| Summary: | Recycled aggregates have high water absorption and crushing index. In order to improve the properties of recycled aggregates in concrete production, various treatments were used to modify the aggregates. In recent years, bio-deposition as a new treatment method of recycled aggregates was environmentally friendly. An improved method of bio-deposition was implemented to modify the properties of recycled mortar aggregates (RMA). O-bio-deposition is based on aerobic bacteria induced CaCO<sub>3</sub> precipitation by respiration by varying the distance between the RMA and the bottom of the container and by adding an oxygen release compound to the culture solution that contains bacteria to promote the induction of CaCO<sub>3</sub>. First, the physical properties, including water absorption, crushing value, and apparent density, of the coarse RMA under different treatment methods were determined, and an o-bio-deposition treatment method was obtained. The fine RMA was treated and compared with the untreated RMA. Concretes were then prepared from the treated coarse RMA, and compressive strength and slump were determined. In addition, the effect of the o-bio-deposition treatment on the RMA surface and the micro-cracks of concretes were observed by scanning electron microscopy (SEM). It was found that the water absorption and crushing index of the coarse RMA treated by o-bio-deposition were reduced by 40.38 and 19.76% compared with untreated RMA, respectively. Regarding the concrete, the slump and the compressive strength (28 d) of concrete were increased by 115% and 25.3%, respectively compared with the untreated concrete.
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