Study On Strain Rate Effect Of Modified Phosphogypsum Under Uniaxial Compression

Phosphogypsum is one of the main solid wastes produced in China’s industrial development.The accumulation of phosphogypsum not only wastes land resources,but also causes irreparable damage to the ecological geological environment.Therefore,the resource utilization of industrial solid waste phosphogypsum is imminent.The application of modified phosphogypsum as an engineering material in mines,buildings and tunnels is one of the effective ways for its harmless disposal.In this thesis,based on the improvement of phosphogypsum solid waste,a modified phosphogypsum formula with hemihydrate phosphogypsum,quicklime and melamine superplasticizer as the main raw materials is proposed.Based on this formula,specimens meeting the requirements of the International Society of Rock Mechanics and Rock Engineering（ISRM）are prepared,and a uniaxial compression acoustic emission monitoring test system was built.On the basis of summarizing the previous research on strain rate division,four groups of loading strain rates suitable for this study were determined,which were 5*10^-6S^-1,10^-5S^-1,2*10^-5S^-1,10^-4S^-1.The uniaxial compression acoustic emission monitoring experiments under different strain rates were carried out to study the strain rate effect of the mechanical properties of modified phosphogypsum.The main results are as follows.（1）Based on the measured data of uniaxial compression in this study,the influence of strain rate on the mechanical properties of modified phosphogypsum under uniaxial compression was revealed.The failure mode of the specimen is not directly related to the strain rate,mainly tensile failure,and the degree of fragmentation is positively correlated with the strain rate.Under different strain rates,the uniaxial compression stress-strain curves of the specimens have obvious stage characteristics,that is,the pre-peak section has experienced compaction stage,elastic deformation and micro-crack stable expansion stage,micro-crack unstable expansion until failure stage.There are obvious differences in the rear section of the peak.The rear section of the middle strain rate peak shows a slow continuous decline,and the rear section of the low strain rate peak shows a steep and stepped drop.The compressive strength of the specimen decreases with the increase of strain rate.In the low strain rate loading range,the brittleness characteristics of the specimen increase with the increase of strain rate,and when the strain rate increases to the medium strain rate,the brittleness characteristics of the specimen are obviously weakened.In the range of low strain rate loading,the elastic modulus and deformation modulus of the specimen increase with the increase of strain rate,while the deformation parameters decrease obviously when the strain rate increases to medium strain rate.（2）Based on the acoustic emission monitoring data of this study,the influence of strain rate on the acoustic emission time series characteristic parameters and damage evolution law of modified phosphogypsum under uniaxial compression was clarified.The response relationship between the time series characteristic parameters of acoustic emission of modified phosphogypsum under different strain rates and its uniaxial compression stress-time curve is good.At the same time,the peak value of acoustic emission ringing count,cumulative ringing count,acoustic emission energy peak and cumulative energy increase with the increase of strain rate.Combined with the damage mechanics theory and the strain-acoustic emission cumulative ring count coupling relationship,the expression of modified phosphogypsum damage variable is proposed.Through the analysis of measured data,it is concluded that the damage variable of modified phosphogypsum under different strain rates has obvious stage change characteristics,and the damage variable at the end of uniaxial loading increases with the increase of strain rate.（3）The closure stress,crack initiation stress,damage stress and peak stress of modified phosphogypsum decrease with the increase of strain rate,but the ratio of each stress threshold to peak stress is almost not affected by strain rate.The ratio of each stress threshold to peak stress can be approximately set to 0.27,0.57 and 0.78,respectively.Combined with data fitting and mathematical derivation,the constitutive relationship of modified phosphogypsum at the front of uniaxial compression peak under different strain rates was preliminarily discussed.The fitting curve of the constitutive relationship of the pre-peak section of the specimen is close to the experimental curve,but the constitutive relationship of the modified phosphogypsum under different strain rates is obviously different.The strain rate has a significant effect on the constitutive relationship of the modified phosphogypsum material.