Study On Shrinkage And Durability Of Composite Limestone Powder Tailings Mixed Sand Concrete

The auxiliary cementitious materials such as fly ash,mineral powder,limestone powder and silica fume have become indispensable components of modern concrete.The incorporation of auxiliary cementitious materials into concrete can not only increase the utilization rate of solid waste,but also improve the material properties of concrete.At the same time,the mixed sand prepared by tailings superfine sand and manufactured sand not only improves the concentration of manufactured sand,but also consumes a large number of useless tailings.Therefore,in this study,multiple auxiliary cementitious materials were used to mix with tailings and manufactured sand.Through physical experiments,theoretical analysis and prediction model establishment,the shrinkage and durability laws and mechanisms of composite limestone powder-tailing mixed sand concrete were discussed.The degradation mechanism of concrete durability was analyzed by studying the phase composition and pore structure of hydration products.The main conclusions and innovations are as follows:（1）The development law of shrinkage performance of composite limestone powder-tailings mixed sand concrete was obtained,and the shrinkage prediction model was established.The self-shrinkage rate of concrete decreases with the increase of water-binder ratio,and the drying shrinkage is proportional to water-binder ratio.Tailings mixed sand instead of natural river sand increases the early autogenous shrinkage of concrete,and slightly reduces the drying shrinkage of concrete.With the increase of the proportion of tailing sand and manufactured sand,the autogenous shrinkage and drying shrinkage of concrete increase.Silica fume increases the autogenous shrinkage of concrete and has a good inhibitory effect on drying shrinkage;With the increase of limestone powder content,the autogenous shrinkage and drying shrinkage of concrete decrease.The autoshrinkage prediction model in the form of hyperbolic function and the dry shrinkage prediction model in the form of logarithmic function of composite limestone powder and tailings mixed sand concrete are established.（2）The deterioration law of freeze-thaw resistance of composite limestone powder-tailings mixed sand concrete was obtained,and the freeze-thaw damage prediction model was established.With the increase of freeze-thaw cycles,the greater the water-binder ratio,the lower the freeze-thaw resistance of concrete.The effect of tailings mixed sand instead of natural river sand on the early frost resistance of concrete is small,and the later frost resistance of concrete is slightly improved.With the increase of the proportion of tailings and manufactured sand,the freeze-thaw resistance of concrete increases first and then decreases.The increase in the proportion of silica fume and limestone powder improves the freeze-thaw resistance of concrete.Considering the influence of water-binder ratio and freeze-thaw cycles on the damage degree of concrete,the freeze-thaw damage prediction model of composite limestone powder-tailing mixed sand concrete was established.（3）The development law of carbonation resistance of composite limestone powder-tailings mixed sand concrete was obtained,and the carbonation depth model was established.The carbonation depth of medium and low strength concrete increased rapidly in the early stage,and the growth rate of carbonation depth decreased gradually in the later stage.The carbonation depth of accelerated carbonation for 14 days reached more than 50%of 60days.No obvious carbonation occurred in high-strength concrete after accelerated carbonation for 60 days.Considering the influence of water-binder ratio and carbonation age on the carbonation depth of concrete,the carbonation depth model of composite limestone powder-tailing mixed sand concrete is established.（4）The influence law of chloride ion penetration resistance of composite limestone powder-tailings mixed sand concrete was obtained,and the life prediction model of chloride ion erosion resistance was established.The chloride ion permeability resistance of concrete is inversely proportional to the water-binder ratio;The resistance to chloride ion erosion of concrete was reduced by replacing natural river sand with tailings mixed sand.The chloride diffusion coefficient of concrete increases with the increase of the ratio of tailings to manufactured sand;With the increase of the proportion of silica fume and limestone powder,the chloride penetration depth and diffusion coefficient of concrete decrease.Considering the binding capacity of concrete and chloride ion and the influence of internal damage of concrete structure,the life prediction model of composite limestone powder-tailing mixed sand concrete against chloride ion erosion is established.（5）The influence of limestone powder on sulfate attack resistance of high strength concrete with composite limestone powder-tailing mixed sand was obtained.At low water binder ratio,the incorporation of limestone powder can improve the sulfate resistance of concrete.With the increase in the number of sulfate dry-wet cycles,the beneficial effect of limestone powder on the sulfate resistance of concrete gradually appears,which improves the sulfate resistance of concrete in the later stage.（6）The relationship between microstructure and macroscopic durability of concrete is obtained,and the influence of hydration products and pore structure on durability is revealed.The durability of concrete is closely related to hydration products and internal microstructure.The higher the content of Ca（OH）₂,the stronger the carbonation resistance of concrete;The increase in the content of C-S-H gel improves the resistance of concrete to chloride ion penetration and sulfate attack.The smaller the average pore size,critical pore size and the most probable pore size,the more reasonable the pore size distribution,which can improve the macroscopic durability of concrete.This paper has 71 figures,76 tables,and 110 references.