance(NMR),and scanning electron microscopy(SEM)were carried out to reveal the macroscopic mechanical behavior and microstructure evolution of the backfill.The results show that the optimal ratio of backfill material is slag content of 50% ,mass ratio of desulfurization gypsum to cement clinker of 3꞉7,and silica fume content of 2.5% .With the increase of the number of dry-wet cycles,the macroscopic performance of the backfill is the reduction of compressive strength and the increase of cumulative conductivity.The inflection point occurs after 10 dry-wet cycles,the mass loss rate and strength loss rate reach the minimum value of-1.91% and-8.36% ,respectively.On the mesoscopic scale,the T2 spectrum distribution form with good inversions.After 10 dry wet cycles,the transverse relaxation time constantly moves right and the spectral area increases,which indicates that the pore size and number of paste filling materials increase with the increase of dry-wet cycles.On the microscopic scale,the expansion and crystallization stress of erosion products such as ettringite,gypsum and calcite accelerates the development of microscopic pore structure,and the repeated dry-wet cycles reduce the cohesion of C-S-H gel,which deteriorates from the initial stacked honeycomb to small lumps.According to the evolution law of macroscopic,mesoscopic,and microscopic structures,the damage evolution of pore structures and the degradation characteristics of mechanical properties have good synchronization.
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