Investigation on deterioration mechanism of geopolymer cemented coal Gangue-Fly ash backfill under combined action of high temperature and salt corrosion environment

The geopolymer cemented coal gangue-fly ash backfill (GCGFB) is pumped to the goaf to maintain the stability of surface subsidence, which is inevitably affected by the high temperature caused by geothermal heat and ion erosion of mine water. To study the deterioration mechanism of GCGFB caused by salt corrosion and high temperature of mine water, in this study, a customized temperature-controlled corrosion chamber was used to carry out the corrosion experiment of GCGFB. A three-factor and three-level orthogonal experiment was designed. The uniaxial compressive strength (UCS), relative dynamic elastic modulus (RDEM) and weight deterioration of GCGFB under different combinations of temperatures, solution concentrations, and solution types were analysed. The deterioration mechanism of GCGFB was analysed through the testing of scanning electron microscopy (SEM)-energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). From the results, the most significant factors on the deterioration of GCGFB is solution type. Cl- and SO42-caused chemical-physical erosion on GCGFB, and the UCS, RDEM and weight of GCGFB increased first and then decreased with ages. The reason for the increase was due to the compacting effect of Friedel 's sat, ettringite, and gypsum filled in GCGFB. The reason for the decrease was due to the expansion pressure on the inner wall of the pores of the GCGFB caused it to crack or even peel off. The GCGFB was corroded in 20 % Na2SO4 solution at 40 degrees C for 90 days, and the UCS increased by 164.90 % compared to that of the uncorroded specimen. Mg2+ brought chemical erosion on GCGFB, and a decreasing law of the UCS, RDEM and weight with ages was shown. During the Mg2+ erosion, there are more and more M-S-H in matrix, which reduced the density of specimen apparently. The GCGFB was corroded in 5 % MgCl2 solution at 40 degrees C for 180 days, and the macropore area distribution of GCGFB is increased to 23.499% than that of uncorroded group. The expansion stress of corrosion products would squeeze the inner pore wall, resulting in microcracks inside GCGFB and high temperatures would accelerate the migration of ions and the formation of corrosion products. Overall, this study can provide a new insight or valuable data for the designing of ultra-high performance, low-caron, eco-friendly, and durable GCGFB structures in the harsh mining environments.