The incorporation of waste materials in concrete is currently a practical solution to solve certain environmental concerns. Because of the property degradation of the produced concrete owing to the addition of waste materials, using pozzolans in the concrete mix can help improve the mechanical performance of recycled concrete. Therefore, in this research, the mechanical specifications of recycled concrete were evaluated at different volume fractions of recycled fine aggregates (RFA) replacing natural fine aggregates (NFA). In addition, the impact of adding colloidal nano-silica particles on the performance of conventional and recycled concretes was investigated. Various parameters comprising compressive strength, pulse velocity, splitting tensile strength, flexural strength, elastic modulus, and water absorption were carefully examined. The attained experimental results reveal that the mechanical and physical characteristics of concrete degrade with increasing the volume fraction of RFA replacing NFA. Also, increasing the weight percentage of nano-silica replacing cement up to 6% in conventional concrete and that containing RFA improves the mechanical and physical properties, the optimum percentage of which was determined as 4.5%. Furthermore, relationships were recommended for predicting the compressive, splitting tensile, and flexural strengths as well as the elastic modulus of concrete mixes by including the RFA and nano-silica replacement levels, which are in acceptable accordance with the experiments in the present study and those available in the technical literature. Finally, to investigate the environmental effects of recycled concrete with different RFA substitution levels and colloidal nano-silica incorporation, the problem-based CML 2000 and the damage-based IMPACT2002 + methods were applied using SimaPro9 software. Thus, environmental parameters such as acidification, global warming potential (GWP), eutrophication, natural resources, ecosystem quality, and human health were inspected. Subsequently, the GWP results were compared and presented using the CML 2000 and the Intergovernmental Panel on Climate Change (IPCC) approaches. To verify the estimates, all the attained results were examined against those captured from the Building for Environmental and Economic Sustainability (BEES) method. Findings show that despite the implementation of different unit measurements in different methods, the environmental effects in the CML 2000 and BEES methodologies are almost the same, suggesting that nano-silica is a raw material with the greatest environmental impact. Additionally, based on the life cycle comparison results of manufacturing one cubic meter of concrete with different mix designs using the damage evaluation approach, IMPACT 2002+, concrete with the RF0NS6 mix design triggered the highest damage extent in natural resource, climate change, ecosystem, and human health categories, respectively. This study fills a significant research gap from an environmental perspective by comprehensively investigating the durability and mechanical specifications of concrete containing RFA and nano-silica pozzolan. The findings presented in this research provide valuable insights into the realm of sustainable construction practices, paving the way for novel advancements in concrete technology.
