The rapid growth of urbanization and the increasing construction and demolition activities have resulted in the generation of large quantities of concrete waste. Therefore, recycling and reusing concrete waste in the production of new concrete have become important approaches in sustainable civil engineering. Recycled Coarse Aggregate (RCA) is one of the most important materials obtained from the processing of concrete demolition waste and can be used as a partial or complete replacement for natural coarse aggregate in concrete. Despite the environmental and economic benefits of using recycled coarse aggregate, its properties differ from those of natural aggregate due to the presence of old adhered cement mortar on the surface of the aggregate particles. This adhered mortar is characterized by relatively high porosity and water absorption, which may reduce density, affect the bond between the aggregate and the new cement paste, and consequently influence the mechanical properties of concrete. 1. Concept of Recycled Coarse Aggregate Recycled coarse aggregate is produced by crushing and processing hardened concrete obtained from demolished buildings and structures or waste generated from concrete production facilities. After crushing and sieving, recycled aggregate generally consists of two main components: Original natural aggregate contained within the old concrete. Adhered old cement mortar surrounding the original aggregate. The adhered mortar is considered one of the most influential factors affecting the quality of recycled aggregate because it increases particle porosity and water absorption and affects the interfacial transition zone between the recycled aggregate and the new cement paste. 2. Importance of Pre-Treatment of Recycled Aggregate When untreated recycled aggregate is used, its high water absorption and the weakness of the adhered old mortar can affect concrete workability, the effective water-to-cement ratio, and the quality of the Interfacial Transition Zone (ITZ). Therefore, pre-treatment mainly aims to: Remove part of the adhered old mortar. Reduce aggregate porosity. Reduce water absorption. Improve aggregate density and surface properties. Strengthen the remaining old mortar. Improve the interfacial transition zone between the recycled aggregate and the new cement paste. Improve the compressive, tensile, and flexural strength of concrete. In general, pre-treatment techniques can be classified into methods for removing old mortar and methods for strengthening the remaining mortar. 3. Main Pre-Treatment Methods 3.1 Mechanical Treatment Mechanical treatment includes grinding, polishing, abrasion, and the use of special equipment such as a ball mill to remove weak mortar surrounding the aggregate. This method reduces the amount of adhered mortar and improves the surface characteristics of the recycled aggregate. Some mechanical treatments have been shown to reduce water absorption and increase aggregate density. However, excessive mechanical treatment may generate microcracks within the aggregate particles and requires additional energy. 3.2 Thermal and Mechanical Treatment In this method, recycled aggregate is heated to weaken the bond between the original aggregate and the adhered old mortar, followed by crushing or mechanical treatment to remove the weakened mortar. This technique can produce recycled aggregate with properties closer to those of natural aggregate. However, it requires thermal energy and appropriate equipment, and the temperature must be carefully controlled to avoid thermal damage to the aggregate. 3.3 Chemical Treatment Chemical treatment uses specific chemical solutions to remove or weaken the adhered cement mortar. These solutions may include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and phosphoric acid (H₃PO₄), while other solutions such as acetic acid have also been investigated. Chemical treatment can dissolve or weaken part of the old cement paste, thereby reducing water absorption and improving the properties of the recycled aggregate. However, acid treatment requires appropriate safety measures and proper disposal of the resulting chemical waste. 3.4 Accelerated Carbonation Accelerated carbonation is considered one of the promising modern techniques for strengthening recycled aggregate. It involves exposing the adhered old cement mortar to carbon dioxide, which reacts with the carbonatable compounds present in the mortar. This process can reduce porosity and water absorption and improve aggregate properties. In addition, it can contribute to the sequestration of a portion of carbon dioxide within the material. 3.5 Treatment Using Pozzolanic Materials and Silica Fume Recycled aggregate can be coated or impregnated with solutions or slurry containing pozzolanic materials such as silica fume. The purpose is to fill the pores within the old mortar and improve its cohesion and strength. Such treatment can improve the compressive strength of concrete, although the degree of improvement depends on the treatment conditions, replacement ratio, and concrete mix design. 4. Effect of Pre-Treatment on the Mechanical Properties of Concrete 4.1 Compressive Strength Compressive strength is one of the most important parameters used to evaluate the effect of recycled aggregate on concrete performance. In untreated concrete, the presence of a large amount of weak adhered mortar may increase porosity and weaken the interfacial transition zone, resulting in lower compressive strength compared with conventional concrete containing natural aggregate. When recycled aggregate is pre-treated, the removal of weak mortar or strengthening of the remaining mortar can improve the bond between the aggregate and the new cement paste. Consequently, several treatment techniques, particularly carbonation, pozzolanic impregnation, and some mechanical treatments, can significantly improve compressive strength. 4.2 Splitting Tensile Strength Splitting tensile strength is strongly affected by the quality of the Interfacial Transition Zone (ITZ). Untreated recycled aggregate may result in a more porous and microcracked interface compared with conventional concrete. Pre-treatment improves the aggregate surface and reduces defects within the adhered mortar, thereby improving stress transfer between the aggregate and cement paste. Consequently, pre-treatment can increase splitting tensile strength, although the magnitude of improvement depends on the treatment method, replacement ratio, and concrete age. 4.3 Flexural Strength Flexural strength depends on the ability of concrete to transfer stresses through the cement paste, interfacial transition zone, and aggregate. When the adhered old mortar is weak, the interface between the recycled aggregate and new cement paste may become a preferential path for crack propagation. Improving the aggregate surface and reducing its porosity can limit this problem and improve flexural strength. 4.4 Modulus of Elasticity Recycled aggregate generally has a lower modulus of elasticity than natural aggregate because of the presence of porous adhered cement mortar. Therefore, increasing the recycled aggregate replacement ratio may reduce the modulus of elasticity of concrete. However, pre-treatment, particularly the removal or strengthening of the old mortar, can reduce this decrease and improve the stiffness of the concrete. 5. Effect of Pre-Treatment on the Interfacial Transition Zone The Interfacial Transition Zone (ITZ) is one of the most important regions governing the mechanical performance of concrete. Concrete containing recycled aggregate may have more than one interfacial zone because of the presence of the old mortar surrounding the original aggregate. These zones are generally more susceptible to porosity and microcracking. Pre-treatment improves the ITZ through the following mechanism: Removal of weak mortar → Reduction in porosity → Improvement of aggregate surface → Stronger bonding → Reduced crack propagation → Improved concrete strength. Therefore, improving the interfacial transition zone is considered one of the main mechanisms responsible for the enhanced performance of concrete containing pre-treated recycled aggregate. 6. Comparison Between Untreated and Pre-Treated Recycled Aggregate 7. Factors Affecting Treatment Efficiency The effectiveness of a particular treatment method depends on several variables, and no single treatment method can be considered optimal for all types of recycled aggregate. The main factors include: Source of the recycled aggregate. Type and amount of adhered old mortar. Aggregate particle size. Replacement ratio of natural aggregate with recycled aggregate. Type of pre-treatment. Treatment duration and solution concentration in chemical treatment. Water-to-cement ratio. Type of cement and supplementary cementitious materials. Concrete age at testing. Curing conditions, humidity, and temperature. Therefore, research findings may differ from one study to another, and a single improvement percentage cannot be generalized to all concrete mixtures. 8. Environmental and Sustainability Aspects The importance of pre-treatment is not limited to improving mechanical properties; it is also closely associated with circular economy principles and sustainability in the construction sector. Reusing concrete waste reduces the demand for natural aggregates and decreases the amount of demolition waste disposed of in landfills. Developing effective treatment methods for recycled aggregate can therefore expand its application in structural concrete. However, the energy and water consumption, chemical requirements, treatment costs, and waste generated by each treatment method should also be considered. The selection of a treatment method should not be based solely on the increase in concrete strength. Consequently, Life Cycle Assessment (LCA) and long-term performance studies are important for determining the actual environmental and economic feasibility of different treatment techniques. 9. Conclusions The following conclusions can be drawn: Recycled coarse aggregate represents an important option for reducing natural aggregate consumption and reusing concrete demolition waste. Adhered old cement mortar is one of the main causes of the reduced quality of recycled aggregate. Pre-treatment can reduce aggregate porosity and water absorption or strengthen the remaining old mortar. Appropriate pre-treatment can improve the compressive, tensile, and flexural strength as well as the modulus of elasticity of concrete. Accelerated carbonation, pozzolanic impregnation, and some mechanical treatment techniques are promising approaches for improving recycled aggregate performance. No single treatment method is suitable for all types of recycled aggregate; the treatment should be selected according to the aggregate source, properties, and replacement ratio. A balance must be achieved between mechanical performance, cost, energy consumption, and environmental impact when selecting a treatment technique. Further long-term studies are required, particularly for structural concrete with high recycled aggregate replacement ratios and for combined treatment techniques. Conclusion Pre-treatment of recycled coarse aggregate is an effective approach for improving the performance of sustainable concrete. It addresses major problems associated with adhered old cement mortar, including high water absorption, high porosity, and weak interfacial transition zones. Recent research indicates that techniques such as mechanical removal, accelerated carbonation, pozzolanic impregnation, and nano-material-based treatments can improve the mechanical properties and durability of recycled aggregate concrete to varying degrees. Nevertheless, the successful use of recycled coarse aggregate depends on the selected treatment method, replacement ratio, concrete mix design, and curing conditions. Therefore, each recycled aggregate concrete system should be comprehensively evaluated before being adopted in structural applications.