Microstructural Responses of Cementitious Materials to Substitutions with Fine Antimicrobial Aggregates

Unlike other construction materials like wood or steel, concrete possesses excellent resilience making it an ideal material for building structures to transport, contain and hold water. However, while concrete has been known for its durability, municipalities and wastewater utilities around the world now recognize that the concrete present in these essential infrastructure elements is becoming severely affected by biogenic corrosion, also known as microbially induced concrete corrosion (MICC). This phenomenon is not new, it has been acknowledged as a potential problem for over a hundred years [1]. The microorganisms responsible for this corrosion, have been linked to the generation of sulfuric acid from common sewer gases. This biogenic acid promotes dissolution of calcium-containing minerals (i.e. calcium-silicate-hydrates (C-S-H) and calcium hydroxides (Portlandite)) responsible for the strength of the concrete structures. In response, concrete protection methods have been developed to include new formulations that obtain more impermeable concrete, protective coatings and paintings on concrete surfaces, and the use of impermeable plastic liners. All these technologies focus on developing acid resistant materials instead of attacking the primary cause: acidogenic sulfur-oxidizing bacteria (SOBs). Little research has been conducted on materials which limit or inhibit the activity of these acidophilic bacteria. One of the most recent and promising approaches to inhibit SOBs is the substitution of metal-laden granular activated carbon (GAC) particles and basic oxygen furnace steel slag (BOF-S) for a fraction of the fine aggregates traditionally used in cement-based materials. While the antimicrobial properties of these replacements have been demonstrated [2], there are no studies related to the effects that these substitutions may have on the microstructural and mechanical properties of the cement-based materials. In response to this research gap, the central aim of this work was to study the effects that these antimicrobial aggregates (i.e. metal-laden GAC or BOF-S particles) have on ordinary Portland cement formulations. A comprehensive characterization of these antimicrobial aggregates was completed along with stringent characterization of the mechanical properties (compressive and tensile strengths), effects on microstructure (porosity, mineralogy), metal mobilization (elemental microprobe analysis), workability (flowability, setting times) and early hydration reactions (isothermal calorimetry).</p

Influence of copper-impregnated basic oxygen furnace slag on the fresh- and hardened-state properties of antimicrobial mortars

Microbially induced concrete corrosion (MICC) is recognized as one of the main degradation mechanisms of sewer infrastructure worldwide. To help control this problem, a beneficial reuse path for basic oxygen furnace slag (BOFS) has emerged in which the incorporation of copper-laden BOFS particles into cementitious materials inhibits the growth of microorganisms responsible for MICC. This study investigated the effect of substituting fine aggregate with copper-laden BOFS particles (0.30–0.85 mm) on the hydration and microstructural evolution of portland cement mortars. In addition, the fate of copper in the cured cementitious matrix is elucidated and reported herein. As revealed by isothermal calorimetry, the total evolved heat at the end of the testing period (118 h) was similar when up to 40% of the fine aggregate mass was replaced with copper-laden BOFS particles of similar size, while delays in setting times were observed. Analysis of microstructural evolution using quantitative X-ray diffraction (QXRD) showed higher C–S–H contents when fine aggregate was replaced with copper-laden BOFS, indicating copper-laden BOFS exhibited some degree of pozzolanic reactivity. Electron microprobe analysis (EMPA) revealed that, while trace amounts of copper could be detected throughout the cement matrix, copper was predominantly localized in a 100 μm spherical region surrounding BOFS particles. Moreover, strong binding capacity of Fe-rich BOFS particles for copper was observed. Finally, compressive strengths of mixtures analyzed herein were not affected by the presence of copper-laden BOFS

Assessing the potential application of bacteria-based self-healing cementitious materials for enhancing durability of wastewater treatment infrastructure

Wastewater treatment plants (WWTPs) around the world are mainly built using concrete. The continuous exposure to wastewater affects the durability of concrete structures and requires costly maintenance or replacement. Concrete production and repair represents ∼8% of the global anthropogenic CO2 emissions due to the use of cement, thus contributing to climate change. Developing a more sustainable cementitious material is therefore required for this vital health infrastructure. In this study, the feasibility of using bacteria-based self-healing (BBSH) cementitious materials for WWTPs is assessed by exposing BBSH mortar prisms to a continuous municipal wastewater flow and comparing their self-healing capacity to equivalent mortar prisms exposed to tap water. Microscopy imaging, water-flow tests and micro-CT analyses were performed to evaluate the self-healing efficiency of the mortar prisms, while SEM-EDX and Raman spectroscopy were used to characterise the healing products. Our work represents the first systematic study of the healing potential of BBSH in mortar exposed to wastewater. The results indicate that the purposely added bacteria are able to induce calcium carbonate precipitation when exposed to wastewater conditions. Moreover, if additional sources of calcium and carbon are embedded within the cement matrix, the rich bacterial community inherently present in the wastewater is capable of inducing calcium carbonate precipitation, even if no bacteria are purposely added to the mortar. The results of this study offer promising avenues for the construction of more sustainable wastewater infrastructure, with the potential of significantly reducing costs and simplifying the production process of BBSH concretes for this specific application

Advancements in bacteria based self-healing concrete and the promise of modelling

In the last two decades self-healing of concrete through microbial based carbonate precipitation has emerged as a promising technology for making concrete structures more resilient and sustainable. Currently, progress in the field is achieved mainly through physical experiments, but their duration and cost are barriers to innovation and keep the number of large scale applications still very limited. Modelling and simulation of the phenomena underlying microbial based healing of concrete may provide a key to complement the experimental efforts, but their development is still in its infancy. In this review, we briefly present the field, introduce some key aspects emerged from the experiments, present the main ongoing developments in modelling and simulation of mineral and microbial systems, and discuss how their synergy may be accomplished to speed up progress in the near future

Air-entraining admixtures as a protection method for bacterial spores in self-healing cementitious composites:Healing evaluation of early and later-age cracks

Costs associated with the encapsulation process of bacterial spores continue to be a limiting factor for the commercialisation of self-healing cementitious materials. The feasibility of using air-entraining admixtures (AEAs) as an economical and straightforward encapsulation method for bacterial spores was evaluated to heal cracks (∼0.50 mm) that were formed at an early (28 days) or later age (9 months). Three AEAs, commonly used in concrete industry, were compared to a successfully proven protection method (i.e., via aerated concrete granules (ACGs)). In this regard, only one of the three AEAs investigated improved the healing performance when compared to an equivalent mix using bacterial spores encapsulated in ACGs. Healing ratios obtained with this successful AEA were 59.6% and 46.2% higher than the results observed for the ACGs-containing mix when the cracking age was 28 days and 9 months, respectively. Moreover, water penetration resistance was increased by 18.1% or presented very similar values (∼84%) after 56 days of healing for early or later-formed cracks, respectively. Moreover, a simple cost analysis was conducted to confirm the significant economic benefits of using AEAs to protect directly added bacterial spores. In this regard, the cost of using AEAs is about 13 times lower than for ACGs. Therefore, this study provides for the first time, evidence of the feasibility of using AEAs to protect bacterial spores, opening the doors to the development of bespoke AEAs to design cost-efficient self-healing cementitious materials.</p

Bacteria-based self-healing concrete− A life cycle assessment perspective

A life cycle assessment (LCA) was utilised to evaluate the environmental impact of bacteria-based self-healing concretes (BBSHCs), where non-ureolytic bacterial endospores are encapsulated in porous calcium silicate granules. Findings reveal that 1 m3 of BBSHC has an overall 85% higher environmental impact than equivalent conventional concrete, primarily due to calcium nitrate and polyvinyl acetate. Furthermore, BBSHC has a 36% larger embodied carbon footprint (120 kg CO2 eq) and a 51% larger water footprint (260 L). However, by selectively incorporating BBSHC in specific areas of reinforced concrete structures, leveraging its inherent self-healing properties to deliberately allow wider crack widths, and consequently, reduce the amount of non-structural steel needed to control early-age cracking, sustainability improvements ranging from 12% to 50% can be achieved depending on the impact category. In this regard, a BBSHC-structure can potentially save up to 51 kg CO2 eq per m3

Dataset for "Air-entraining admixtures as a protection method for bacterial spores in self-healing cementitious composites: Healing evaluation of early and later-age cracks"

This dataset contains the experimental results obtained when evaluating the feasibility of using air-entraining agents (AEAs) as an economical and straightforward encapsulation method for bacterial spores in bacteria-based self-healing cementitious materials when cracks (~ 0.50 mm) are formed at an early (28 days) or later age (9 months). The data included here corresponds to the following test methods: 1) Determination of air-content (BS EN 1015-7 (Method A- Pressure method)). 2) Flowability (BS EN 1015-3). 3) Compressive and flexural strength of mortar specimens (BS EN 1015-11). 4) Images and data used for determining the mean healing ratio of the complete cracks using image binarization. 5) Crack healing (%) as a function of the initial crack width for mortar specimens cracked after 28-days or 9-months of curing. 6) Improvement in water penetration resistance following RILEM test method 11.4. 7) Raman spectroscopy for 9-months-old mortar specimens after a healing period of 56 days. 8) Images and data used for determining the micro-bubbles total area and size distribution.The methodology for this dataset is contained in the associated paper