Investigation Of Carbonation Cracking Behavior And Carbonation Mechanism Of Concrete

Concrete may be eroded by carbon dioxide(CO2)in nature,and the concrete will shrink after carbonization,or even crack in severe cases,which seriously affects the durability of concrete.However,due to the limitation of research methods(such as,destructive research methods or holistic research of complex systems),there is still a lack of understanding of the cracking conditions,cracking characteristics and carbonation erosion mechanism of concrete carbonation.Based on this,this thesis has studied the carbonization cracking behavior and carbonation mechanism of cement-based materials based on new research methods and research ideas.Firstly,the feasibility of using micrometer X-ray computed tomography(ACT)technology to study the carbonation performance of cement-based materials was analyzed,and the extraction of cracks in the carbonized zone and the segmentation of the uncarbonized zone were optimized.The research in this part lays the foundation for the subsequent analysis of carbonation cracking features.Next,the effect of pre-drying on the carbonization cracking performance of cement paste was studied,and the carbonation characteristics of the paste were analyzed under pre-drying and non-pre-drying conditions,which provides a reference for the identification of carbonation characteristics in nature.Finally,the carbonation mechanism of the unhydrated cement component and the hydrated products were analyzed respectively,which laid the foundation for the control,utilization of carbonation and carbonation prediction modelThe results show that compared with the traditional threshold segmentation technology,the introduction of ocular vessel segmentation technology(B-COSFIRE filtering)into the crack segmentation in the carbonized area in the μCT images and the edge recognition technology into the segmentation of the uncarbonized area in the μCT images,these two method can improve the accuracy of the extraction of cracks in the carbonized area and the extraction of the area in the non-carbonized areaFrom the carbonization cracking conditions,after pre-drying,the cement-based material is more prone to crack,and compared with the sample without pre-drying,the crack width generated under this condition is larger,but the number of cracks is reduced.At the same time,pre-drying can accelerate the carbonization rate of cement paste.Therefore,if you want to avoid the carbonization shrinkage cracking of cement,you should try to avoid excessive drying before carbonization under laboratory conditions;in actual engineering,especially for prefabricated components,you should also try to avoid placing them in a dry environment for a long time before putting them into engineering applications.About the characteristics of carbonization cracking,the carbonization cracks extend from the outside to the inside,and the cracks are rimous.The cracks only propagate in the carbonized zone and rarely enter the uncarbonized zone.But long-term carbonization will also form through cracks.Observing the surface cracks,triangular radial cracks appear on the surface and the ground of specimens and the side surfaces are mainly longitudinal cracks.The width of the surface cracks first increased and then decreased.From the view of the carbonization mechanism of each component before and after hydration of cement,the carbonization of unhydrated cement is slow under anhydrous conditions.Even if the surrounding environment has a high humidity,it will not significantly accelerate the carbonization rate.Therefore,carbonization can only occur if enough water is added to stir to form a cement slurry.After adding water,carbonization can promote the degree of hydration of cement and generate more carbonation products to fill the pores in cement paste,thereby effectively reducing the porosity.It is also observed that the C3S paste produces dense carbonized products after carbonization,which can effectively inhibit the further progress of carbonization;compared with C3S,the carbonization activity of C2S is higher,but the carbonized products are far less dense than C3S carbonized products,so CO2 gas can be directly enters the interior of the samples;the carbonization activity of C3 A the lowest,and there is very little calcium carbonate produced,but also due to the low density of carbonation products,the depth of carbonation increases faster;C4AF carbonization products are not as dense as C3S carbonization products,but the hydration activity of C4AF is very high,a large number of hydrated products can be generated in the early stage to fill the pores,and improve the carbonation resistance of cement paste.Therefore,from the perspective of suppressing the carbonization shrinkage of concrete,it is an appropriate method to increase the content of C3S and C4AF in the cement and reducing the content of C2S and C3A to reduce the degree of carbonation shrinkage of concrete and improve the carbonation resistance of Portland cement.