Creep characterisation and microstructural analysis of municipal solid waste incineration fly ash geopolymer backfill

In this work, an alkali-activated municipal solid waste incineration (MSWI) fly ash-based filling material was prepared with MSWI fly ash as the raw material and slag as the auxiliary material. The filling body experiences long-term creep, which may have a direct effect on the stability of the overlying strata of the mine goaf. The long-term mechanical properties of the fly ash-based filling materials were tested with a triaxial rheological apparatus. First, uniaxial creep testing was carried out at five levels of axial stress: 50%, 60%, 70%, 80% and 90% of the uniaxial compressive strength (UCS). Then, triaxial creep testing was carried out by considering the geological environment of the goaf. The creep characteristics of fly ash-based filling materials under a three-dimensional stress state were explored. The results indicate that (1) under different stress levels, the creep curves of fly ash-based filling materials can be divided into three types: decelerated creep‒stable creep, decelerated creep‒constant creep, and decelerated creep‒constant creep‒accelerated creep. (2) The total creep deformation of the fly ash-based filling material is 0.46 ~ 0.78%, which is similar to the creep deformation of soft rock. The instantaneous deformation during loading contributes most of the total deformation. (3) The polymerization products generated in the fly ash-based filling material system can effectively cement the raw material particles, and the presence of gel can effectively delay the accelerating creep process of the material. (4) A nonlinear fractional-order model composed of an Abel dashpot can fully describe the complete process of decelerating creep-constant creep-accelerating creep.