A cost-effective bacteria-based self-healing cementitious composite for low-temperature marine applications

Bacteria-based self-healing concrete is an innovative self-healing materials approach, whereby bacteria embedded in concrete can form a crack healing mineral precipitate. Structures made from self-healing concrete promise longer service lives, with associated economic benefits [1]. Despite concretes susceptibility to marine-based degradation phenomena [2], and much of the world’s marine infrastructure being located in cool with freezing climatic zones (annual average temperature < 10°C and average summer temperature generally < 20 °C) [3], research on the development of bacteria-based self-healing concrete has been largely restricted to room temperature freshwater studies [4-14]. The objective of the current project was, therefore, to develop a cost-effective bacteria-based self-healing cementitious composite for application in low-temperature marine environments. The current thesis charts the development of this composite. In Chapter 2 the autogenous healing capacities of ordinary Portland cement (OPC) and blast-furnace slag (BFS) cement mortar specimens submerged in fresh and seawater, are visually quantified and characterised. The BFS cement specimens healed all crack widths up to 104 µm, and OPC specimens healed all crack widths up to 592 µm, after 56 days in seawater. BFS cement specimens healed all crack widths up to 408 µm, and OPC specimens healed all crack widths up to 168 µm, after 56 days in freshwater. OPC specimens in seawater displaying the higher crack healing capacity also demonstrated considerable losses in compressive strength. Differences in performance are attributable to the amount of calcium hydroxide in these mortars and specific ions present in seawater. Chapter 3 reports on the crack healing capacity of seawater submerged mortar specimens with the aid of a crack permeability test. Cracks of defined widths were created in BFS cement specimens allowing reference crack permeability values to be generated for unhealed-specimens against which healed-specimens were quantified. Specimens with 0.2 mm wide cracks demonstrated no water flow after 28 days submersion. Specimens with 0.4 mm cracks demonstrated decreases in water flow of 66% after 28 days submersion and 50 to 53% after 56 days submersion. Chapter 4 presents a modified permeability test for generating crack permeability data for cementitious materials. To gauge for any improvement both the modified and unmodified tests were tested and compared. Cracks were generated in mortar specimens using both tests, the accuracy of these cracks was analysed through stereomicroscopy and computer tomography (CT), and the water flow through the cracks determined. Reduction factors and crack flow models were generated, and the accuracy and reliability of the predictions assessed. All of the models had high predictive accuracies (r2 = 0.97-0.98), while the reliability of these predictions was higher for the models generated with the crack width analysis through stereomicroscopy. The cracks generated by the modified test were more accurate (within 20 µm of the desired crack widths) than those of the unmodified test. The modified test was 30% quicker (10 hours for twenty-one specimens) than the unmodified test at generating the crack permeability data. Further, crack width analysis through stereomicroscopy is currently/generally quicker than analysis through CT. Chapter 5 presents a bacterial isolate and organic mineral precursor compound, as part of a cost-effective healing agent for low-temperature marine concrete applications. Organic compounds were screened based on their cost and concrete compatibility, and bacterial isolates based on their ability to metabolise concrete compatible organic compound and to function in a low-temperature marine concrete crack. Magnesium acetate was the cheapest organic compound screened, and when incorporated (1% of cement weight) in mortar specimens had one of the lowest impacts on compressive strength. Bacterial isolate designated psychrophile (PSY) 5 demonstrated very good growth under saline (3%), high pH (9.2), low-temperature (8ºC) conditions, with sodium lactate as an organic carbon source; and good growth at room temperature using magnesium acetate as an organic carbon source. Further, PSY 5 also demonstrated good spore production when grown on monosodium glutamate at room temperature. Chapter 6 presents a bacteria-based bead for realising self-healing concrete in low-temperature marine environments. The bead, consisting of calcium alginate encapsulating bacterial spores and mineral precursor compounds, was assessed for: oxygen consumption, swelling, and its ability to form an organic-inorganic composite in a simulative marine concrete crack solution (SMCCS) at 8ºC. After six days in the SMCCS, the bacteria-based beads formed a calcite crust on their surface and calcite inclusions in their network, resulting in a calcite-alginate organic-inorganic composite. The beads swell by 300% to a maximum diameter of 3 mm, while theoretical calculations estimate that 0.1 g of the beads are able to produce ~1 mm3 of calcite after 14 days submersion. Swelling and the formation of bacteria induced mineral precipitation providing the bead with considerable crack healing potential. It is estimated, based on the bacteria-based beads costing roughly 0.7 €.kg-1, that bacteria-based self-healing concrete made using these beads would cost 135 €.m-3. Chapter 7 presents a bacteria-based self-healing cementitious composite for application in low-temperature marine environments. The composite was tested for its crack healing capacity with the water permeability test presented in Chapter 4, and for its strength development through compression testing. The composite displayed an excellent crack healing capacity, reducing the permeability of cracks 0.4 mm wide by 95%, and cracks 0.6 mm wide by 93%, following 56 days submersion in artificial seawater at 8ºC. Some conclusions were drawn based on the results obtained during the development of the bacteria-based self-healing cementitious composite: • Visual crack closure is not a measurement for the regain of functional properties such as strength. Visual crack closure, therefore, should only be conducted as a complementary method when measuring the regain of such a property. • The capacity of a cementitious material to heal a crack depends on the width of the crack, thermodynamic considerations, the presence of water and the amount of ions available in the crack. Autogenous crack healing for seawater submerged cementitious materials is principally attributable to the precipitation of aragonite and brucite in the cracks. • The crack healing capacity of a bacteria-based cementitious composite is directly related to the amount of organic carbon available to the bacteria, and so the cheaper the organic mineral precursor compound, the cheaper the bacteria-based self-healing technology in general. Further, the compound must not have an adverse effect on concrete properties when included and must be readily metabolised by the bacteria as part of the healing agent. Magnesium acetate, in the current study, best balanced these criteria making it a good candidate as the organic mineral precursor compound for the healing agent. • A large number of specimen replicates (≥ 7) are required to generate reliable crack permeability data, and hence to quantify the crack healing capacity of cementitious materials through their functional water tightness. • The bacteria-based self-healing cementitious composite displayed an excellent crack healing capacity, reducing the permeability of cracks 0.4 mm wide by 95% and cracks 0.6 mm wide by 93%, following 56 days submersion in artificial seawater at 8ºC. This crack healing capacity was attributable to: mineral precipitation as a result of chemical interactions between the cement paste and seawater; bead swelling; magnesium-based precipitates as a result of chemical interactions between the magnesium of the beads and hydroxide ions of the cement paste; and bacteria-induced mineral precipitation. • The 28-day compressive strength of mortar specimens incorporated with beads was 55% of plain mortar specimens. Reducing the amount of bacteria-based beads will likely increase the compressive strength of the bacteria-based self-healing cementitious composite. Such a reduction, given the swellability of the beads, may have relatively little impact on the healing capacity of the composite. • The bacteria-based self-healing cementitious composite shows great potential for realising self-healing concrete in low-temperature marine environments, while the organic-inorganic healing material formed by the composite represents an exciting avenue for self-healing concrete research. I hope that the work presented herein provide a valuable reference for those interested in bacteria-based self-healing concrete, particularly for application in marine environments, and more generally for those interested in the wider field of self-healing materials research.Materials and Environmen

Bacteria-based self-healing concrete for application in the marine environment

Marine concrete structures are exposed to one of the most hostile of natural environments. Many physical and chemical phenomena are usually interdependent and mutually reinforcing in the deterioration of marine exposed concrete: expansion and microcracking due to physical effects increases concrete permeability paving the way for harmful chemical interactions between seawater, concrete and embedded steel reinforcement. Early research in self-healing concrete has focused on the autogenous ability of hydrates to heal cracks over time, this form of healing is however restricted to early and small cross sectional crack width reductions, while limited research is available on the autogenous healing of concrete incorporating GBFS (Ground blast furnace slag). A novel approach to self-heal concrete is a bioinspired technique, where bacteria immobilized in the concrete are activated through crack induced water ingress, forming a mineral healing precipitate [1]. The current study characterises the autogenous healing of blast furnace slag cement (CEM III/B 42.5 N) mortar cubes submerged in both fresh- and synthetic sea- water, as the first step towards developing a bacteria-based self-healing concrete for application in the marine environment.Structural EngineeringCivil Engineering and Geoscience

Tunable chitosan-alginate capsules for a controlled release of crystallisation inhibitors in mortars

Plasters and renders used in historic monuments are vulnerable to degradation caused by salt weathering. Crystallisation inhibitors (molecules/ions that alter salt crystallisation) mixed into mortars have shown promising results in mitigating salt damage by inhibiting salt crystallisation, promoting salt transport to the evaporating surface, and modifying crystal habit. However, past research suggests that inhibitors easily leach out from mortars, meaning their long-term positive effect is lost. Encapsulation of an inhibitor within a mortar is a potential solution to minimise leaching. Herein, capsules composed of a polyelectrolyte complex of calcium alginate coated in chitosan are investigated for the controlled diffusive release of sodium ferrocyanide, a known NaCl crystallisation inhibitor. Capsules with varying chitosan-calcium alginate ratios are prepared using the extrusion dripping technique. The release of the inhibitor from capsules in solutions of various pH values ranging from 7–13 is investigated. Results show that increasing the capsule’s chitosan to calcium alginate ratio reduces the inhibitor release for all studied solution pH values compared to pure calcium-alginate capsules. Therefore, a controlled inhibitor release can be obtained by tuning the chitosan-alginate ratio. In future, additional tests will be performed to find suitable capsule compositions for optimising their performance when mixed in mortars.Heritage & ArchitectureMaterials and Environmen

An Improved Test for Generating Rapid, Accurate, and Reliable Crack Permeability Data for Cementitious Materials

Presented is a modified test for generating crack permeability data for cementitious materials. Single-parallel cracks were generated in mortar specimens. The width of the cracks was analysed through stereomicroscope and computer tomography, and the water permeability of the cracks was determined. Reduction factors and crack flow models were generated, and the reliability of those predictions was assessed. Cracks analysed through stereomicroscope produced reliable crack permeability predictions (r 2 = 0.97–0.98), highlighting the importance of testing multiple (≥ 7) replicates. The modified test produced accurate cracks (i.e., cracks that were within 20 µm of their desired crack width) and was easy to use allowing rapid permeability data (i.e., 10 h for 21 specimens) to be generated. The modified test will be of great use for those wanting to generate rapid, accurate, and reliable crack permeability data for cementitious materials. Materials and Environmen

Microbially mediated carbonation of marine alkaline minerals: Potential for concrete crack healing

Concrete constructions in the marine environment suffer from chemical attack of sea salts which can induce damage to both the concrete matrix and embedded steel reinforcement. For example, ingress of sulfate and chloride ions can respectively result in detrimental ettringite formation and enhanced corrosion of the steel rebars. The first degradation mechanism is due to development of expansive minerals within the concrete matrix, a process where increased internal pressure can result in crack formation. Cracking dramatically increases matrix porosity further enhancing the ingress rate of detrimental chemicals. However, with controlled mineral expansion comes the possibility of an employable mechanism for autonomous sealing of cracks. In this research project our aim is to study the potential for carbon dioxide-producing bacteria to act as an agent to control expansive carbonation reactions following serpentinization (hydration reactions) of alkaline precursor minerals in marine concrete. Early experimental results showed that seawater derived magnesium ions and carbon dioxide molecules show high potential for self-healing due to the formation of crack-filling expansive minerals. Environmental scanning electron microscopy combined with X-ray element analysis revealed that dolomite (CaMg(CO3)2) formation via intermediate conversion of brucite (Mg(OH)2) is the mechanism underlying this process. Several previously documented studies have reported that dolomite formation, analogous to delayed ettringite formation, can be detrimental to concrete as it can in fact induce crack formation due to increased internal stresses. In this project our aim is to control expansive mineral formation by bacterial CO2 production channelling the process for healing- but not for causing cracks in concrete constructions in the marine environment.Structural EngineeringCivil Engineering and Geoscience

ESEM-BSE coupled with rapid nano-scratching for micro-physicochemical analysis of marine exposed concrete

Ordinary Portland cement (OPC) mortar specimens submerged in sea-water were analysed through environmental scanning electron microscopy (ESEM) in back scattered electron (BSE) mode and nano-scratching. Results from both sets of analysis show the presence of distinct phases associated with aragonite, brucite and cement paste. Phases associated with porosity and aggregates were also distinguishable through the BSE analysis and less defined in the nano-scratch data. This study indicates the powerful nature of coupling BSE image analysis with nano-scratching to obtain information on the quality of concrete. Work is underway to improve the method in order to apply it for better understand on the micro-physicochemical properties of marine exposed concrete.Structural EngineeringCivil Engineering and Geoscience

Optimization of the calcium alginate capsules for self-healing asphalt

It has been demonstrated that calcium alginate capsules can be used as an asphalt healing system by pre-placing rejuvenator (healing agent) into the asphalt mix and releasing the rejuvenator on demand (upon cracking). This healing mechanism relies on the properties of capsules which are determined by the capsule preparation process. In this study, to optimize the calcium alginate capsules, capsules are prepared using varying Alginate/Rejuvenator (A/R) ratios. Light microscope microscopy and Environmental Scanning Electron Microscope (ESEM) are employed to characterize the morphology and microstructure of these capsules. Thermal stability and mechanical property are investigated by thermogravimetric analysis (TGA) and compressive tests. The testing results indicate that higher alginate content results in smaller diameter and lower thermal resistance, but higher compressive strength. The optimum A/R ratio of calcium alginate capsules is found to be 30/70. To prove the effectiveness of the optimized capsules, the capsules are embedded in asphalt mortar beams and a bending and healing program is carried out. The effect of capsule shell material on the mechanical response of asphalt mixture is evaluated through three-point bending on the mortar beams embedded with blank capsules (without the healing agent). Aged mortar beams containing alginate capsules encapsulating rejuvenator demonstrate a higher strength recovery after bending tests, which indicates effective healing due to the release of the rejuvenators from the capsules.Materials and EnvironmentPavement Engineerin

Gel-mediated chemo-mechanical control of calcium carbonate crystal formation

The production of synthetic crystals with controlled shapes and properties is an enticing prospect, yet, the production of such materials is an ongoing challenge. Here, we present a strategy for chemo-mechanically directing the growth of crystals with non-equilibrium structures using a custom-designed double-diffusion cell. We combine chemical additives (e.g., Mg2+ ions) and mechanical confinement (e.g., hydrogel networks) to modulate the growth of calcium carbonate crystals. Specifically, the combination of Mg2+ ions with a strong agarose gel results in calcitic structures, at the gel-glass slide interface, with distinct fried egg-like morphologies and radial or Maltese-cross extinction patterns. In contrast, precipitation with only Mg2+ or agarose results in aragonite spherulites or squished calcite rhombohedra, respectively. Raman spectroscopy and energy dispersive spectroscopy of the “fried eggs” reveals that they are composed of Mg-calcite, which becomes less disordered over time, and the “egg whites” make this transition before the “yolks”. We propose that the “fried eggs” form due to a spherulitic growth process molded by the crystallization-induced delamination of the gel away from the glass slide at the gel-glass interface. In support of the importance of the gel-glass interface, the “fried eggs” do not form when the glass slide is treated with a hydrophobic silane, suppressing heterogeneous nucleation and weakening the interfacial adhesion between the gel and glass, making it easier for the gel to delaminate, thus reducing the confinement effect. As such, this work highlights the important chemo-mechanical role that gel environments can play in crystal formation.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen

Volume fraction, thickness, and permeability of the sealing layer in microbial self-healing concrete containing biogranules

Autonomous repair systems in construction materials have become a promising alternative to current unsustainable and labor-intensive maintenance methods. Biomineralization is a popular route that has been applied to enhance the self-healing capacity of concrete. Various axenic microbial cultures were coupled with protective carriers, and their combination appears to be useful for the development of healing agents for realizing self-healing concrete. The advantageous traits of non-axenic cultures, such as economic feasibility, self-protection, and high specific activity have been neglected so far, and thus the number of studies investigating their performance as healing agents is scarce. Here we present the self-healing performance of a mortar containing a healing agent consisting of non-axenic biogranules with a denitrifying core. Mortar specimens with a defined crack width of 400 μm were used in the experiments and treated with tap water for 28 days. Self-healing was quantified in terms of the crack volume reduction, the thickness of the sealing layer along the crack depth and water permeability under 0.1 bar pressure. Complete visual crack closure was achieved in the bio-based specimens in 28 days, the thickness of the calcite layer was recorded as 10 mm and the healed crack volume was detected as 6%. Upon self-sealing of the specimens, the water permeability decreased by 83%. Overall, non-axenic biogranules with a denitrifying core shows great potential for development of self-healing bioconcrete.Materials and Environmen

Forming Anisotropic Crystal Composites: Assessing the Mechanical Translation of Gel Network Anisotropy to Calcite Crystal Form

The promise of crystal composites with direction-specific properties is an attractive prospect for diverse applications; however, synthetic strategies for realizing such composites remain elusive. Here, we demonstrate that anisotropic agarose gel networks can mechanically "mold" calcite crystal growth, yielding anisotropically structured, single-crystal composites. Drying and rehydration of agarose gel films result in the affine deformation of their fibrous networks to yield fiber alignment parallel to the drying plane. Precipitation of calcium carbonate within these anisotropic networks results in the formation of calcite crystal composite disks oriented parallel to the fibers. The morphology of the disks, revealed by nanocomputed tomography imaging, evolves with time and can be described by linear-elastic fracture mechanics theory, which depends on the ratio between the length of the crystal and the elastoadhesive length of the gel. Precipitation of calcite in uniaxially deformed agarose gel cylinders results in the formation of rice-grain-shaped crystals, suggesting the broad applicability of the approach. These results demonstrate how the anisotropy of compliant networks can translate into the desired crystal composite morphologies. This work highlights the important role organic matrices can play in mechanically "molding" biominerals and provides an exciting platform for fabricating crystal composites with direction-specific and emergent functional properties.Accepted Author ManuscriptMaterials and EnvironmentBT/Environmental Biotechnolog