Microbially mediated calcium carbonate precipitation on normal and lightweight concrete
Highlights
► Ca(CO)3 crystals precipitated by bacteria had different shapes by types of bacteria. ► Bacterial treatment reduced water absorption of both normal and lightweight concrete. ► Bacillus sphaericus had higher treatment efficiency for concrete surface than Sporosarcina pasteurii.
Introduction
The application of microbiology to the field of construction has been increasingly popular as its high potential to overcome many limits of the conventional technology. This application, called bioremediation, can make concrete structure environmentally friendly. One promising application is to cleanup unwanted and/or harmful contaminants on concrete. For example, hydrolytic enzymes, such as lipase that cut ester bonds, have been tried to remove aged acrylic resin coatings [3]. A sulfur-oxidizing bacterium Thiobacilli was employed to clean fouled concrete surfaces [6]. Pseudomonas species were also proposed to remove toxic contaminants, such as n-hexadecane, naphthalene, and 2,4-dinitrotoluene (DNT) from concrete [2], [21]. Pseudomonas putida was also proposed to remediate contaminated concrete [21].
The most promising and feasible application, however, is believed to enhance durability of concrete structures via microbially-mediated formation of nano-sized particles such as calcium carbonate (CaCO3) crystals. Various products, such as water repellents, pore-blockers, and coatings, have been applied to diminish the uptake of water or to protect the chemical deterioration of concrete [8], [9], [12], [15]. However, these conventional products, such as organic materials, have distinct disadvantages including: (1) different thermal expansion rates between the product and concrete surface; (2) degradation and/or delamination over time; and (3) the risk of environmental pollution [8], [9]. Compared with treatment methods using conventional organic products, the microbial treatments for enhancing durability of concrete structures have a number of advantages such as: (1) similar thermal expansion properties between the microbially precipitated calcium carbonate and concrete surface; (2) environmentally friendly characteristics; (3) potential for self-healing [8], [9].
The precipitation of calcium carbonate crystal particles via microbes, a process known as bio-mineralization is well understood [19], [24]. Considering that the sizes of the responsible bacterial cells are around 1 μm both the cells and their media containing reactants such as urea and calcium ion can permeate deep into micro- and macro-pores and the interface between aggregate and paste (Interfacial transition zone, ITZ) of concrete structures [23]. Once the pores and the ITZ are filled with microbially-precipitated calcium carbonate, some characteristics of the concrete, such as durability and/or water resistant have known to be enhanced [24].
To unravel the mechanism, Ramachandran et al. [23] proposed a filling method for cracks in concrete using sand mixed with Bacillus pasteurii. Ghosh et al. [11] and Ghosh et al. [10] studied the effects of Shewanella species on the strength and chemical composition of hardened cement mortar. De Muynck et al. [8] and De Muynck et al. [9] found that calcium carbonate precipitation by Bacillus sphaericus could improve the durability of hardened cement paste and the performance was compared with that provided by conventional treatments. Qian et al. [22] applied B. pasteurii as corrosion protection for the surface of cementitious materials. Jonkers et al. [13] introduced the application of Bacillus pseudofirmus and Bacillus cohnii as self-healing agents for the development of sustainable concrete. Van Tittelboom et al. [26] used B. sphaericus and silica gel to repair cracks in concrete. Wiktor and Jonkers [27] reported experimental results on the crack-healing in concrete with lightweight aggregates containing Bacillus alkalinitrilicus. Chahal et al. [4] studied the effect of the addition of S. pasteurii with different concentration on the compressive strength, water absorption and rapid chloride permeability of fly ash concrete.
This study compares the characteristics of calcium carbonate microbially-precipitated on concrete by two bacterial species, Sporosarcina pasteurii and B. sphaericus, both of which are widely used to make cementitious composites [8], [9], [22]. Two types of concrete, normal and lightweight concrete, were used. A macrographic study of the distributions of calcium carbonate precipitation on the concrete specimens was carried out to investigate the effects of the bacteria. Scanning electron microscope (SEM) images and energy dispersive spectroscopy (EDS) spectra were also employed to analyze the shapes and distributions of calcium carbonate crystals at a microscale level. The X-ray diffraction (XRD) analysis was carried out to characterize the crystalline phases of the calcium carbonate crystals formed in liquid medium with and without cells. In addition, capillary water absorption tests of the concrete specimens were conducted to evaluate the effects of microbiological precipitation of calcium carbonate on the properties of moisture transport, which is related to the durability of the concrete.
Section snippets
Bacterial strains and growth medium
Two types of ureolytic bacteria, S. pasteurii (ATCC 11859) and B. sphaericus (ATCC 13805), were selected for this study. It has been reported that these bacteria form CaCO3 crystals [8], [9], [22] with causing limited health effects such as only isolated cases of mild eye and skin irritation [25]. Note that the American Type Culture Collection (ATCC) numbers, given the above, denoted the specific codes assigned for a certain microbial strain.
Liquid medium contained the following ingredients [8]
Distributions of calcium carbonate precipitation on concrete
Fig. 3 presents the distributions of calcium carbonate precipitation on the top surfaces of normal and lightweight concrete specimens with various treatment methods.
As shown in Fig. 3c and d, for the cell-free medium, the calcium carbonate precipitated on the top surface of the concrete specimens and formed a thick layer, which was easily peeled off by hand. Whereas the calcium carbonate only formed a thin, white layer over the top surface of the normal concrete specimens, it was found that
Conclusion
In this paper results of the investigation on characteristics of microbiological precipitation of calcium carbonate on normal and lightweight concrete by two types of bacteria, S. pasteurii and B. sphaericus are presented. Concrete specimens were treated by pure water, a cell-free medium and medium with cells; and macro- and micro-graphic studies were carried out to analyze the shapes and distribution of calcium carbonate crystals. In addition, a capillary water absorption test of the concrete
Acknowledgments
This research was sponsored by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2012008855). The authors would like to thank Ms. M.R. Kim, Dr. K.M. Jeong and Mr. N.K. Lee at KAIST for helping in the preparation of the experiments and for advising in the analysis of the experimental results.
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