microstructure evolution of sandstone cemented by microbe cement using x-ray computed tomography

1134 Vol.28 No.6 RONG Hui et al: Microstructure Evolution of Sandstone Cemented by Micr...

Microstructure Evolution of Sandstone Cemented by Microbe Cement Using X-ray Computed Tomography

RONG Hui1,2,3, QIAN Chunxiang 1,2,3*

(1. School of Materials Science and Engineering, Southeast University, Nanjing 211189, China; 2. Jiangsu Key Laboratory of Construction Material, Nanjing 211189, China; 3. Research Institute of Green Construction Materials, Southeast University, Nanjing 211189, China)

Abstract: The bio-sandstone, which was cemented by microbe cement, was fi rstly prepared, and then the microstructure evolution process was studied by X-ray computed tomography (X-CT) technique. The experimental results indicate that the microstructure of bio-sandstone becomes dense with the development of age. The evolution of inner structure at different positions is different due to the different contents of microbial induced precipitation calcite. Besides, the increase rate of microbial induced precipitation calcite gradually decreases because of the reduction of microbe absorption content with the decreasing pore size in bio-sandstone.

Key words: microbe cement; bio-sandstone; microstructure evolution; x-ray computed tomography; calcite

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013(Received: Oct. 8, 2012; Accepted: Feb. 17, 2013)

RONG Hui (荣辉): Postgraduate; E-mail: [email protected]*Corresponding author: QIAN Chunxiang (钱春香): Prof.; Ph D;

E-mail: [email protected] by the National Natural Science Foundation of China

(No. 51072035), the Ph D Program’s Foundation of Ministry of Education of China (No. 20090092110029), the Research Innovation Program for College Graduates of Jiangsu Province (No. CXZZ_0145) and the Scientific Research Foundation of Graduate School of Southeast University (Nos. YBJJ1127 and YBPY1208)

DOI 10.1007/s11595-013-0833-z

1 Introduction

In recent years, microbe cement has experienced an increased level of interest for reducing the high energy requirements and environmental costs of cement manufacture. Microbe cement consists of three materials: alkalophilic microbes, substrate solution and calcium ion solution. This cementation type of process is more environmentally friendly than conventional treatment methods and often occurs in naturally cemented deposits. To be specific, the cementation mechanism of microbe cement has a complex process. Firstly, the bacteria solution is injected into the loose particles and after the solution is fully exudative, numerous bacteria will stay on the surface of sand. Then the mixture of substrate and calcium ions solution is injected into the sand. During reaction time, the

substrate solution is firstly broken down into carbon dioxide by the bacteria-producing urease according to the reaction[1-3]:

(1)

Secondly, the value of pH around surrounding environment increases through the breakdown of substrate. At the meantime, calcium ions in the solution are attracted to the bacteria cell wall due to the negative charge of the latter and upon addition of substrate to the bacteria. The equations for the precipitation of calcium carbonate at the cell surface serving as the nucleation site are as follows:

(2)

(3)

These chemical ions diffuse through the cell wall of the bacteria and into the surrounding solution. Two reactions spontaneously occur in the presence of water. Thirdly, in the presence of calcium ions, this could result in a local supersaturation and hence heterogeneous precipitation of calcium carbonate on the bacterial cell wall. After a while, the whole cell becomes encapsulated, limiting nutrient transfer, resulting in cell death. According to this circulation,

Journal of Wuhan University of Technology-Mater. Sci. Ed. Dec.2013 1135

the microbial induced calcium carbonate precipitation on the surface of sand particles and between loose sand grains become more and more so that the loose sand grains are connected to form a whole sand body with a certain degree of strength. The state before and after cementing loose sands is given in Fig.1.

The application of microbe cement has been widely used. Several researchers have shown that microbe cement can be used to improve the mechanical properties (cohesion, friction, stiffness, strength) and decrease the permeability of porous materials[4-10]. Moreover, microbe cement has been investigated for its potential to reinforce or repair construction materials such as limestone and cement-based materials or concretes[11-15]. A series of studies has been aimed at modifying the properties of soil or sand[16] while other work has targeted the field of bioremediation[17-21]. Recently, Rong H et al[22] have introduced the method of binding loose sand particles to sandstone using microbe cement and studied the influence of molding process on mechanical properties of bio-sandstone. However, the research on microstructure of sandstone cemented by microbe cement is very little, especially in microstructure evolution. In order to further explore the cementation process of microbe cement and the change process of microbial induced calcium carbonate precipitated in bio-sandstone, so the microstructure evolution law of bio-sandstone is investigated by X-ray computed tomography (X-CT) in this study.

In this paper, fi rstly, the bio-sandstone (sandstone cemented by microbe cement) was prepared and then the microstructure of the bio-sandstone was monitored by X-CT. Finally, the microstructure evolution at different cross-sections, 3D-reconstruction evolution and the evolution of 3D defect volumes distribution in bio-sandstone were presented, respectively.

2 Experimental

2.1 MaterialsMicrobe cement only consists of three materials:

alkalophilic microbes, substrate solution and calcium

ion solution. A microbial culture with optical density (at 600 nm wavelength) value of 1.7 and enzyme urease activity value of 4.1 mmol/L•min was used. Cultivation of the organism was conducted in a medium containing 3 g/L yeast extract and 5 g/L peptone. The pH of the medium was adjusted to 7 by sodium hydroxide, and then the medium was placed at autoclave at 121 ℃ for 25 min. After storage at autoclave of 121 ℃ for 25 min, the medium contained microbe was put in a shaking table, where temperature was set at 30 ℃ and the vibration frequency was 170 r/min. After 24 h, the microbe solution was acquired. In general, the harvested microorganisms were stored at 4 ℃ prior to use. The concentrations of substrate solution and calcium ion solution were 1 mol/L.

The aggregate used was quartz sand (grain size characteristics: d10=150 μm (10% of the grains were smaller than this diameter); d90=300 μm).2.2 Column parameters and molding tec-

hnique50 mL plastic syringes (internal diameter 3.0 cm,

length 11.0 cm) were used. The columns were packed in the following order: first a layer of approximately 0.5 cm of gauze was placed at bottom of the syringes, followed by 6.0 cm of quartz sand. Packing was performed under water to avoid the inclusion of air pockets. On top of the sand another layer of approximately 1.0 cm of gauze sheet was placed. The column was closed and positioned vertically. Before experiments were conducted, tap water was flushed through the sands two or three times to drive away the extra air. All experiments were performed at ambient temperature of 25±3 ℃.

The molding technique was performed as follows: first, a pump that could regulate the flow rate was connected to an injection point at the bottom of the column. Second, microbial solution was injected into the sand at a constant flow rate of 5 mL/min. When the microbial solution was fully loaded into the sand, the flow was stopped for 2 h to fix more bacteria to the sand. After 2 h of fixation time the cementation solution was fl ushed through the sand at a stable fl ow rate of 10 mL/min. After the fi rst batch of cementation fl uid was injected into the sand column, the fl ow was paused for 6 h. After 6 h of reaction time a second batch of microbial solution was flushed through the sand column again. The above steps were repeated two times a day. Then the sample was placed into the X-CT system for investigating the inner structure evolution of bio-sandstone. The placed position of sample was


the same every time to ensure the studied position of inner structure in bio-sandstone to be unchanged each time. After the sample was tested using X-CT, the microbial solution and the cementation fl uid were injected into the sample according to the above step, and then the sample was placed into the X-CT system again to study in further the inner structure of the bio-sandstone. The above steps were repeated until microbe cement could not be injected. This implies that the cylindrical specimen, 30 mm diameter × 60 mm long were successfully cemented using microbe cement after seven days.2.3 X-ray computed tomography (X-CT)

X-ray computed tomography (X-CT), which is a powerful non-intrusive experiments method, measures the three dimensional internal structure of a material’s X-ray absorption. X-rays passed through an object can be transmitted, scattered and absorbed. To construct a CT image, X-rays are passed through a single plane of a specimen and the resultant decreases in intensities are measured on all detectors. In order to complete the scan of a slice, multiple views must be acquired each at a different angular orientation. The distribution of the X-ray attenuation in the slice plane was reconstructed by a mathematical process involving an algorithm known as filtered back projection[23]. The schematic illustration of X-CT system is given in Fig.2.

In the X-CT, the resulting intensity, which is known unidirectional (X-axis) X-ray beam intensity after absorption by the material, and for different directions of irradiation (θ), named projections[24], was measured by a detector. According to Beer-Lambert’s law, for each angle of projection θ, the resulting intensity I (y, z, θ) on each pixel of the detector (coordinates y, z) is given by:

(4)

where, I0 is the intensity of the beam before the specimen, and μ(x, y, z, θ) for a great number of angles θ is named the Radon transform of the attenuation coefficient of the sample. For a sufficient specimen sampling, the projection-slice theorem ensures that the 3D map of μ can be reconstructed[25]. The attenuation coefficient mainly depends on density of the specimen[26].

3 Results and discussion

3.1 Microstructure evolution at different cross-sections of bio-sandstone

The microstructure evolution law of cross-section at 17 mm in bio-sandstone is shown in Fig.3. From Fig.3 it could be seen that the number of pores at the cross-section of 17 mm decrease with time. Furthermore, it could also be observed that the maximum defect volume of bio-sandstone decreases from 41.71 mm3 at the initial state to 4.19 mm3 at the


final state, indicating that the microstructure of the cross-section at 17 mm becomes dense. The reason of the above phenomenon is that the pore of cross-section at 17 mm is gradually filled by microbial induced precipitation calcium carbonate so that the number of pores is decreased. Meantime, the size of defect volume is also reduced with the increase of calcium carbonate content. Moreover, it is observed from Fig.4 that the average gray values of cross-section at 17 mm gradually increase from 268 to 331. Generally, gray values are dependent on the density of samples, and the latter increases with the former. The consequence is that the density of cross-section increases with the development of age.

The microstructure evolution law of cross-section at 37 mm is given in Fig.5, from which we could see that the evolution process of the cross-section is similar to that of the cross-section at 17 mm. That is to say, the pore content of the cross-section is gradually reduced with the increase of microbial induced precipitation calcite content. The size of maximum defect volume is also decreased due to the calcite precipitation. Besides, the change of average gray values of the cross-section at 37 mm with age is present in Fig.6. It could also be seen from Fig.6 that the average gray values of the cross-section at 37 mm gradually changes with time from 261 to 310. This also indicates that the density of the cross-section is increased due to the formation of microbial induced precipitation calcite.

From Fig.4 and Fig.6 it is also observed that the average gray values of cross-section at 37 mm at each stage is smaller than that of cross-section at 17 mm. To be specifi cally, the average gray values of cross-section at 37 mm at each stage are 261, 268, 288, 302 and 310, respectively. Whereas, the average gray values of cross-section at 17 mm are 268, 274, 294, 314 and 331, respectively. The experimental results mean that

the microbial induced precipitation calcite content at 17 mm cross-section is higher than that at 37 mm cross-section. That is to say, the structure of bottom zone is superior to that of top zone, which indicates the microstructure of bio-sandstone is not uniform. Moreover, this phenomenon shows that the absorption of microbe in different zones is different, which is responsible for the heterogeneity of microstructure in bio-sandstone. In addition, Fig.4 and Fig.6 all display


that both of the increase rates of average gray values of the two cross-sections decrease with time. This indicates that the microbial induced precipitated calcite content at these cross-sections is reduced step by step. It might be related to the large number amount of pores in the initial inner structure of bio-sandstone, which make large number of microbe absorb at the surface of sand particles. Hence the quantity of microbial induced precipitation calcite is more in the initial state. However, the increase rate of microbial induced precipitation calcium carbonate content is reduced step by step due to the reduction of the content and the size of pore in bio-sandstone. Moreover, it could also be seen from Fig.4 and Fig.6 that the increase degree of average gray values at 37 mm cross-section is smaller than that at 17 mm cross-section. This also demonstrates that the structure of bottom zone is superior to that of top zone and the inner structure of bio-sandstone is heterogeneous.3.2 3D-reconstruction evolution of bio-

sandstone

The 3D-reconstruction evolution of bio-sandstone is presented in Fig.7. From Fig.7 it could be seen that the maximum defect volume of bio-sandstone decreases with the increasing age, which is from 50 mm3 to 6 mm3. This might be related to the increase of microbial induced precipitation calcite content.

Moreover, it could be also seen from Fig.7 that there are a lot of defects in the bottom zone and with the increasing age, the defects content in the bottom zone are obviously decreased because the defects of bio-sandstone are constantly fi lled by the microbial induced precipitation calcite. Besides it shows the heterogeneity of the initial inner structure of bio-sandstone. Although gradually fi lled by the microbial induced precipitation calcite, non-uniformity of the inner structure still exists. 3.3 The evolution of 3D defect volumes

distribution in the bio-sandstone

The evolution of 3D defect volumes distribution in the bio-sandstone is given in Fig.8, which displays the defect volume distribution of bio-sandstone concentrates on the smaller defect volume less than 0.5 mm3. It is also observed from Fig.8 (a) that the content of the smallest defect volumes (0.01 mm3) in bio-sandstone obviously decreases from 35000 to 10000. That implies that the smaller defect is easier to be filled with the microbial induced precipitation calcite so that the defect content is gradually reduced. In addition, evolution law of the bigger defect volume distribution (from 0.5 to 45 mm3) in the bio-sandstone is presented in Fig.8 (b). From Fig.8 (b) it could be also seen that with the development of age, the maximum defect volume is reduced from 41.71 mm3 to 4.19 mm3


and the defect content is decreased to 1 from 14. This indicates that the defect gradually decreases because the microbial induced calcite is continuously precipitated in bio-sandstone. Meantime, it also indicates that the microbial induced calcite could fi ll in both the smaller defect volume (less than 0.5 mm3) and the bigger defect volume (more than 0.5 mm3). Finally, the results show that the inner structure of bio-sandstone is being compacted step by step due to the formation of calcite that is precipitated in the defect.

4 Conclusions

The microstructure evolution process of bio-sandstone was investigated by X-ray computed tomography technique. The results obtained can be summarized as follows:

a) With the increasing age, the defect of sandstone cemented by microbe cement gradually reduces so that the inner structure of bio-sandstone is being compacted step by step.

b) With the development of age, the microbial induced precipitation calcium carbonate content of different zones in bio-sandstone is different because the absorption of microbe at different zones is different. That is to say, the change degree of inner structure at different positions is different. So the microstructure of bio-sandstone shows heterogeneity.

c) With the increasing age, the increase rate of microbial induced calcite precipitation in bio-sandstone gradually decreases because the absorbed microbe content reduces with the decreasing pore size in bio-sandstone.

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microstructure evolution of sandstone cemented by microbe cement using x-ray computed tomography

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