Biomineralization Process of CaCO3 Precipitation Induced by Bacillus mucilaginous and Its Potential Application in Microbial Self-healing Concrete

Microbial induced calcium carbonate precipitation (MICP) is widely common in nature, which belongs to biomineralization and has been explored carefully in recent decades. The paper studied the effect of temperature, initial pH value and Ca2+ concentration on bacterial growth and carbonic anhydrase activity, and then revealed the biomineralization process through the changes of Ca2+ concentration and calcification rate in alkali environment. Meanwhile, microbial healing agent containing spores and calcium nitrate was prepared and used for the early age concrete cracks repair. The self-healing efficiency was assessed by crack closure rate and water permeability repair rate. The experimental results showed that when the optimal temperature was 30 °C, the pH was 8.0-11.0, and the optimal Ca2+ concentration was 0-90 mM, the bacteria could grow better and the carbonic anhydrase activity was higher. Compared with reference, the crack closure rate with the crack width up to 0.339 mm could reach 95.62% and the water permeability repair rate was 87.54% after 28 d healing time of dry-wet cycles. XRD analysis showed that the precipitates at the crack mouth were calcite CaCO3. Meanwhile, the self-healing mechanism of mortar cracks was discussed in detail. In particular, there is no other pollution in the whole mineralization process, and the self-healing system is environmentally friendly, which provides a novel idea and method for the application of microbial self-healing concrete.

Exploring a high-urease activity Bacillus cereus for self-healing concrete via induced CaCO3 precipitation

The structural integrity and esthetic appeal of concrete can be compromised by concrete cracks. Promise has been shown by microbe-induced calcium carbonate precipitation (MICP) as a solution for concrete cracking, with a focus on urease-producing microorganisms in research. Bacillus cereus was isolated from soil and employed for this purpose in this study due to its high urease activity. The strain exhibited strong tolerance for alkaline media and high salt levels, which grew at a pH of 13 and 4% salt concentration. The repair of concrete cracks with this strain was evaluated by assessing the effects of four different thickeners at varying concentrations. The most effective results were achieved with 10 g/L of sodium carboxymethyl cellulose (CMC-Na). The data showed that over 90% repair of cracks was achieved by this system with an initial water penetration time of 30 s. The study also assessed the quantity and sizes of crystals generated during the bacterial mineralization process over time to improve our understanding of the process. KEY POINTS: • MICP using Bacillus cereus shows potential for repairing concrete cracks. • Strain tolerates alkaline media and high salt levels, growing at pH 13 and 4% salt concentration. • Sodium carboxymethyl cellulose (CMC-Na) at 10 g/L achieved over 90% repair of cracks.

The viability of spores is the key factor for microbial induced calcium carbonate precipitation

While previous studies mainly focused on the total number of spores as an index to predict the calcium precipitation activity (CPA) of bacterial strains, the effect of viability of spores on microbial-induced calcium precipitation (MICP) has remained highly ignored. Therefore, for the first time, we have attempted to optimize the sporulation process in terms of viable spore production and, most importantly, aimed to build a correlation between viable spores and CPA. The results have shown that for the sporulation of Bacillus sp. H4, starch and peptone are the optimal carbon and nitrogen sources, respectively. One gram per liter of sodium chloride promotes CPA and production of viable spores, whereas an increase of sodium chloride concentration beyond 8 g L^-1 significantly reduces CPA without reducing the quantity of viable spores. Exogenous conditions such as seed age, inoculation quantity, and liquid volume only pose slight influence on the sporulation and CPA. Conclusively, the spores produced under optimized conditions are more morphologically uniform and display a 20% increase in CPA compared to pre-optimized spores. Furthermore, by combining the results of heatmap analysis, it can be concluded that not only the quantity, but also the quality of viable spores is important for bacterial strain to develop high CPA and effective MICP process. This study sheds light on the breadth of biomineralization activity based on viable spores and is an imperative step toward the intelligible design of MICP-based engineering solutions. KEY POINTS: • Viability of spores is a key controlling factor in calcium precipitation activity (CPA). • Spores produced under optimized conditions display a 20% increase in CPA. • Quality of viable spores is imperative for bacterial strains to develop high CPA.

Recycling industrial wastes into self-healing concrete: A review

Self-healing concrete is an innovative construction material designed to repair its cracks autogenously or autonomously. The self-healing effect reduces the need for maintenance and increases the longevity of concrete structures, bringing environmental and economic benefits. However, the developed methods to improve self-healing performance, e.g., incorporating advanced techniques or expensive chemical healing agents, significantly increase the cost of concrete manufacture. There is worldwide interest in using waste materials to reduce the cost of self-healing concrete, and a significant amount of studies have been performed on this topic. A review of research on waste-derived self-healing concrete is presented in this paper. The wastes were used in both autogenous and autonomous self-healing approaches, such as mineral admixture, bacteria-based technology, and engineered cementitious composite; different environmental conditions may significantly influence self-healing efficiency due to different reaction mechanisms. In general, waste materials could be reused to manufacture self-healing concrete if adopting appropriate mix design and treatment methods. Self-healing concrete made with various industrial wastes is an efficient way to reduce the manufacturing cost and promote its application in practice.

A review on the potential of filamentous fungi for microbial self-healing of concrete

Concrete is the most used construction material worldwide due to its abundant availability and inherent ease of manufacturing and application. However, the material bears several drawbacks such as the high susceptibility for crack formation, leading to reinforcement corrosion and structural degradation. Extensive research has therefore been performed on the use of microorganisms for biologically mediated self-healing of concrete by means of CaCO₃ precipitation. Recently, filamentous fungi have been recognized as high-potential microorganisms for this application as their hyphae grow in an interwoven three-dimensional network which serves as nucleation site for CaCO₃ precipitation to heal the crack. This potential is corroborated by the current state of the art on fungi-mediated self-healing concrete, which is not yet extensive but valuable to direct further research. In this review, we aim to broaden the perspectives on the use of fungi for concrete self-healing applications by first summarizing the major progress made in the field of microbial self-healing of concrete and then discussing pioneering work that has been done with fungi. Starting from insights and hypotheses on the types and principles of biomineralization that occur during microbial self-healing, novel potentially promising candidate species are proposed based on their abilities to promote CaCO₃ formation or to survive in extreme conditions that are relevant for concrete. Additionally, an overview will be provided on the challenges, knowledge gaps and future perspectives in the field of fungi-mediated self-healing concrete.

Complementing urea hydrolysis and nitrate reduction for improved microbially induced calcium carbonate precipitation

Microbial-induced CaCO₃ precipitation has been widely applied in bacterial-based self-healing concrete. However, the limited biogenetic CaCO₃ production by bacteria after they were introduced into the incompatible concrete matrix is a major challenge of this technology. In the present study, the potential of combining two metabolic pathways, urea hydrolysis and nitrate reduction, simultaneously in one bacteria strain for improving the bacterial CaCO₃ yield has been investigated. One bacterial strain, Ralstonia eutropha H16, which has the highest Ca²⁺ tolerance and is capable of performing both urea hydrolysis and nitrate reduction in combined media was selected among three bacterial candidates based on the enzymatic examinations. Results showed that H16 does not need oxygen for urea hydrolysis and urease activity was determined primarily by cell concentration. However, the additional urea in the combined medium slowed down the nitrate reduction rate to 7 days until full NO₃^- decomposition. Moreover, the nitrate reduction of H16 was significantly restricted by an increased Ca²⁺ ion concentration in the media. Nevertheless, the overall CaCO₃ precipitation yield can be improved by 20 to 30% after optimization through the combination of two metabolic pathways. The highest total CaCO₃ precipitation yield achieved in an orthogonal experiment was 14 g/L. It can be concluded that Ralstonia eutropha H16 is a suitable bacterium for simultaneous activation of urea hydrolysis and nitrate reduction for improving the CaCO₃ precipitation and it can be studied later, on activation of multiple metabolic pathways in bacteria-based self-healing concrete.

Experimental data of bio self-healing concrete incubated in saturated natural soil

The provision of suitable incubation environments is vital for successful implementation of bio self-healing concrete (bio-concrete). We investigated the effect of soil incubation to examine if the self-healing process can be activated in comparison with the conventional incubation environment (water). The data was collected from laboratory-scale experiments conducted on mortar specimens. The mortar was impregnated with Bacillus subtilis and this bacteria was encapsulated in calcium alginate for protection from the production process. The mortar specimens were mechanically cracked and then incubated within fine-grained fully saturated natural soil for about 4 weeks. The cracks were inspected before and after incubation by light microscopy to evaluate the healing ratio. The mineral precipitations on crack surfaces were examined by Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectrometry (EDX). The data reflects the efficiency of bio-concrete for certain structures such as tunnels and deep foundation, where concrete elements are exposed to ground conditions.

Current challenges and future directions for bacterial self-healing concrete

Microbially induced calcium carbonate precipitation (MICP) has been widely explored and applied in the field of environmental engineering over the last decade. Calcium carbonate is naturally precipitated as a byproduct of various microbial metabolic activities. This biological process was brought into practical use to restore construction materials, strengthen and remediate soil, and sequester carbon. MICP has also been extensively examined for applications in self-healing concrete. Biogenic crack repair helps mitigate the high maintenance costs of concrete in an eco-friendly manner. In this process, calcium carbonate precipitation (CCP)-capable bacteria and nutrients are embedded inside the concrete. These bacteria are expected to increase the durability of the concrete by precipitating calcium carbonate in situ to heal cracks that develop in the concrete. However, several challenges exist with respect to embedding such bacteria; harsh conditions in concrete matrices are unsuitable for bacterial life, including high alkalinity (pH up to 13), high temperatures during manufacturing processes, and limited oxygen supply. Additionally, many biological factors, including the optimum conditions for MICP, the molecular mechanisms involved in MICP, the specific microorganisms suitable for application in concrete, the survival characteristics of the microorganisms embedded in concrete, and the amount of MICP in concrete, remain unclear. In this paper, metabolic pathways that result in conditions favorable for calcium carbonate precipitation, current and potential applications in concrete, and the remaining biological challenges are reviewed.