Study On Mechanical Properties And Micro Mechanism Of Composite Limestone Powder-tailings Mixed Sand Concrete

In recent years,the development of green buildings in China has achieved fruitful results,and the application of high strength concrete has become more and more extensive.However,new problems such as lack of raw materials and low waste utilization rate have become increasingly apparent.Using mineral admixtures and tailings mixed sand to prepare new concrete not only improves the utilization rate of solid waste,but also effectively solves the problem of shortage of natural sand resources.Therefore,this paper studies the working performance,mechanical properties,uniaxial compression performance and microscopic mechanism of composite limestone powder-tailings mixed sand concrete,and provides theoretical reference for the application of new concrete materials in the field of green building.The main conclusions and innovations are as follows:(1)The variation of working performance of composite limestone powder-tailings mixed sand concrete is obtained.The slump loss rate,cohesiveness and water retention of the mixture increased with the decrease of water-binder ratio.The working performance of quaternary concrete with ordinary strength is better than that of ternary concrete.The workability index of high strength concrete decreases with the increase of silica fume content,increases first and then decreases with the increase of limestone powder content,and reaches the optimum when the content is 5%.With the increase of tailing sand proportion,the slump of the mixture increases first and then decreases,the expansion decreases sharply,and the viscosity increases.(2)The variation law of basic mechanical properties and strength prediction model of composite limestone powder-tailing mixed sand concrete were obtained.The failure process of ordinary strength concrete is similar to that of high strength concrete,and the higher the strength is,the greater the cracking sound is.The failure surface also changes from the interface between aggregate and slurry to the joint fracture of aggregate and cement stone.The strength of concrete increases with the decrease of water-binder ratio and the increase of age,but the growth rate slows down.In ordinary strength concrete,the gain of mineral powder on the later compressive strength of concrete is higher.In high strength concrete,the strength increases first and then decreases with the increase of silica fume content,and the optimal content is 10%.The influence of stone powder content on compressive and flexural strength is similar to that of silica fume,but it is unfavorable to splitting tensile strength.The strength decreases with the increase of tailing sand blending ratio,and the optimal blending ratio is 25%.The contribution of mineral powder and silica fume to splitting tensile strength and flexural strength is greater than compressive strength.The split tensile-compressive strength relationship model is fts=0.20fcc0.77(normal strength)and fts=0.07fcc0.99(high strength).The strength growth prediction models established by the relationship formula f=A lnt+B are in good agreement.(3)The variation law and constitutive model of uniaxial compression performance of composite limestone powder-tailings mixed sand concrete are obtained.Uniaxial compression test specimens are mostly destroyed along the diagonal direction,and the higher the strength is,the more sudden the failure is,and the greater the cracking sound is.The change law of the failure surface is similar to the cube compressive test.The peak strain and elastic modulus increase with the decrease of water-binder ratio,and are basically positively correlated with the peak stress.The ordinary strength of limestone powder-fly ash concrete strength is significantly lower than the other two.The peak strain of high strength concrete increases first and then decreases with the increase of silica fume and stone powder content,but it is opposite with tailings,but the difference is not big.The elastic modulus increases first and then decreases with the increase of stone powder content,and decreases gradually with silica fume and tailings.The characteristic parameters of ordinary strength prism specimens are larger than that of the cylinder,while in high strength concrete,the peak stress and strain of the prism are smaller than those of the cylinder,and the elastic modulus is slightly larger.The strength relationship model between the two is fc=0.96935f’.The relationship model between compressive strength and elastic modulus,peak stress and peak strain is fitted with formulas Ec=105/[a+(b/fcu)]and ε0=(a+b(?))×10-6,and a constitutive model of concrete uniaxial compression is established at the same time,with a high degree of fit.(4)The hydration product composition and interface microstructure of composite limestone powder-tailings mixed sand concrete were obtained.The hydration products of concrete mainly include calcite,ettringite,calcium hydroxide and hydrated calcium silicate.With the decrease of water-binder ratio,the hydration process of concrete is accelerated,and the contents of lamellar Ca(OH)2 and C-S-H gels are increased,but the generation of AFt crystals is inhibited.The particle distribution of cementitious materials is not uniform,and the porosity increases.The structure of interfacial transition zone is more likely to be destroyed than that of other regions.The appropriate incorporation of silica fume and limestone powder can optimize the pore structure of concrete and accelerate the overall hydration rate of concrete,while the increase in the proportion of tailing sand delays the process of hydration reaction.(5)The microscopic mechanism of macroscopic properties of composite limestone powder-tailings mixed sand concrete is revealed.The workability of the mixture is improved due to the small and smooth particle size morphology of the cementitious material.The higher the chemical activity of the material,the more significant the loss of working performance over time.The decrease of water-binder ratio increases the relative content of cementitious materials in the system,increases the time of continuous hydration of the system,and the concrete strength increases due to the filling of pores in the gel.The nucleation,chemical and dispersion effects of limestone powder can accelerate the reaction rate to some extent.Fly ash and mineral powder with volcanic ash and micro-aggregate effect can strengthen the bonding of the system.The high activity and capillary filling effect of silica fume can greatly improve the density of the system.The tailings mixed sand is hard and rough,which is more conducive to the bonding of the slurry.The microscopic mechanism of uniaxial compression performance is similar to the basic mechanical properties.The incorporation of limestone powder dilutes the cement in the system,which increases the ductility of concrete.The characteristics of high activity and small particle size of silica fume increase the brittleness of concrete.This thesis has 78 figures,38 tables,and 131 references.