Experimental Study On Physical Mechanism And Hydration Characteristics Of Gypsum Rock Mechanical Properties

The gypsum deposited underground is usually exploited by room-pillar,that is to say,the way of "retaining pillar after mining room" is used to treat the goaf.The pillar is the main carrier to support the overlying strata,of which bearing capacity directly determines the stability of the goaf.The particularity of the structure and composition of gypsum rock determines that the change of its mechanical properties is affected by many factors.Therefore,it is of great significance to explore the internal and external factors affecting the mechanical properties of gypsum rock and their rules of action for the design of the pillar size and the safety control measures of the goaf.In this paper,the physical properties and hydration characteristics of gypsum rock were studied.The mechanical properties and mechanism of gypsum rock under the influence of various internal and external factors were studied by means of basic mechanical experiments,microscopic analysis experiments and analytical chemistry methods.The effect of different microscopic structure and component content on the mechanical properties of gypsum rock was studied;the regularity of deterioration characteristics of immersed,dry-wet cycles and gypsum rocks decrystallized water was obtained;The evolution of long-term stress-strain and energy damage of gypsum rock under various water-bearing conditions was investigated.(1)Based on the Shimadzu test system,EDS-XRD experimental system,scanning electron microscopy(SEM)system and analytical chemistry experiment methods,the Brazilian test and uniaxial compression experiments of fibrin gypsum rock,transparent gypsum rock,alabaster rock and ordinary gypsum rock were carried out;four kinds of gypsum rock micro-material composition,element content and other experimental analysis were carried out.The differential characteristics of calcium sulfate dihydrate in four gypsum rocks and the correlation law with tensile strength were obtained.The correlation between microstructural characteristics and macroscopic failure modes of gypsum rock was studied.The results show that the content of calcium sulfate dihydrate is negatively correlated with the strength of gypsum.The internal microstructure of gypsum is an important factor affecting its strength.(2)The basic mechanical characteristic experiments of gypsum rocks with different immersion time and dry-wet cycling were carried out;the uniaxial compression experiments of decrystal lized water gypsum rocks were carried out;the internal microstructures of three groups of experimental rocks were analyzed by scanning electron microscopy(SEM);and the effects of hydration on the mechanical properties and micro-structural characteristics of gypsum rocks were obtained by ICP test of immersion solution.The results show that the mechanical properties of gypsum rocks under water immersion and dry-wet cycling are deteriorating,and the effect of dry-wet cycling is more obvious.Compared with the other two hydration states,the mechanical parameters of gypsum rocks decrystallized water are greatly reduced,but the peak strain is the largest.The development of internal defects in gypsum rocks gradually strengthens with the increase of hydration time;the failure modes of gypsum rocks are also quite different under different hydration conditions.Statistical analysis of the experimental phenomena and results shows that the deterioration laws of gypsum rock by water can be attributed to the structural particularity of gypsum,swelling of absorbent substances,dissolution of soluble substances,erosion and denudation laws.(3)Based on acoustic emission(AE),DVC(camera)and scanning electron microscopy(SEM),creep tests of natural,dried and immersed gypsum under step loading were carried out.The creep strength and deformation characteristics of gypsum rock in different water-bearing states were studied.The AE characteristics and damage energy evolution of gypsum rock creep experiment was analyzed.The macro-creep damage characteristics and micro-morphology of gypsum rock were obtained.The results show that the difference of strength of gypsum rock is not synchronous with the state of water content;the creep strength of gypsum rock is obviously higher than its uniaxial compressive strength,that is,the creep macroscopic hardening characteristics are produced;the variation law of single stress and horizontal strain of gypsum rock in three different water-bearing states is basically the same,and all of them are U-shaped;there are obvious differences in creep AE characteristic parameters and failure modes among three groups of gypsum rocks;after creep failure,the internal microstructure integrity of gypsum rocks is stronger than that of uniaxial compression rock samples,and more debris particles are found in creep samples.