Study On Shrinkage And Durability Of Composite Limestone Powder Concrete

With the development of modern concrete and the increased demand for infrastructure construction,a variety of mineral admixtures?such as limestone powder,fly ash,slag,etc.?have gradually replaced cement as a composite auxiliary cementitious material.This greatly reduces environmental pollution and waste of resources.Research shows that compared with ordinary concrete,the concrete added with a reasonable mineral admixture has little or even a slight advantage in mechanical properties,but the cement system becomes more complicated with the addition of various mineral admixtures,and the concrete is durable.The degradation law and mechanism of performance are also more complicated.At the same time,the shrinkage cracks generated by concrete are closely related to the durability of concrete.Therefore,it is of great significance to study the shrinkage and durability of composite limestone powder concrete.In this paper,physical experiments,theoretical analysis,and model building methods of double-limestone limestone powder,fly ash and triple-limestone limestone powder,fly ash,and slag composite limestone powder concrete are carried out.The effect of shrinkage and durability of limestone powder concrete,and the analysis of the change law of durability performance from the perspective of microscopic pore structure.Finally,a feasibility study and economic benefit analysis of composite limestone powder concrete were conducted.The main conclusions and innovations are as follows:?1?The evolution law of shrinkage performance of composite limestone powder concrete is obtained,and a shrinkage prediction model is established.The development curve of the self-shrinkage rate and drying shrinkage rate of concrete is relatively fast in the early stage,and the development curve of the self-shrinkage rate tends to be gentle in the later period.The self-shrinkage rate of 24h can reach about 80%of 72h,and the development rate of drying shrinkage rate has slowed down,but there is still a certain rate of development.The self-shrinkage rate of concrete increases with the decrease of the water-binder ratio;the drying shrinkage rate of the concrete is less affected by the water-binder ratio in the early stage,and decreases with the decrease of the water-binder ratio in the middle and later stages.Mineral admixtures have a good inhibition effect on the shrinkage of concrete?self-shrinkage,drying shrinkage?,the shrinkage rate has decreased,and the anti-shrinkage performance of concrete has been improved.Through comparison and analysis of traditional prediction models,combined with experimental data to modify the hyperbolic function formula,a composite limestone powder concrete shrinkage?self-shrinkage,drying shrinkage?prediction model is established,which has a high degree of fit and can accurately reflect the composite limestone powder concrete shrinkage The relationship between rate,age and water-to-rubber ratio.?2?The evolution law of carbonization resistance of composite limestone powder concrete was revealed,and a carbonation depth prediction model was established.The growth rate of concrete carbonation depth developed rapidly in the early stage and gradually slowed down in the later stage.Decreasing the water-binder ratio can improve the carbonization resistance of the concrete.When the water-binder ratio is?0.40,the carbonization resistance of the concrete is greatly improved.The secondary hydration of the mineral admixture consumes Ca?OH?₂,reduces the alkalinity inside the concrete,thereby reducing the ability to absorb CO₂,and reducing the carbonization resistance of the concrete.Through regression analysis of experimental data,a prediction model of carbonization depth of composite limestone powder concrete is established.The development curve of concrete carbonation depth has a good correlation with d_t=a?W/B?^bt^c,which can better express the relationship between carbonization depth,carbonization time,and water-binder ratio.?3?The degradation law of freeze-thaw resistance of composite limestone powder concrete was revealed,and a freeze-thaw damage model was established.At the initial stage of freezing and thawing,the amount of water absorption caused by the increase of the internal porosity of some concrete specimens is greater than the amount of mass loss,and the mass increases.As the number of freeze-thaw cycles increases,the apparent morphological damage of concrete specimens gradually increases,mass loss gradually increases,and the relative dynamic elastic modulus continues to decrease.The smaller the water-binder ratio,the smaller the mass loss rate of the concrete specimen,the larger the relative dynamic elastic modulus,and the stronger the freeze-thaw resistance of the concrete.Although the addition of mineral admixtures increases the mass loss of concrete to a certain extent,it increases the relative dynamic elastic modulus of concrete,and enhances the freeze-thaw resistance of concrete.Based on the theory of concrete damage degree,a freeze-thaw damage model of composite limestone powder concrete was established.The model can better express the relationship between the damage degree,the number of freeze-thaw cycles and the water-binder ratio.?4?The change law of chloride ion penetration resistance of composite limestone powder concrete was obtained,and a life prediction model was established.The smaller the water-binder ratio,the stronger the resistance of concrete to chloride ion attack.The effect of water-binder ratio on concrete with mineral admixture is greater than that of benchmark concrete.For concrete with mineral admixture,the resistance to chloride ion penetration can be effectively improved by reducing the water-binder ratio.Under the combined effects of the physical and chemical aspects of mineral admixtures,concrete with mineral admixtures has higher compactness and better bonding properties,and the concrete's resistance to chloride ion penetration has been greatly improved.Based on Fick's second diffusion law,considering the effects of chloride ion binding capacity,time-varying effects of internal damage to concrete structures,and water-binder ratio,a prediction model for chloride ion erosion life of composite limestone powder concrete was established.?5?Through the analysis of the pore structure,the correlation between the micro pore structure and the macro durability of the concrete is obtained,and the micro mechanism of the durability of the composite limestone powder concrete is revealed.The addition of mineral admixtures optimizes the pore structure inside the concrete and improves the durability of the concrete.?6?Based on the acquisition of raw materials,work performance,mechanical properties,shrinkage performance and durability,the feasibility of composite limestone powder concrete was verified,and the cost was reduced by more than 5%compared with the benchmark concrete.And compared with the current commercial concrete,its cost performance is higher.This paper has 58 pictures,48 tables,and 126 references.