Scientists in China have found a way to strengthen sand moulds engineered with the help of microbes, delivering a way to make low-carbon cement-based composites and reducing the environmental impact of traditional construction materials.

silk material

The team, from the China campus of Technion - Israel Institute of Technology, discovered that incorporating silk fibroin cross-linked with genipin into the process of microbially induced carbonate precipitation (MICP) significantly improves the engineering properties of sand moulds.

The results are published in a paper, ‘Self-assembled silk fibroin cross-linked with genipin supplements microbial carbonate precipitation in building material’, which was recently accepted for publication by Environmental Microbiology, an Applied Microbiology International publication.

Strength and durability

The findings can contribute to the improvement of engineering properties in building materials, particularly in terms of strength and durability, while also addressing environmental concerns associated with traditional MICP methods.

“Microbially induced carbonate precipitation (MICP) has been shown to enhance the mechanical properties of building materials. Silk fibroin (SF) also displays excellent mechanical properties and can complement the MICP process. The research aims to address the need for enhancing the biomineralization and compressive strength of sand, a fundamental construction material, by utilising SF and MICP,” said Professor Varenyam Achal, a Professor in Environmental Engineering at the Institute.

“This novel biological system resulted in a compressive strength improvement of over 50% compared to control specimens, with a higher calcite content. These modified specimens also had a higher calcite content. These findings suggest that silk fibroin cross-linked with genipin coupled with MICP can play a vital role in developing low-carbon cement-based composites.”

Ureolytic bacterium

To conduct the experiment, the researchers used the ureolytic bacterium Bacillus megaterium CGMCC 1.1741, which showed urease activity in the nutrient broth urea (NBU) media. They also tested the toxicity of genipin on bacterial cells to ensure its safety. 

The genipin cross-linking reaction with silk fibroin was carried out, and the resulting solution was added to the bacterial culture. The sand mould specimens were prepared by layering sand with the optimised concentrations of silk fibroin solution, genipin, and bacterial culture. Control specimens were also prepared using different treatments. 

The sand specimens were then tested for their compressive strength, calcite content, and porosity. 

Compressive strength testing

Unconfined compressive strength testing showed that the sand specimens with the cross-linked silk fibroin and MICP had a significant improvement in strength compared to the control specimens. The calcite content in the modified specimens was also higher, indicating a greater degree of biomineralization. 

The porosity and pore size distribution of the specimens were determined using mercury porosimetry. Microstructural analyses were conducted to analyse the functional groups associated with biomineralization reactions and the surface topography, morphology, and mineralogical composition of the precipitates. 

FTIR analysis showed the presence of specific functional groups related to the biomineralization process coupled with the cross-linked silk fibroin. SEM analysis provided visual insights into the surface characteristics of the samples, while XRD analysis helped identify the specific biominerals present. 

Successful precipitation

X-ray photoelectron spectroscopy (XPS) experiments were conducted to confirm the presence of calcium carbonate in the sand specimens. The results further supported the successful precipitation of calcium carbonate in the modified specimens.

“In conclusion, this study demonstrated that the addition of cross-linked silk fibroin to the MICP process can significantly improve the compressive strength and calcite content of sand mould specimens,” said Professor Achal.  

“The combination of silk fibroin and MICP shows promise in developing low-carbon cement-based composites with enhanced engineering properties. Further research and development in this area can lead to the practical application of this method on a larger scale.”

Significant improvement

One surprising finding, he said, was the significant improvement in compressive strength of sand mould specimens by over 50% with the addition of silk fibroin cross-linked with genipin. 

This level of improvement surpassed previous research findings, indicating the effectiveness of this modified biological system in enhancing the engineering properties of building materials.

“Another surprising discovery was the higher calcite content observed in the modified specimens compared to the control specimens,” Professor Achal said.

“This suggests that the combination of silk fibroin and MICP not only improves the strength but also promotes the formation of calcium carbonate, which further contributes to the enhancement of the material’s properties.

Potential toxicity

“Furthermore, the research highlighted the potential toxicity of genipin on bacterial cells. Although genipin is generally considered to have low acute toxicity, its inhibitory effect on bacterial growth could impact the process of carbonate precipitation. 

“This unexpected finding emphasises the need for careful consideration and optimization of genipin concentrations to ensure the efficacy of the MICP process.

“In summary, the surprising outcomes of this study shed light on the significant potential of silk fibroin cross-linked with genipin in improving the engineering properties of building materials through MICP. These unexpected findings provide valuable insights for future research and the development of innovative and sustainable building materials.

Revolutionising construction

“By utilising the combination of silk fibroin and microbially induced carbonate precipitation (MICP), the compressive strength of sand mould specimens showed a remarkable improvement of over 50%, offering the potential to revolutionise the construction industry as it provides a simple and effective method to enhance the strength of building materials without the need for energy-intensive processes,” Professor Achal said.

“Furthermore, the higher calcite content observed in the modified specimens suggests that this approach not only improves strength but also promotes the formation of calcium carbonate, which can contribute to the durability and stability of the materials. 

“This is particularly relevant in the real world, where sustainable and eco-friendly solutions are increasingly sought after. Moreover, the utilisation of silk fibroin cross-linked with genipin in MICP offers a low-carbon alternative for cement-based composites, reducing the environmental impact associated with traditional construction materials. “This research paves the way for the development of innovative, low-carbon building materials with enhanced engineering properties, addressing the pressing need for sustainable construction practices.

Practical implications

“Overall, the findings of this study have practical implications and hold the potential to transform the construction industry by providing a more eco-friendly and efficient approach to enhancing the strength and engineering properties of building materials. This research contributes to the development of sustainable and low-carbon construction practices, which is increasingly important in addressing environmental challenges.”

The improved compressive strength and higher calcite content observed in the modified specimens demonstrate the effectiveness of the combination of silk fibroin and MICP, not only improving the performance and durability of building materials but also offering a greener alternative to traditional cement-based composites, Professor Achal said. 

“The real-world significance lies in the potential to reduce energy consumption, lower carbon emissions, and promote more sustainable construction practices, thus contributing to a more environmentally conscious and resilient built environment.”

Further exploration

To build on the findings of this research, he said, several areas need further exploration. 

“First, more in-depth studies are required to optimise the concentrations of silk fibroin and genipin for cross-linking, as well as the bacterial culture conditions, to ensure the highest level of effectiveness in improving the engineering properties of building materials,” Professor Achal said. “Further investigation into the long-term durability and stability of the modified specimens is also necessary to assess their performance under various environmental conditions.

“Additionally, it would be beneficial to conduct further research on the scalability and feasibility of implementing this method on a large scale. Exploring potential applications beyond sand moulds, such as in other cementitious materials or construction components, can expand the practicality and versatility of the silk fibroin-genipin-MICP approach.

Toxicity concerns

“Future studies should also focus on addressing any potential toxicity concerns related to genipin by studying its impact on different bacteria strains and evaluating its overall safety. This will help ensure that the process of carbonate precipitation is unhindered by any adverse effects on bacterial growth. 

“Overall, ongoing research and development efforts should aim to translate these promising findings into practical solutions for the construction industry.”

This research was led by Prof. Varenyam Achal who has extensive expertise in the field of microbially induced carbonate precipitation in building materials. His knowledge and experience guided the research process and ensured the rigour and quality of the study. The study was also supported by Master student Ms Jiayu Li who contributed to data collection, analysis, and experimentation. The study was funded by Key Discipline Fund provided by Guangdong Department of Education.

‘Self-assembled silk fibroin cross-linked with genipin supplements microbial carbonate precipitation in building material’ recently appeared in Environmental Microbiology, an Applied Microbiology International publication.

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