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Microbially Induced Calcium Carbonate Precipitation (MICP) is a natural biocementation that takes place in corals, stromatolites and beach rocks. In recent years, researchers have explored the emulation of this process as a sustainable alternative of engineered cement. Although the natural process is undoubtedly sustainable, its engineered variant deviates substantially from the natural process. In this paper, we investigate the environmental and economic performance of the engineered biocementation process vis-à-vis present manufacturing of calcium carbonate. SimaPro 8.0 software and the Ecoinvent V2.2 database were used for materials inputs and AUSLCI along with Cumulative Energy Demand 2.01 software were used for carbon footprint and eutrophication potential. Our results show that different metabolic pathways of MICP have considerably varying environmental impact. We observe that nature performs MICP sustainably at ambient conditions and geological time scales utilizing naturally occurring sources of carbon and calcium at micromoles concentrations. Due to the mandate on duration of construction projects, highly purified reactants in a high concentration are used in the engineered process. This has a negative environmental impact. We conclude that the sustainability of engineered MICP is directly impacted by the metabolic pathway of bacteria as well as the purity of the input chemicals. A few biotic processes are superior to the present industrial process for manufacturing calcium carbonate if ingredients of laboratory grade purity are replaced by industrial grade products. A bigger dividend can be obtained by introducing industry by-products as nutrients. The results of this study help to direct future research for developing sustainable biocement for the construction industry.
Microbially Induced Calcium Carbonate Precipitation (MICP) is a form of mineralisation that is responsible for major carbonate formations in nature such as corals, stromatolites and beach rocks [1]. Similar to industrial cement, the grains of sand can be bound together through MICP. Thus, MICP is biocementation that occurs at ambient conditions with no additional source of energy and with water as the solvent. The construction industry, on the other hand, is heavily reliant on ordinary Portland cement (OPC) that produces roughly the same amount of greenhouse gases as its own weight [2]. Worldwide, nearly 3.6 billion tonnes of OPC is produced, which accounts for approximately 6% of anthropological greenhouse gases. Researchers are exploring the emulation of the natural cementation process as a means of achieving sustainability in construction. Dejong, et al. [3] conclude that harnessing the biological processes that occur in natural formations is the next transformative practise for geotechnical engineering. Biocementation is envisaged to be sustainable due to several factors such as a low embodied energy, reversibility and recyclability and self-healing [4, 5, 6, 7]. However, there has been little attempt to objectively examine the sustainability of engineered biocementation. Biocementation has been happening in nature for millions of years, and there is no doubt that the process is sustainable. However, it occurs over geological timeframes utilizing naturally available reactants often at micromolar concentrations (Figure 1a). When emulating biocementation for construction applications, it becomes necessary to enrich both the bacteria and the nutrient media to allow the process to fit it into the speed, reliability and performance mandates of the construction industry [8, 9, 10, 11, 12] (Figure 1b). Investigators have typically used purified laboratory grade chemicals at a much higher concentration than what occurs in nature [8, 9, 10, 11, 12, 13, 14, 15]. The purification process of the chemicals is likely to consume considerable energy. Thus, it is important to evaluate the input media to ascertain the environmental impact of biocementation. Clearly, biocementation can be performed in a number of ways, and a methodology to evaluate the processes to identify the best among several alternatives is essential. Life cycle analysis has emerged as a great tool for evaluating sustainability [16], particularly in the areas of sustainable housing technologies, building assessments [17, 18, 19, 20] and commonly used construction materials such as cement [21, 22, 23] and plastic wastes [24] aided by the enormous amount of field data over a long time. There are few studies on industrial-scale projects with biocement to conduct a full-scale life cycle analysis, although the importance of such a study cannot be overestimated. An initial embodied energy analysis on biocementation by urea hydrolysis reveals that the manufacturing processes for urea and calcium chloride are the key contributors to embodied energy, while for ordinary Portland cement, the key contributor to energy usage is the burning of limestone during the calcination process [25]. The production of ordinary Portland cement is estimated to consume approximately 6.21 MJ/kg [26] of energy while urea consumes 30.54 MJ/kg [27] and calcium chloride 11.76 MJ/kg [28]. Field scale experiments demonstrate that 0.6 kg urea and 1.1 kg calcium chloride is required to produce 1 kg of calcium carbonate for biocement through the ureolytic pathway [10, 29]. Thus, there is an imperative of examining the sustainability of engineered biocementation. Early indications on the likely pathways to sustainable options based on already available extensive laboratory experiments would be valuable to chart the research directions towards sustainable biocement technologies for the field. This paper employs the life cycle analysis methodologies on the available data to identify the developments that are likely to have the deepest impact on the sustainability of biocement technology.
In biocementation, the bacteria nucleate the conversion of a water-soluble source of calcium such as calcium chloride into an insoluble one such as calcium carbonate. The bacterial cells secure themselves in the grooves of the substrate and act as the nucleation site for the growth of the calcium carbonate crystals (Figure 2a). Thus, the crystals grip the grains at several places and coat it and gradually grow into mesocrystals (Figure 2b). When the mesocrystals from different sand grains join together, they become cemented (Figure 2c). Through quantitative scans of the substrate, the extent of cementing of the grains can be established (Figure 2d).
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During MICP, naturally occurring microbes act as a biocatalyst for the precipitation of calcium carbonate [3] (Figure 2). MICP can occur via autotrophic or heterotrophic pathways (Table 1), although presently, MICP via urea hydrolysis is the most investigated route for construction applications due to its simplicity [31]. In the case of heterotrophs, mineralization takes place as a by-product of the metabolic activity of the bacteria. In these cases, heterotrophic bacteria utilize organic compounds, such as urea, for energy and cellular material [31, 32]. The metabolic activity of the bacteria causes a rise in the pH of the surrounding pore water, resulting in supersaturated conditions and allowing for the precipitation of carbonates when in the presence of an inorganic calcium source. In the cases of autotrophs, bacteria obtain energy from the sun and reduce the atmospheric carbon (in the form of carbon dioxide) for energy and cellular material [1]. Heterotrophic pathways include urea hydrolysis, denitrification, ammonification and methane reduction, while autotrophic pathways include photosynthesis and MICP through carbonic anhydrase-producing bacteria. In both heterotrophic and autotrophic pathways, the rate of MICP is controlled by factors such as the metabolic activity of the bacteria, the availability of calcium, the dissolved inorganic carbon source and the pH of the surrounding environment [3, 12, 13, 33]. Due to differing levels of nutrients and waste products produced during the reaction, different pathways to MICP are likely to have varying impacts on the environment. Their efficacy in terms of sustainability is an important question to answer, particularly when developing MICP as a construction technique for industrial applications.
The economic cost of biocement has been estimated by a few researchers. De Muznck, et al. [43] conducted an economic assessment of biocement as a surface treatment for building materials. The urea/calcium chloride cementation media was found to be the highest contributor to costs, responsible for approximately 47% of the overall cost of the treatment. The cost of application and added value of the product accounted for 41% of the overall cost while the bacterial growth media was approximately 12% or the total cost. The overall cost of biocement as a surface treatment was 23–28 EUR/sqm, (AUD 34–42).
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