Amirkabir University of Technology Amirkabir Journal of Civil Engineering 2588-297X 57 2 2025 04 21 Effect of Interfacial Transition Zone on Properties and Microstructure of Concrete Effect of Interfacial Transition Zone on Properties and Microstructure of Concrete 291 316 5647 10.22060/ceej.2025.23347.8147 FA Seyed Fathollah Sajedi Professor, Department of Civil Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran 0000-0003-3933-2793 Seyed Qasem Mirahmadi PhD candidate, Department of Civil Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Journal Article 2024 07 15 The interfacial transition zone (ITZ) is a relatively heterogeneous region with a thickness of 50 micrometers, whose physical and mechanical properties are determined descriptively due to the fine length scale and heterogeneous characteristics. In this research, by focusing on the ratio of different elements forming the ITZ and simultaneous examination of microscopic images, it was possible to numerically investigate the effect of changes in the ratio of elements on the properties and microstructure of concrete. In order to investigate the effect of the ITZ on concrete properties by examining the rheology of fresh concrete, 30 standard cubic samples were examined in the form of 10 mixing plans, and then the selected mixing plan was determined by trial and error. Then, 54 cubic and cylindrical samples of aggregates with high silica and w/c as 0.45 were made, and tests were conducted to determine the mechanical properties and durability of concrete in the age range of 7 to 180 days; finally, the microstructure of the mixtures was investigated. With increasing age, hydrated calcium silicate and calcium carbonate increased by 24% and 21.5%, respectively, and accordingly, the compressive strength (CS) and specific electrical resistances also increased by 10.3% and 48.6%, respectively, and the accelerated penetration of chloride ions decreased by 5.2%. In the limit of 30 micrometers from the aggregate boundary, due to the disturbance in the boundary region, no acceptable correlation relations between the ratio of different constituent elements and concrete properties were obtained. From the limit of 30 to 50 micrometers, linear relationships with coefficients of determination of 0.85 were established between the weight ratio of calcium to silicon with CS and the accelerated penetration of chloride ions, so that by reducing the said weight ratio by 82%, the CS increased by 10.3% and penetration decreased by 5.2%. The interfacial transition zone (ITZ) is a relatively heterogeneous region with a thickness of 50 micrometers, whose physical and mechanical properties are determined descriptively due to the fine length scale and heterogeneous characteristics. In this research, by focusing on the ratio of different elements forming the ITZ and simultaneous examination of microscopic images, it was possible to numerically investigate the effect of changes in the ratio of elements on the properties and microstructure of concrete. In order to investigate the effect of the ITZ on concrete properties by examining the rheology of fresh concrete, 30 standard cubic samples were examined in the form of 10 mixing plans, and then the selected mixing plan was determined by trial and error. Then, 54 cubic and cylindrical samples of aggregates with high silica and w/c as 0.45 were made, and tests were conducted to determine the mechanical properties and durability of concrete in the age range of 7 to 180 days; finally, the microstructure of the mixtures was investigated. With increasing age, hydrated calcium silicate and calcium carbonate increased by 24% and 21.5%, respectively, and accordingly, the compressive strength (CS) and specific electrical resistances also increased by 10.3% and 48.6%, respectively, and the accelerated penetration of chloride ions decreased by 5.2%. In the limit of 30 micrometers from the aggregate boundary, due to the disturbance in the boundary region, no acceptable correlation relations between the ratio of different constituent elements and concrete properties were obtained. From the limit of 30 to 50 micrometers, linear relationships with coefficients of determination of 0.85 were established between the weight ratio of calcium to silicon with CS and the accelerated penetration of chloride ions, so that by reducing the said weight ratio by 82%, the CS increased by 10.3% and penetration decreased by 5.2%. https://ceej.aut.ac.ir/article_5647_06563f3b418fe57f8fc331872343ce44.pdf