Performance enhancement of recycled concrete aggregates using sodium alginate-aided microbial-induced carbonate precipitation

Recycled concrete aggregates (RCA) offer an environmentally friendly alternative to natural aggregates (NA), but their inferior physical properties limit their widespread adoption. While conventional and alginate-aided Microbial-Induced Calcite Precipitation (AIMICP) have shown promise, a systematic optimization of the treatment process has been lacking. This study advances the state-of-the-art by performing the first systematic comparison of six distinct application methods, including spraying, soaking, and varied sequential applications. These methods are tested across a wide range of treatment solution concentrations (up to 20-fold increases) using two bacterial strains, Lysinibillus sphaericus and Priestia megaterium . Measurements of mass gain, water absorption, and abrasion resistance evaluated the effectiveness of these methods. The optimal protocols involved applying the alginate-bacterial solution, by soaking or spraying, either before or simultaneously with the calcium source. These optimized treatments significantly enhanced RCA properties, increasing mass by up to 6 %, reducing water absorption by up to 88 %, and decreasing abrasion mass loss by up to 53 %. Microstructural analysis confirmed the formation of a dense, well-adhered calcite layer that sealed surface pores and micro-cracks. By identifying the most effective application strategies and concentrations, this work establishes an optimized and versatile bio-modification protocol, significantly improving RCA quality and promoting its use in sustainable construction. • Sodium alginate improves microbial-induced calcite precipitation (MICP) in RCA. • Treatment increased mass of RCA by up to 6 %, reducing water absorption by 88 %. • Soaking/spraying RCA with alginate before calcium source yielded superior results. • DSM 28 performed better than DSM 32, evident by mass gain and water absorption. • SEM and XRD confirmed the formation of calcite and its adhesion to RCA surfaces.