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Self-healing geological construction materials and structures

Periodic Reporting for period 1 - GEOHEAL (Self-healing geological construction materials and structures)

Período documentado: 2018-01-22 hasta 2020-01-21

Natural stone is one of the most widely used geological construction materials. Although stone masonry structures have the potential to survive over centuries, they may be subject to significant damage and deterioration. Various conservation treatments have been explored for modifying the characteristics of stone, often in the layer closer to the surface. However, treatments may limit the breathability of the material triggering further damage.
Microbially induced carbonate precipitation by bacteria is a promising breathable alternative for the protection of building stone from deterioration. The mineralogical composition and pore structure of most stone types used in construction are favourable for the growth of bacterial communities. However, the protection layer will deteriorate in the same manner as the host material, and although it will give enhanced protection of the stone, it is not infinitely durable. By giving this microbiological protection system the ability to ‘self-heal’ (i.e. respond automatically to heal damage and deterioration, without human intervention), it has the potential to considerably extend the durability of masonry protection.
During the GEOHEAL project, our objectives focused on the development, application and assessment of biological healing and self-healing mechanisms for repairing and preventing damage in cementless construction materials, such as natural stone. The direct compatibility of the biological healing products, such as calcite, with stone materials, enables their application in heritage conservation, and significant reduction of maintenance costs in both existing and new structures. There is the potential for other minerals to act in a similar way, and which may be more durable (e.g. oxalates).
Sporosarcina pasteurii, an aerobic, ureolytic bacterium was used as one of the model microorganisms on two different types of stone. The specific needs of bulk materials and existing structures required the determination of an appropriate protocol for the application and assessment of biological healing. The results demonstrated that S. pasteurii induced sufficient cementation in the near surface region of the specimens to an extent that could be considered protective, yet compatible with the materials’ natural properties.
Calcite precipitation by Sporosarcina ureae, a spore-forming ureolytic bacterium, was instigated in calcitic limestone specimens by supply of an appropriate nutrient solution. Due to its ability to form spores (S. pasteurii did not form spores under the conditions tested in our laboratory), S. ureae is a useful model to demonstrate self-healing using sporulation/germination – exposure due to damage can cause spore germination leading to bacteria which cause mineral formation, whilst spores can survive in the mineral generated. Damage due to salt crystallisation was induced and vegetative cells eliminated (chemical or heat treatment), mimicking worst-case environmental conditions. Further cycles of immersion in cementation solution followed to facilitate spore germination and healing by S. ureae. The results demonstrated the potential of the system to change the microstructure of porous natural stone and therefore increase its resilience to deterioration without adversely affecting its response to moisture cycling. The outcomes of this study also suggest that the system can survive harsh weathering conditions and reactivate, enabling cyclic healing.
Pseudomonas fluorescens was investigated for oxalate formation as an alternative to calcium carbonate precipitation. Laboratory experiments took place to test the ability of the bacteria to induce oxalate precipitation in liquid samples that contained different concentrations of calcium concentrate aqueous solutions. The mineralogical analysis of the filtrated samples showed some promising evidence of oxalate formation. The results are encouraging for further investigation which could lead to the development of a methodology appropriate for stone surfaces’ applications.
Also, research was carried out on the delivery mechanisms employed for nutrients. In so doing, 3-D printed microvascular networks, as well as encapsulation with alginate gel beads were developed and studied in the laboratory. We also designed, produced and tested novel lime-based mortars using the developed biological mechanisms. The results showed the potential of the biological self-healing system to restore efficiently the flexural strength of the lime-based mortars when damage occurred.
The work has produced working healing and self-healing systems for application in masonry and similar materials, as described above, and has led to a number of developments.
Collaboration between Cardiff School of Engineering and Cadw, the Welsh Government's historic environment service, was established to deliver research impact through testing on heritage masonry materials. A heritage site in Wales which exhibits masonry deterioration was identified and following site visits and investigations representative stone types from the site were selected and subjected to experimentation which proved the efficiency of the developed biological system to change the microstructure of the materials exposed to weathering. Yet, the applied treatment did not seem to alter the treated zone of the stone specimens to an extent that could be proven incompatible with the substrate or any adverse aesthetic impact (extremely important in heritage structure management).
The developed biological system was also investigated as a method for repairing and preventing further damage on sedimentary stone-built heritage that has been impacted by ballistics. The biological treatment was applied on stone specimens previously subjected to simulated ballistics under laboratory conditions with the resulting biomineralization sufficient to significantly reduce water absorption, improving durability and suggesting that the method can be considered to sensitively repair structures without necessarily obscuring the damage, considered part of the history in regions that experienced armed conflict.
The GEOHEAL project has attracted interest from wide ranging media sources such as BBC Wales, Forbes, The Engineer, New Civil Engineer, TECH Explorist and others. The GEOHEAL research work has been widely disseminated through articles, workshops/seminars, international conference presentations, invited lectures and Youtube.
Bacterially healed limestone (top) compared to non-healed specimen.