Evaluating the influence of treatment techniques of recycled aggregate on properties of concrete towards sustainable development

The sustainability of concrete is increased by using more sustainable components. Recycled aggregates generated from demolishing old concrete buildings are an environmentally friendly replacement for natural aggregate. This paper treated the recycled aggregate using six methods: soaking in bacteria solution, directly mixing with bacteria, mechanical treatment in a drum mixer, treating aggregates with dry-ice CO₂, directly mixing with dry-ice CO_2, and soaking in acetic acid. Various parameters, including physical, mechanical, durability, and microstructural properties, were investigated for the different treatment options. The properties of treated recycled aggregates were compared with each other, as well as with those of natural aggregate and untreated recycled aggregate. Hardened concrete properties with natural aggregate, untreated recycled aggregate, and treated recycled aggregate were investigated to see the effect of each aggregate type on them. Various parameters were examined for the different treatment options to find the optimum condition for each alternative. The quality of the untreated recycled aggregate, as well as that of the concrete produced with it, was inferior to that of the natural aggregate and the corresponding concrete. However, the reduction in quality was generally slight. For example, the compressive strength of untreated recycled aggregate was only 8% lower than that of natural aggregate concrete, which indicates that the quality of the recycled aggregate used in this study was acceptable. The most efficient treatment in terms of aggregate and concrete properties was mechanically treating the aggregate, and the most effective duration of this treatment was two hours; this treatment was able to improve all the studied properties. Treating the aggregate by submerging it in 2% bacteria by aggregate weight for seven days and directly adding 3% bacteria by cement weight during mixing also gave good concrete mechanical properties. However, it was inefficient in improving the remaining properties, unlike mechanical treatment, which was efficient in improving all properties. In this study, treating the aggregate with dry ice and acetic acid was generally ineffective.