We present two novel experimental methods to follow global and local pH changes on a microscale in bio-cementation processes.

Microbial-induced calcium carbonate precipitation (MICP) is a biological process inducing biomineralization of CaCO3. This can be used to form a solid, concrete-like material. To be able to use MICP successfully to produce solid materials, it is important to understand the formation process of the material in detail. It is well known that crystallization surfaces can influence the precipitation process. Therefore, we present in this contribution a systematic study investigating the influence of calcite seeds on the MICP process. We focus on the changes in the pH and changes of the optical density (OD) signal measured with absorption spectroscopy to analyze the precipitation process. Furthermore, optical microscopy was used to visualize the precipitation processes in the sample and connect them to changes in the pH and OD. We show, that there is a significant difference in the pH evolution between samples with and without calcite seeds present and that the shape of the pH evolution and the changes in OD can give detailed information about the mineral precipitation and transformations. In the presented experiments we show, that amorphous calcium carbonate (ACC) can also precipitate in the presence of initial calcite seeds and this can have implications for consolidated MICP materials.

Biocementation is commonly based on microbial-induced carbonate precipitation (MICP) or enzyme-induced carbonate precipitation (EICP), where biomineralization of $$\text {CaCO}_{3}$$ CaCO 3 in a granular medium is used to produce a sustainable, consolidated porous material. The successful implementation of biocementation in large-scale applications requires detailed knowledge about the micro-scale processes of $$\text {CaCO}_{3}$$ CaCO 3 precipitation and grain consolidation. For this purpose, we present a microscopy sample cell that enables real time and in situ observations of the precipitation of $$\text {CaCO}_{3}$$ CaCO 3 in the presence of sand grains and calcite seeds. In this study, the sample cell is used in combination with confocal laser scanning microscopy (CLSM) which allows the monitoring in situ of local pH during the reaction. The sample cell can be disassembled at the end of the experiment, so that the precipitated crystals can be characterized with Raman microspectroscopy and scanning electron microscopy (SEM) without disturbing the sample. The combination of the real time and in situ monitoring of the precipitation process with the possibility to characterize the precipitated crystals without further sample processing, offers a powerful tool for knowledge-based improvements of biocementation.

Concrete is the second most consumed product by humans, after water. However, the production of cement, which is used as a binding material in concrete, causes more than 5% of anthropogenic CO 2 emissions and has therefore a significant contribution to climate change and global warming. Due to increasing environmental awareness and international climate goals, there is a need for emission-reduced materials, that can replace conventional concrete in certain applications. One path to produce a solid, concrete-like construction material is microbial-induced calcium carbonate precipitation (MICP). As a calcium source in MICP, crushed limestone, which mainly consists out of CaCO 3 , can be dissolved with acids, for example lactic acid. The pH evolution during crystallization and dissolution processes provides important information about kinetics of the reactions. However, previous research on MICP has mainly been focused on macro-scale pH evolution and on characterization of the finished material. To get a better understanding of MICP it is important to be able to follow also local pH changes in a sample. In this work we present a new method to study processes of MICP at micro-scale in situ and in real time. We present two different methods to monitor the pH changes during the precipitation process of CaCO 3 . In the first method, the average pHs of small sample volumes are measured in real time, and pH changes are subsequently correlated with processes in the sample by comparing to optical microscope results. The second method is introduced to follow local pH changes at a grain scale in situ and in real time. Furthermore, local pH changes during the dissolution of CaCO 3 crystals are monitored. We demonstrate that these two methods are powerful tools to investigate pH changes for both MICP precipitation and CaCO 3 dissolution for knowledge-based improvement of MICP-based material properties.