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The use of mixed recycled aggregates (RMA) for concrete is limited according to current German standards (DIN 1045-2). The coarse natural aggregate is only allowed to be replaced proportionally. RMAs contain a high amount of brick material, which results in high porosity and water absorption properties. This primarily influences the consistency of fresh concrete. If recycled aggregate consists exclusively of crushed bricks or masonry construction and demolition waste, it is also referred to as recycled brick aggregate (RBA), which is not yet regulated for use in recycled aggregate concrete. For this reason, a biodeposition approach was chosen to optimize the properties of the RBA. There are various applications based on microbial-induced calcium carbonate precipitation (MICP), whose promising approaches in construction have already proven effective [1]. This study tested an MICP treatment designed to optimize the water absorption properties of RBA. A bacterial culture of Sporosarcina pasteurii DSM 33 was used in combination with urea and calcium chloride to precipitate calcium carbonate. The aim is to use the CaCO$_{3}$ precipitate to form a layer on the surface of the RBA, thereby filling the pore space and significantly reducing the porosity [2]. For the treatment of RBA, a process with multiple short immersion intervals and intermediate vacuum extraction was used to apply the liquid MICP components. Up to 5 treatment intervals were carried out, and the water absorption was determined according to DIN EN 1097-6:2022-05 after each step. The results show a trend toward a steady reduction in water absorption, depending on the number of MICP treatments, where the initial water absorption can be reduced by 40.6%. García-González et al. [2] found similar results and stated that ceramic aggregate may offer particular advantages for MICP treatment due to its high surface roughness. In addition, changes in bulk density and apparent grain density were determined, which are directly associated with a reduction in porosity. According to Sun et al. [3], the reduction in porosity primarily affects pores in the range of 10 – 300 nm, with capillary pores or large pores (>1000 nm) decreasing to a lesser extent. Mineralogical investigations (SEM and XRD) confirm the formation of CaCO$_{3}$ on the surface of the RBA, whereas mainly vaterite crystals could be detected. MICP treatment of recycled aggregate appears to be an effective approach for reducing porosity and water absorption. However, further research is needed to investigate the pore space filling mechanism with precipitated CaCO$_{3}$ in order to optimize the MICP treatment method.
| References | [1] De Belie, N. (2016). Application of bacteria in concrete: a critical evaluation of the current status. RILEM Technical Letters, 1, 56-61. [2] García-González, J., Rodríguez-Robles, D., Wang, J., De Belie, N., Morán-del Pozo, J. M., Guerra-Romero, M. I., & Juan-Valdés, A. (2017). Quality improvement of mixed and ceramic recycled aggregates by biodeposition of calcium carbonate. Construction and Building Materials, 154, 1015-1023. [3] Sun, Y., Liu, K., Sun, D., Jiang, N., Xu, W., & Wang, A. (2024). Evaluation of urea hydrolysis for MICP technique applied in recycled aggregate: concentration of urea and bacterial spores. Construction and Building Materials, 419, 135366. |
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| Country | Germany |
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