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DOI 10.1007/s11595-016-1410-z

Distribution of Calcium Carbonate in the Process of Concrete Self-healing

QIAN Chunxiang1,2, LI Ruiyang1,2, LUO Mian1,2, CHEN Huaicheng1,2

(1.School of Materials Science and Engineering, Southeast University, Nanjing 211189, China; 2.Research Institute of Green Construction Materials, Southeast University, Nanjing 211189, China) Abstract: The complete deposition distribution process of calcium carbonate is summarized in three directions of cracks. Distribution of calcium carbonate in the self-healing process of microbial concrete is studied in detail, with the help of a variety of analytical techniques. The results show that carbonate deposits along the x-axis direction of the cracks. The farther from the crack surfaces of concrete matrix in x-axis direction, the more the content of the substrate, the less content of calcium carbonate. Gradual accumulation of calcium carbonate along the y-axis direction is like building a house with bricks. Different repair points are gradually connected, and ultimately the whole of cracks are completely filled. In the z-axis direction, calcium deposits on the surface of fracture direction, when the crack is filled on the surface, because the internal crack hypoxia in the depths of cracks hardly produces calcium carbonate. Key words: microbial concrete; cracks; self-healing; calcium carbonate; deposition

1 Introduction Concrete is engineering material which is used widely and its usage is the largest. But the modulus of elasticity and tensile strength are very low[1]. As a result, the surface of concrete often produces some micro-cracks which are visible to the naked eye. However, the durability of concrete will be adversely affected, including the impermeability of concrete, resistance to chloride ion erosion and anti-carbonation. Repair of cracks on the concrete surface can prevent harmful substances from invading the inside of concrete and improve the service life of the concrete structure. The method of Microbial Remediation can solve the above problems and do not require manual inspection and repair. The mechanism of the microbial mineralization generating calcium carbonate has been studied extensively [2-3]. Microbial self-healing of concrete

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2016 (Received: Oct. 17, 2015; Accepted: Feb. 4, 2016) QIAN Chunxiang(钱春香): Prof.; Ph D; E-mail: cxqian@seu. edu.cn Funded by the National Natural Science Foundation of China (No. 51178104), 333 Project of Jiangsu, and Ph D Program’s Foundation of Ministry of Education of China (No. 20110092110033)

cracks is based on the mechanism of microbial mineralization. Mineralization mechanism can be expressed by Formula (1): (1) In 1995, Gollapudi and Bang[4,5] first proposed that the micro-organisms can be used for self-repair of cracks in concrete. The ability of microbial repairing concrete cracks attracts a large number of researchers. Based on the mechanism of microbial mineralization, microorganisms and substrate were added to the cement matrix. University of Delft[6-9], The Netherlands and the University of Ghent[11-13], Belgium, have made much research progress. When micro-cracks appear on the concrete surface, cracks can be repaired by unhydrated cement particles. But the repairing rang is very small. Jonkers [10] et al found that when the concrete crack width is less than 0.21 mm, cracks can be completely repaired by secondary hydration. Cracks can be completely repaired by microorganisms when the crack width is less than 0.47 mm. Although microorganisms self-healing of concrete cracks has been studied intensively, distribution of calcium carbonate deposition in self-healing has not been studied extensively. In order to improve the self-healing effect of concrete cracks, we studied the distribution of calcium carbonate deposition in the self-healing

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process of microbial concrete.

2 Experimental 2.1 Bacteria The strain selected should have a strong alkali resistance because of the high alkali environment in the matrix of the cement-based materials. Bacteria of alkalophilic Bacillus were extracted from the soil of high alkaline salt lake. By repeated alkali resistance breeding, a strongly alkali resistant and improved basophils Bacillus that can produce calcium carbonate crystals was bred. 2.2 Specimen The cement used in this experiment was PⅡ52.5; The ratio of water-cement was 0.36; The amount of substrate accounted for 2% of the mass of cement. The ratio of raw materials is shown in Table 1.

In Table 1, bacterial sludge was concentrated broth after centrifugation; The size of specimen is 40 mm × 40 mm × 160 mm; The number of bacteria in the specimen is approximately 2×108 /cm3. Specimen was cured 21 days in the curing room. The curing temperature was 20 ℃ and the humidity was greater than 90%. 2.3 Creation of cracks Wi t h t h e h e l p o f t h e a u t o m a t i c b e n d i n g compression testing machine, the uniform distributed pressure is applied to the test piece at both ends, respectively. By controlling the loading speed and loading time, the specimen surface will generate visible micro-cracks. In order to prevent the test piece from fracturing during the loading process, we should clean the surface of the specimen and stick tape on the upper and lower surfaces before loading. The metal rod of a certain diameter was embedded in both ends of the micro-cracks that can produce a relatively uniform width cracks. 2.4 Curing In order to provide a suitable environment for microorganisms, the specimen was cured in a water tank of 30 ℃. When the crack is filled with water, the substrate may be dissolved into the water and

transported to crack surfaces through the pores of the concrete. The bacteria also can reach the surface of the crack through the pores, and then undergo resurrection and mineralization. Ultimately the substrate provided raw materials for the mineralization of the bacteria.

3 Results and discussion In order to characterize the distribution of calcium carbonate, we simulated the three directions (x-axis, y-axis and z-axis) of the cracks in the specimen. The x-axis indicates the width direction of the crack. The y-axis represents the longitudinal direction of the cracks. The z-axis represents the direction of the depth of the cracks, as shown in Fig.1.

3.1 Concrete self-healing of the x-axis direction 3.1.1 Matrix component analysis of cracks surface in the x-axis direction After the specimen was broken apart along the crack, we took one side to analyse the distribution of calcium carbonate along the x-axis direction of cracks by TG. We made use of the rasp to grind cement matrix along the fracture surface of the crack, and collected powder. We made use of a vernier caliper to measure the remaining thickness of the substrate, and calculate the interval scope of the powder in matrix. The results are shown in Table 2. Quantitative analysis of calcium carbonate on the cracks x-axis direction was conducted. As can be seen from Fig.2, the weight loss of Sample 1 and Sample 2 is the largest at about 700 ℃. Sample 3 and Sample 4 followed and the weight loss of Sample 5 is the Minimum. Due to the sample 1 and sample 2 in the vicinity of the crack surface, the substrate is almost completely exploded; Because the specimen only added to the mass ratio of 2% of the substrate, we can consider that a small amount of

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substrate remaining in the sample 3 and sample 4 has little effect on the TG curve and also can be neglected. In addition, sample 5 is virtually free of calcium carbonate, so sample 5 was taken as a reference. We believe that the calcium carbonate content in sample 5 is 0% and from this calcium carbonate content in the other samples can be obtained. The calculation results are shown in Table 3.

are shown in Table 4. In contrast of Fig.3 and Table 4, we can find that with the extension of the maintenance period, the content of calcium carbonate crystals increases in the x-axis direction of the cracks and the crack width becomes smaller. Cracks are completely filled by calcium carbonate crystals after the specimen is cured for 40 days.

The mass loss of the final stage occurred at about 700 ℃ in the TG curve. We believe that all the mass loss of sample 5 is from the cement paste and the extra mass loss of the other samples compared to sample 5 is from the decomposition of calcium carbonate, and it is thus possible to calculate the calcium carbonate content and the mass fraction of each sample. Fig.2 and Table 3 show that: the content of calcium carbonate showed a decreasing trend along the x-axis direction. The content of calcium carbonate is higher than 20% in the range of distance of 1.5 mm from the fracture surface; The content of calcium carbonate is only about 8% at a distance of 1.5 to 2.0 mm from the cross-sectional surface; When the fracture surface range is greater than 2.0 mm, there is almost no calcium carbonate formation. 3.1.2 Concrete crack repair process of the x-axis direction The width of the A, B, C was detected by Crack Width Gauge at different curing time and the results

3.1.3 Concrete crack repair results of the x-axis direction The results of Fig.2 and Table 3 are obtained by thermal gravimetric analysis and differential scanning calorimetry analysis. It is not difficult to find that the farther from the crack surfaces of concrete matrix in x-axis direction, the more the content of the substrate, and the less content of calcium carbonate. In the process of forming , the substrate is uniformly added to the cement, so the substrate is uniformly distributed in the concrete matrix. When the sample was placed with water, the substrate is transported to the surface of cracks due to the difference in concentration. The substrate is formed of calcium carbonate by microbial mineralization, as shown in Fig.4. Fig.3 and Table 4 show that the process of selfrepair in concrete cracks is random. Firstly the repairing process starts at some point, but not a whole cracks. With the restoration prolonging, crack width gradually reduces at repair point until the crack is completely

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closed. The reason why concrete cracks are repaired at some point firstly is mainly because these points favor microorganisms and substrate adhesion, enabling the successful completion of the mineralization process, but these points are also provided for the attachment of calcium carbonate.

3.2 Concrete self-healing of the y-axis direction 3.2.1 Concrete crack repair process of the y-axis direction Length of crack at different curing time is shown in Fig.5 and Fig.6. With the extension of self-healing time, crack length becomes small. When the repair time reaches 40 days, the crack is completely repaired. 3.2.2 Concrete crack repair results of the y-axis direction

As shown in as Figs.5 and 6, the y-axis direction will continue restoration after the concrete cracks are completely repaired at some point. Gradual accumulation of calcium carbonate along the y-axis direction is like building a house with bricks. Different

repair points are gradually connected, and ultimately the whole of cracks are completely filled. This process is shown in Fig.7.

After the whole crack is completely filled, microbial mineralization does not end. With the substrate and bacteria continue to precipitate from the matrix, the density of calcium carbonate increases on the surface of cracks. Calcium carbonate covers the outer edge of cracks, as shown in Fig.5. The substrate has been precipitated from the matrix, resulting in microbial mineralization of calcium carbonate kept rich. Therefore, in order to save repair costs for the smaller width of the cracks we can reduce the amount of substrate added in concrete. 3.3 Concrete self-healing of the z-axis direction 3.3.1 Component distribution in the z-axis direction of cracks As shown in Fig.10, we take 7 observation points (marked with a circle) along the depth direction (z-axis direction) of cracks with distance of the crack opening from 0 to 14 mm. And SEM photos of the seven points are 500 times magnification. There are mainly two

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kinds of particles that can be found on the surface of the fracture. One kinds of particles are blocks, others are strips. It can be concluded from Fig.8 and Fig.9 that two kinds of particles are calcium carbonate and substrate. There is a clear trend that can be found along the crack depth direction (z-axis): Massive particles of calcium carbonate in the cracks opening, there is no residual unreacted substrate; Along the positive direction of z-axis , the number of calcium carbonate particles reduce gradually; A small amount of substrate can be found when the depth reaches 6 mm; there is a clear mutation significantly and particulates of calcium carbonate reduce significantly, when the depth reaches 10 mm; By scanning electron microscopy, we can find that the crack filler may be calcium carbonate crystals. Fig.8 shows that small needles located at the cement surface are decomposed substrate. Taking points along the z-axis direction the analysis shows that the calcium carbonate is generated along the z-axis direction from 0 to 10 mm and its content decreases with depth. Microbial mineralization requires proper environmental conditions. At the crack mouth, oxygen content

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is the highest; Microbial enzyme catalysis is the most intense; The decomposition rate of substrate is the highest. With increasing depth of the crack along the z-axis direction, environmental conditions are more demanding, that is not conducive to the growth of microorganisms. The corresponding yield of calcium is lower, but the number of substrate decomposition increases. 3.3.2 Repair process of the z-axis direction We can directly observe the filling condition along the x-axis and y-axis, but we can’t directly observe fracture filled of z-axis direction. That can be scanned through the CT imaging of concrete cracks. CT scanning photos are shown in Fig.11. There is similar mushroom-shaped filler on the surface of crack and the gray value is similar to the cement matrix. So crack is filled by calcium carbonate on the surface of crack and Fig.3 and Fig.4 match the phenomenon. The gray value of internal cracks is almost similar to the gray value of the air, which can determine virtually that there is no calcium carbonatefilled in the internal cracks. Results are similar to Fig.10.

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microbial mineralization. As a result, internal cracks can not be filled. The process is shown in Fig.12.

4 Conclusions

3.3.3 Concrete crack repair results of the z-axis direction With the extension of the repair period, not only the surface of the specimen fractures may be filled by calcium carbonate, but also cracks within the sample can be filled. However, Fig.10 and Fig.11 show that carbonate distribution along the z-axis direction of the crack is in a concentration decreasing trend. And CT scan map does not detect the internal cracks repaired by calcium carbonate. The reason may be summarized as follows: a) After the formation of cracks, the concentration of moisture and oxygen presented reduced concentration distribution along the z-axis direction of fracture. Because of better growth and reproduction of microorganisms, mineralization reaction is more easy when closer to the crack. b) With the entry of moisture, the substrate is dissolved in the pore water and will continue to migrate. And the substrate accumulates in the region which is closer to the cracks surface. c) Calcium carbonate produces in the cross sections of cracks where there are most suitable microorganisms for mineralization reaction. That will make the fracture cross-section more dense and prevent moisture or oxygen entering. That also causes the difficulty of microbial mineralization. At last the content of calcium carbonate shows a distribution trend of decreasing along the z-axis direction.

In the self-repairing process of concrete, microbial and substrate gather on the surface of micro-cracks after the water gets into the cracks. Firstly, they attach to some point for mineralization. Calcium carbonate will continue to accumulate along the x-axis direction until two surfaces of cracks are completely connected. Then the calcium carbonate continues to accumulate along the y-axis direction, until it completely fills cracks on the sample surface. In the z-axis direction, calcium deposits on the surface of fracture direction, when the crack is filled on the surface, because the internal crack hypoxia in the depths of cracks hardly produces calcium carbonate. Therefore, in order to improve selfrepairing effect, we can find microorganisms who has stronger mineralization ability and even fewer oxygen anaerobic to replace the current test bacteria. References [1]

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Surface cracks can easily be filled. Cracks within the sample produce only small amounts of calcium carbonate, because they can’t satisfy the conditions for

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