Engineering Microbial-Induced Carbonate Precipitation via Meso-Scale Simulations

Ageing infrastructure is an increasing economic and environmental problem. Economic because, while the production cost of one cubic metre of concrete varies between £45 - £55, it is estimated that currently the direct cost for repairing/maintaining one cubic metre of the same material is around £100. Environmental because production of cement generates 5 to 8% of the world's carbon dioxide emissions. Counteracting the degradation of concrete would lower the requirement for new materials and thus reduce the consumption of resources and the emission of greenhouse gases.

Engineers have proposed a revolutionary solution, which was inspired by nature: self-healing materials able to self-repair as a result of the metabolic activity of bacteria. The main mechanism of concrete healing is the microbial-induced precipitation of calcium carbonate (MICP), which fills the cracks of the damaged material. However, the current approach in microbial self-healing concrete technology is to identify a few species of bacteria that work for limited sets of concretes and environments, and to optimise their MICP performance incrementally by experiments. This leads to solutions that are poorly transferable to new applications, unless new costly experimental campaigns are undertaken. In this proposal we aim to provide a new theoretical basis to predict the most promising combinations of bacteria and concrete, once the application-specific chemical compositions of the concrete of the surrounding environment are identified. This will establish a new paradigm for the digital design of concrete-bacteria systems and will enable technology transfer across the constructions sector.

The approach we propose entails two main steps:
1) developing and validating the world-first simulator of bacterial self-healing in concrete, starting from the length-scale of a single crack (1-100 micrometres) and then transferring information on the kinetics of self-healing to macroscale simulations of concrete mechanics;
2) using the new simulator to inform an experimental campaign aimed at optimising the formulation of self-healing concrete for application in the aggressive chemical environment of an industrial wastewater treatment.

The new simulator will be obtained by building on three existing state-of-the-art simulators that have been very recently developed at Newcastle and Cardiff universities and that model, to date separately, the three main steps involved in self-healing: i) bacterial growth; ii) kinetic evolution of an aggregate of mineral particles immersed in a solution; and iii) macro-mechanics of concrete elements with evolving strength and stiffness.

The experiments will first provide inputs to the simulations and data for their validation. These experiments will be carried out in university laboratories and will address all the relevant length scales, from the nanoscale of the morphology of the mineral phases in concrete, to the microscale of the self-healing process inside single cracks, to the macroscale of self-healing concrete samples.

The validated simulations will be run predictively to simulate the environmental conditions inside a wastewater treatment plant. The simulations will identify the best combinations of bacteria and concrete chemistry to ensure self-healing in such conditions, and the final experiments will produce the simulation-guided self-healing concrete and test their performance in the facilities of our industrial partner Northumbrian Water.

If successful, this project will provide a completely new way to approach the design of self-healing materials via simulations. This would drastically reduce the cost, time, and uncertainty related to developing these materials, enhancing the rate of progress in the field by orders of magnitude and putting the UK at the forefront worldwide in this new technology.

This project will generate impact in key areas identified by the EPSRC Delivery Plan 2016-2020:

1) New materials for a more "sustainable society, with focus on the circular economy". Bacterial self-repair can make concrete structures more durable and resilient, cutting down on monitoring, maintenance, and replacement costs. This, along with developing "greener" cement chemistries and using biofuels for cement production, forms the three pillars to address the challenge of cutting CO2 emissions by 80% by 2050. This will require a large-scale mobilisation in the construction industry and legislation, beyond our project alone. Our project, however, will contribute to and foster this process by targeting the more direct impacts below.

2) A new digital approach to self-healing material design, generating "innovative and disruptive technology". This project will combine cutting-edge modelling techniques from the fields of microbiology and cement science. The resulting simulations, along with appropriate and innovative experiments at all scales, will enable us to design effective biomineralisation strategies depending on the chemistry of the material to be healed and on the surrounding environment. The dissemination of our results and of our multi-scale bio-cement simulator will also stimulate research beyond the directly impacted field of concrete self-healing (see Academic Beneficiaries). The collaboration with Northumbrian Water Group and RM4L partner organisations (which include Costain, Arup, CEMEX and Highways England) will provide a direct industrial demonstration of our simulation-guided approach in the key area of wastewater treatment, validating our new approach at scale in a realistic context.

3) Affordable solutions for national needs, particularly in the infrastructure sectors. Bacterial self-healing can boost the durability and reduce the permeability of damaged structures. Our project will directly address applications to the water infrastructure system, starting with wastewater treatment plants that we can access via the ongoing EPSRC-funded NUFEB project at Newcastle University. With our industrial partner Northumbrian Water Group, we foresee the next application to be on water distribution, as leakages in the UK amount to 20%, or enough to supply around 13 million people. This will require a more thorough analysis of bacteria compatibility and health assurance compared to wastewater treatment, but the new approach in this project will provide the fundamental understanding of self-healing systems that is key for such critical applications. Success in the area of water infrastructure will stimulate applications to other critical infrastructures too, including ground engineering and remediation, transportation, and nuclear waste storage.

4) New opportunities for UK small business to develop application-tailored self-healing solutions. Our new simulations will provide a rapid and economical way to combine bacterial species, nutrients, and cement chemistries, for effective self-healing depending on application-specific environments. This will enable local business and SMEs to specialise in regional applications (e.g. local water or soil chemistries) while small biotechnology labs could find their niche by cultivating application-tailored bacteria. This aligns with the EPSRC "Make it Local, Make it Bespoke" approach. To this end, new links with industries will be created: following this project, our aim will be to develop a first partnership between engineering and biotech companies, leveraging our collaborations with the Northern England N8 partnership network of SMEs and with industrial partners from RM4L.