Physical and mechanical properties of municipal solid waste incineration residues with cement and coal fly ash using X-ray Computed Tomography scanners

Abstract

A significant volume of Municipal Solid Waste incineration bottom ash and fly ash (i.e., incineration residues) are commonly disposed as landfill. Meanwhile, reclamation of landfill sites to create a new land space after their closure becomes an important goal in the current fewer and fewer land availability scenario in many narrow countries. The objective of this study is to reclaim incineration residue materials in the landfill site by using cement and coal fly ash as stabilizers aiming at performing quality check as new developed materials before future construction. Indeed, physical and mechanical properties of these new materials should be initially examined at the micro scale, which is the primary fundamental for construction at larger scale. This research examines quantitative influences of using the combination of cement and coal fly ash at different ratio on the internal structure and ability of strength enhancement of incineration residues when suffering from loading. Couple of industrial and micro-focus X-ray computed tomography (CT) scanners combined with an image analysis technique were utilized to characterize and visualize the behavior and internal structure of the incineration residues-cement-coal fly ash mixture under the series of unconfined compression test and curing period effect. Nine types of cement solidified incineration residues in term of different curing period (i.e., 7, 14, 28 days) and coal fly ash addition content (i.e., 0%, 9%, 18%) were scanned before and after unconfined compression tests. It was shown that incineration residues solidified by cement and coal fly ash showed an increase in compression strength and deformation modulus with curing time and coal fly ash content. Three-dimension computed tomography images observation and analysis confirmed that solidified incineration residues including incineration bottom and fly ash as well as cement and coal fly ash have the deliquescent materials. Then, it was studied that stabilized parts play a more important role than spatial void distribution in increment or reduction of compression strength.

Keywords

mechanical property / municipal solid waste incineration residues / coal fly ash / unconfined compression test / image analysis / X-Ray Computed Tomography scanners

Introduction

Incineration is a widely-used and effective method for global increase in waste which has become a major concern in many countries. It enables to reduce waste mass by 70% and volume up to 90% [¹]. Thus, landfill is commonly known as the final disposal site for residues from incineration [²]. A considerable volume of Municipal Solid Waste (MSW) incineration residues are disposed in landfill. In the limited land availability scenario, reclamation of landfill site after closure to open new land for future construction becomes an important choice. In order to improve quality of landfill ash material as well as perform proper and stable design on future settlement lying these materials, physical and mechanical characteristics of solidified incineration residues at the micro scale which is an indispensable fundamental for construction at the larger scale should be explicitly evaluated. MSW incineration residue is a complicated material that contains heterogeneous bottom incinerator ash [³] and a large amount of salt which can be effectively stabilized by cement and coal fly ash [⁴,⁵]. For these reasons, it is important to investigate compression strength, deformation properties and internal three-dimension (3-D) visualization of these residues solidified by cement and coal fly ash when suffering heavy loading, hence which provides deep insights on the bearing capacity of the landfill.

A variety of researches for investigating compression strength and deformation properties of MSW incineration residues are available. The strength was examined by determining effects of seasonal variation [⁶], curing age [⁷], curing condition [⁸], particle scale [⁹], additional pozzolanic content [¹⁰] when subjecting materials to unconfined compression test or triaxial compression test. Waste materials sampled and assessed are abundant including MSW incineration bottom ash [⁶]; concrete mixture formulated with MSW incineration residues and air pollution control (APC) fly ash [¹]; MSW incineration waste mixture [⁷,⁹]. However, there is few research evaluating compression strength as well as other deformation properties of the mixture of inhomogeneous MSW incineration residues stabilized with cement and coal fly ash through visualizing, quantifying and verifying its internal structures.

X-ray Computed Tomography (CT) scanner is a powerful non-destructive 3-D visualization and qualification tool providing better insights on deformation processes which are crucial for the physical and mechanical characteristics of materials. Application of X-ray CT was emphasized in many fields of geoscience including 3D pore characterization [¹¹,¹²], 3D grain analysis [¹³,¹⁴], fracture analysis [¹⁵,¹⁶], fluid flow analysis [¹⁷,¹⁸]. Particularly, Otani et al. [¹⁵] conducted X-ray CT scanning to visualize and confirm the physical and mechanical properties of in-situ light weight soils with air foams during unconfined compression. Peth et al. [¹⁹] examined local strain and changes in soil structure resulting from hydraulic and mechanical stresses based on X-ray CT data. Promentilla and Sugiyama [²⁰] tried to characterize the internal structure of mortars that were exposed to freezing-thawing action by using X-ray CT. As the result, coupled with image analysis, the void space obtained from micro-CT was characterized in three dimensions (3D) in terms of void fraction and air void size distribution, as well as, the crack width and tortuosity of the connected crack network. The recent application of X-ray CT scanner in reconstructing the pore geometry and originally developed a void segmentation technique was also reported by Taylor et al. [²¹] and Mukunoki et al. [²²]. In geo-waste science, few researches utilize X-ray CT scanner as a useful tool for examining internal structure of waste materials. Therefore, in this research, with the certain curing age, hetehomogeneous concrete mixture constituted from MSW incineration residues solidified by cement and coal fly ash was investigated in the macro and micro scale by using an industrial X-ray CT and a micro-focused X-ray CT possessed by X-Earth Center of Kumamoto University. In order to elucidate deformation properties of this concrete composite under loading, density distribution and pore-size distribution in three dimensions need to be visualized explicitly and evaluated specifically thanks to advantages of X-Ray CT scanners.

Materials and methods

Material preparation

Sample composition

MSW incineration bottom ash, MSW incineration fly ash and coal fly ash were sampled from R incineration facility in F city, Japan. In order to systematically explore physical and mechanical behaviors of solidified MSW incineration residues, a series of artificial specimens with 10% of cement and different percentage of MSW incineration residues and coal fly ash were concerned. Considering the presence of coal fly ash, three crucial types of specimens are produced with the increasing addition of coal fly ash, namely 0% (C0), 9% (C9), 18% (C18) which are presented in Table 1. Due to the considerable water content of ash, cases of C0 and C9 are dispensable to add more water, but 29 kg/m³ water was supplemented in C18 in order to guarantee precise solidification of the concrete specimens. The cement type of Blast –furnace Slag Cement Type B and the cement-water ratio (c/w=0.40) was kept constant.

Sample stabilization

In general, cement and coal fly ash are used as the stabilizers to mix with incineration residues. This approach depends on the volume of water to increase the fluidization of materials. Excessive addition water will cause reduction of strength so eventually excessive cement to keep enough strength as the material. In recent, a superfluid method has been proposed. This method makes mixed materials of cement and coal fly ash have high fluidization not applying water but vibration. Coal fly ash particles are composed of silica and well enough range of particle size to cause bearing effect. In this study, the collected coal fly ash are also stabilized by the superfluid method.

Unconfined compression (UC) test

The mixture was deposited in cylindrical molds of 50 mm diameter and 100 mm in height. Subsequently, these specimens were cured in a humidity controlled room at a constant temperature of 20°C. The studied ages are 7, 14, and 28 days. After curing, some specimens were subjected to unconfined compression (UC) test that was performed according to ASTM D 2166. The load was applied so as to produce an axial strain and loading continued until the load values decreased with increasing strain. The axial strain was measured by displacement transducer and was automatically recorded by a data logger.

Image acquisition using X-ray CT scanners

Table 2 shows the summary of each sample condition and evaluation method. Scanning of cylinder specimens cured for 7 days and 28 days was performed using an industrial X-ray computed tomography (as referred ICT, herein) scanner (Toshiba, TOSCANER-20000RE) at Kumamoto University, Japan in order to detect the effects of curing time on density variation of each location in the mixture. Under a source radiation of 300 kV and 2 mA, X-Ray CT images composing of the unit called voxel instead of pixel because of three-dimension were obtained. In scans of the entire cross section of the specimens, 2048×2048 voxels were used and the size of the voxel from ICT is 0.073×0.073 mm² with 1 mm in width which is equal to the thickness of the X-ray beam. It should be noted that the resolution of 0.073 mm was chosen despite the much larger scanning area than the specimen because the minimum radiographic field of view of this ICT is 150 mm, one of the geometric limitations in this study. However, ICT takes 5 minutes to scan one cross-sectional image.

Furthermore, while the advantage of ICT is able to scan the materials including high density in shorter time than MXCT, hence to detect the density variation accurately in the materials, micro-focused X-ray CT scanner (as referred MXCT, herein) is capable of precisely mapping the internal microstructure of materials. Hence, MXCT has much better spatial resolution than ICT, however ICT is good at representing slight changes of the density. As the result, MXCT, type (TOSCANER-32300) was used for these cylinder specimens. Cases of different coal fly ash content (0% and 18%) and before/after UC test (14 days and 28 days) were performed. In this experiment, a power setting of 140 kV and 200 µA was used for a full (360°) cone-beam scan with 1200 projection views. The specimen was set in a holder mounted on a precision rotation table and then the table position was adjusted to fit the image within the field-of-view (see Fig. 1). Calibration and setting of scan parameters and conditions prior to scanning were performed to obtain high resolution images as well as to reduce the noise and artifacts during image acquisition. As the above specimens concerned, 2048×2048 voxels were used and the size of the voxel from MXCT is 0.08×0.08 mm² with 0.08 mm in width. Total 1250 contiguous slices were generated to reconstruct solid and pore network structure in three dimensions. The image acquisition time for each specimen that includes both scanning and reconstruction time was about 2 hours, hence is much longer than ICT. To observe the inner condition of unknown materials, authors used ICT to scan the sample in two dimensions at the first step to confirm whether MXCT could scan the sample well; and then, it was confirmed that metals were not included very much caused artifact in the sample. Before analyzing these images, they are applied the function of median filter for noise removal in the software Image J, which is presented in the section 3.1.2.

In this research, image processing and analysis were done using public domain program as Image J. X-ray CT images compose of voxels called CT-value which is considered to be a value proportional to the material density, with high CT values correlating to high density (white in images), low CT values corresponding to low density (black in images) [¹⁵,²³]. Thus, they give the density distribution of a sample, therefore it becomes possible to monitor density change in the solidified MSW incineration residues during 7 to 28-day curing. In addition, to quantitatively study composition of each material, it is important to set an appropriate threshold values for each object in the CT images during image processing. This paper quantifies voids and aggregates composition by using a histogram of CT values obtained from planar images. 3-D visualization and analysis of pore space as well as failure/deformation characteristics of each case of specimen before and after UC test were also explored.

Results and discussion

Physical characteristics

Effects of curing on density variation

From the laboratory test, it could be concluded that average density of the whole specimen remained stable during 14-day-curing; however, it is unclear that how density variation of each part of the specimen was. Apparently, X-ray CT images from ICT are possible to provide the density information of different locations along the height axis of the specimen. The visualization of the tomographic images and the density distribution cross sections from reconstructed images, located at three positions of the top, central and bottom of solidified MSW incineration residues sample in term of different curing time (7 and 28 days) and coal fly ash content (0%, 9%, and 18%) were shown. It was indicated that solidified MSW incineration residues materials, despite various coal fly ash percentage, whose mean CT value or mean density changes insignificantly during 7-to-28-day curing at each part of specimens (see Table 3). In normal, CT values indicate relative value with respect to density because an X-ray tube aged due to generating photon so it is not possible to produce exactly same energy of X-ray beam each scan test. CT value during 7-to-28-day curing at each part of specimens changed less than 50 so it should be evaluated that Table 3 indicates there is negligible density difference due to curing.

Figure 2 shows the vertical and horizontal cross-sectional images of specimen with curing term of 7 days. Density distribution was presented by gray level colored images (high density in white and low density in black). Mean density of samples are similar each other; however, CT images give us status of spatial variation of voids (black) and metals (white) in each sample. In normal, MSW incineration bottom ash includes metals which cannot be burned in the incineration process were displayed clearly as the white parts in some locations in all samples. The particle density of coal fly ash, MSW fly ash and MSW bottom ash are similar to each other; hence, the white parts with the lines radiated around observed in the CT images of C18 could be the lumped cement and some metals like iron by accident. They are called “diamond artifact” and sometimes they are caused by the existence of large density difference in the scanned area. In this case, the density of cement is 1.37 to 1.46 times greater than the density of these ash so that diamond artifact was caused.

On the other hand, based on the visual observation, the CT images of C18 show some larger voids and greater density (white) than C0 and C9. According to Table 1, the sample of C0 has the greatest MSW bottom ash in other samples (i.e., C9 and C18). Meanwhile, the sample of C18 has the greatest coal fly ash in other samples (i.e., C0 and C9). Hence, all samples (i.e., C0, C9, and C18) have more or less voids and especially, the sample of C18 has the larger voids. The superfluid method gives the vibration to the materials however; it is not easy to confirm to remove air enough during the making process of sample. The large void may cause the weak point of sample due to loading and it will be discussed later section.

Determination of threshold value for quantitative discussion

Figure 3 shows horizontal cross section of CT images with 16 bits before and after threshold segmentation obtained from micro-focused X-ray CT scanner. In order to analyze the pore and crack structure, binarization of CT image should be required and then, image segmentation can be applied to the binarized CT image. Segmentation algorithms are aimed at extracting structural features such as pore or/and crack network from the volume. To facilitate segmentation precisely, median filter was applied to reduce the spread and overlap of the distribution of the CT value of the different components inside an image by replacing each voxel value by the median of its neighboring voxels. Figure 4 shows the histogram of CT value as shown in Fig. 3(a) after filtering operation. The data is thresholded to separate voids from the solids in the original grayscale image. This calculation was based on a binarization of the images into solid and pore space. All the voxels with value less than the threshold were changed into “0” (black color – B representing for pore) and the voxels with value above the threshold are converted into “1” (white color – W typical for solid), resulting in a B/W image. As mentioned earlier, the threshold in the present paper is determined through the histogram which shows two distinct peaks are associated with the void and solid matrix, respectively. The threshold value is determined as the valley value of the bimodal histogram, determined as 61. Figure 3(b) shows the binary images based on thresholding 61. Based on the visual inspection of CT images, this threshold value was found to be adequate and reasonable for this research. Figure 4 indicates a representative slice of the specimen cured for 14 days without coal fly ash before and after UC test as well as their binary images at threshold value of 61. Thus, it can be observed clearly pore and crack space on these slices (black voxel).

As the threshold value of 61 was concerned, the porosity was easily quantified by diving the total number of internal void voxel with the total number of foreground voxels. Table 4 summarizes the porosity of the scanned specimens in different cases. An increase in the porosity after UC tests would indicate the formation of cracks through the specimen which is shown in the section 3.2.2.

Effects of coal fly ash on pore size distribution

The spatial distribution of pore size can be obtained from the analysis of the CT images. 3D object counter, which is the plug-in function of Image J software, was applied to conduct the calculation related to pore space and determine the void size distribution. Note that the equivalent diameters were calculated from mean distance to surface of pore (average distance from the geometric center of the pore to its surface). In this paper, the minimum pore size in the sample is assumed as 0.08 mm which is equal to the voxel size. Hence, the equivalent void diameter of the specimens in this paper is ranged from 0.08 to 11.0 mm but mostly distributed in 0.1–0.2 mm as shown in Fig. 5.

It can be seen from Fig. 5 that at the age of 14 days, there is no much difference on frequency of all pore sizes between the absence (C0) and presence (C18) of coal fly ash. On the other hand, after 14 passed days the pore size distribution is quite much different. At the age of 28 days, most frequency of the pore sizes of C18 is higher than C0. As the matter of fact that solidified incineration residues including incineration fly and bottom ash as well as cement and coal fly ash have the deliquescent materials such as NaCl, CaCl₂, KCl, hence these samples also have this same property. Normally, the deliquescent materials enable to absorb moisture from air so that water such as condensation water can be observed on surface of the sample and then, those water filled with voids in the sample. At initial state, the effect of these deliquescent materials on each sample is unobvious, however, this effect is manifested obviously after curing time. C18 indicates 18% coal fly ash and less incineration residues than C0 which involves 0% of coal fly ash and more incineration residues. The frequency of the pore size of C18 is higher than C0 at the age of 28 days, hence C18 received more condensation water from air than C0 after curing time, as the result that coal fly ash suffers from deliquescent effects greater than incineration residues. It is noted that the term “pore size” hereby is for voids whose equivalent diameter is equal to or larger than 0.08 mm because the resolution of X-Ray CT image from MXCT is the X-Ray beam thickness or voxel size of 0.08 mm. Therefore, voids whose equivalent diameter is smaller than 0.08 mm could not be detected as the inherent limitation of X-Ray CT method in this research. However, the results from the effect of coal fly ash on pore size distribution is one of the important points in using coal fly ash as stabilizer in the mixture of incineration residues.

Mechanical characteristics

Unconfined compression strength

As far as the mechanical properties of the MSW incineration residues solidified by cement and coal fly ash were concerned, the results of unconfined compression test measurement were analyzed in different cases of curing period and percentage of such additional material as coal fly ash. Figure 6 shows the stress-strain relationship of MSW incineration bottom and fly ash samples cured for 14 and 28 days with the presence of coal fly ash at 0%, 9%, and 18%. It could be seen that the stress-strain curve is not completely smooth as usual for such all cases, which is the result from the heterogeneity of MSW bottom and fly ash materials. In both 14 and 28 days age, the compression strength of MSW incineration ash rose with the more addition of coal fly ash. This trend can be observed clearly through Fig. 7, which shows that increment tendency of peak stress and young modulus when adding more and more coal fly ash. In term of 18% of coal fly ash, the peak stresses for samples cured for 14 and 28 days are 2.95 MPa and 3.82 MPa, respectively while the figures for absence of coal fly ash are very low, at 1.70 MPa for 14 days and 1.96 MPa for 28 days curing. The analysis is based on the assumption that the increase strength of solidified MSW incineration ash is an additional strength due to the hydration products formed from the pozzolanic material – coal fly ash. Figures 6(a) and 6(b) indicate that most specimens have similar strains (around 1.5%) except C18D28 (28 days curing) with strain nearly 1.0%. The above-mentioned results presented that C18D28 reaches the highest peak stress of all cases thanks to the stronger stabilization and longer curing time, hence, the specimen will be broken and difficult to scan X-ray CT if they continue being applied strain that is higher than 1.0%. Figure 7 indicated that the more the coal fly ash, the more the difference of peak stress and young modulus between 14 and 28 ages. In addition, the increment rate of compression strength due to coal fly ash content at 28 days of age is slightly higher than 14 days. Indeed, the greater the slope angles of the increasing trend-narrow connecting 3 values are, the faster the increment rate of compression strength is.

Cracking behavior after UC using X-ray CT image analysis

Results from both the laboratory test and CT image test indicate the higher porosity after UC test than before UC test in all cases of specimens. The appearance of crack after UC test shown in CT cross sections is considered as the evidence for this output.

It can be seen from Fig. 6(a) that C0 and C18 cured for 14 days have the same strain (1.5%), therefore, X-ray CT image processing analysis of two these cases should be implemented to detect the changing mechanism of specimens. Figure 8 presents horizontal cross sections at the height of 20, 40, 60, and 80 mm of the specimens C0 and C18 cured for 14 days before and after UC tests. As a consequence, from cross sections observation,

1) Crack density is higher in the upper part of specimen and some cracks come from voids or distribute in the position near or at both small and large voids but some others have less relation with voids (white dash-line circles in Fig. 8);

2) Some small voids become bigger voids called cracking of voids (white solid-line circles in Fig. 8);

3) Some voids become smaller or even disappear called compression of voids (white solid-line square in Fig. 8);

4) Some metals were divided into some smaller pieces or move to other positions (white dash-line square in Fig. 8).

Figure 9 shows horizontal cross section of CT images with 16 bits before and after threshold segmentation obtained from micro-focused X-ray CT scanner. Specific characteristics of void and crack distribution system would be examined if 3D visualization is promoted. Figure 10 and Figure 11 present the visualization of the voids and crack in three dimensions of specimens cured for 14 days without coal fly ash content (C0D14) under four viewpoints, namely front to back, back to font, left to right, and right to left. Similarly, the 3-D visualization video can be completed for fully detail observation in all other specimen cases. It is calculated that the biggest and smallest void size of C0D14 are 1.07 cm and 0.008 cm, respectively. According to Fig. 10 and Fig. 11, biggest and smallest voids can be observed by the naked eyes using both 2D and 3D images of X-ray CT. Black solid-line squares on Fig. 10 and Fig. 11 mark voids in the size range of 0.7–1.07 cm. Most of them concentrates on the upper part of the specimen. It can be seen that most cracks arise from both small and big void positions stretching from the left of top to the right of bottom of specimen. On the other hand, it seems that some relatively big voids in the left of bottom part isolate from crack. This observation implies that relatively big voids do not always become a trigger of crack generation.

Changes of empty space after compression test (changed porosity) due to more void and crack addition in different cases of curing time and coal fly ash content are illustrated in Fig. 12. In all cases, the changed porosity hardly exceeds over 4%.

Curing time’s impacts (different curing days of 14 and 28 days) on the changed porosity of specimen shown in case 1 – Fig. 12(a). Most changed porosity of C0D14 is three times as many as that of C0D28 along the height of specimen. Particularly, the empty space after compression test seems to change quite slightly and evenly along the height. Additionally, the negative changed porosity is quite abundant, hence, the phenomenon of compression of void in this case seems to be more common than cracking of void. As a consequent, the specimen cured for 28 days has enough steady matrix and hardness to suffer the loading.

Case 2 (Fig. 12(b)) indicates curing time of 14 days but different coal fly ash content (C0D14 and C18D14), most changed porosity of C0D14 is twice as many as that of C18D14 and the fluctuation of changed porosity along the specimen height is more different and significant at C0D14 than C18D14, especially at the height of 60–70 mm. The more significant the fluctuation, the more considerable density variation of specimen matrix. Thus, the UC load damaged the voids and impacted on such a loose matrix leading to the increase of cracks and which consequently causes to lower strength. It can be noted from Fig. 12(b) that there is little negative number at the height of 22 mm because some small voids disappeared or some voids decrease the volume, which is in accordance with the observation of mentioned earlier 2D and 3D images. Indeed, in order to limit the creation of void or the initiation of cracks for the specimens cured for 14 days, an addition of coal fly ash is required.

Based on all the above described results, Table 5 presents the summary of physical and mechanical properties of studied materials on each curing time.

It may be concluded that crack appearance has less tight relationship to the relatively big voids as shown in Figs. 8, 10, and 11; namely, the density variation of matrix part caused the strain localization. Otani et al. [15], who studied light-weight stabilized soil material with air foam, also had similar conclusion. This conclusion should be related to the amount of cement. If the mass of cement is little, voids will become a trigger to generation of collapse.

Conclusions

Industrial and micro-focused X-ray CT scanners were applied to the field of waste management. X-ray CT scanners are useful tools in waste-material field for not only visualizations but also for the quantitative discussion of waste material properties; the change of density distribution during curing time and the nondestructive 3D visualization of internal void and crack structure under the load from unconfined compression test regardless of some their inherent limitations. The relationship between compression strength and 3D internal characteristics of materials constituting of municipal solid waste incineration residues solidified by cement and coal fly ash were discussed based on image analysis. The conclusions drawn from this study are shown as follow:

Physical properties:

1) Based on the image analysis using the industrial X-ray CT scanner, there was negligible change of the density distribution at three different locations (upper, middle and lower parts) of the specimen made from MSW incineration residues solidified by cement and coal fly ash during the 14 to 28-day curing period;

2) Micro X-Ray CT images elucidates greater deliquescent effects of stabilized incineration residues with coal fly ash than those without coal fly ash, hence coal fly ash increases the frequency of the pore sizes at 28 days curing time.

Mechanical properties:

3) MSW incineration residues samples solidified by cement and coal fly ash showed an increase in compression strength and young modulus with curing time and coal fly ash content;

4) Internal void and crack structure of MSW incineration residues mixture solidified by coal ash and cement were investigated visually in three dimensions and were evaluated quantitatively;

5) The results of image analysis using micro-focused X-ray CT scanner indicated that 14-day cured and solidified MSW incineration residues specimens promoted the changed porosity as well as cracking appearance after UC test while 28-day cured and solidified MSW incineration residues specimens indicate less changed porosity three times than samples of 14-day curing due to steady matrix and hardness to suffer the loading in case of the absence of coal fly ash;

6) Coal fly ash presence decreased the changed porosity and cracking areas after unconfined compression tests for the specimens cured for 14 days, consequently, coal fly ash is able to limit the creation of void or the initiation of cracks in case of 14-day curing;

7) Distribution of pose size analyzing 3-D CT images verified that void did not always have influence in the crack generation of these materials;

8) Cement and coal fly ash play a more important role than spatial void distribution in increment or reduction of compression strength in case of 14 and 28 days curing age.

References

[1] Del Valle-Zermeño R, Formosa J, Chimenos J M, Martínez M, Fernández A I. Aggregate material formulated with MSWI bottom ash and APC fly ash for use as secondary building material. Waste Management, 2013, 33(3): 621–627

[2] Birgisdóttir H, Bhander G, Hauschild M Z, Christensen T H. Life cycle assessment of disposal of residues from municipal solid waste incineration: recycling of bottom ash in road construction or landfilling in Denmark evaluated in the ROAD-RES model. Waste Management, 2007, 27(8): S75–S84

[3] Xiao Y, Oorsprong M, Yang Y, Voncken J H L. Vitrification of bottom ash from a municipal solid waste incinerator. Waste Management, 2008, 28(6): 1020–1026

[4] Kamon M, Katsumi T, Sano Y. Stabilization of municipal waste incineration fly ash. In: Proceedings of the First National Symposium on Environmental Geotechnology, Japanese Geotechnical Society, 1994, 231–238 (in Japanese)

[5] Kamon M, Katsumi T, Sano Y. MSW fly ash stabilized with coal ash for geotechnical application. Journal of Hazardous Materials, 2000, 76(2–3): 265–283

[6] Arm M. Variation in deformation properties of processed MSWI bottom ash: results from triaxial tests. Waste Management, 2004, 24(10): 1035–1042

[7] Lan N C, Inui T, Ikeda K, Katsumi T. Aging effects on the mechanical property of waste mixture in coastal landfill sites. Soils and Foundations, 2015, 55(6): 1441–1453

[8] Sato K, Matsumura K, Yoshida N, Shimaoka T, Miyawaki K, Hanashima M. Dynamic characteristics of incineration residue under different curing conditions. In: Proceedings of the 4th JGS Symposium on environmental mental Geotechnics, Japanese Geotechnical Society, 2001, 55–58 (in Japanese)

[9] Lan N C, Inui T, Ikeda K, Katsumi T. Scale effects on the shear strength of waste in coastal landfill sites. In: Proceedings of the 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering. Japanese Geotechnical Society Special Publication, 2016, 1824–1828

[10] Shibi T, Kamei T. Effect of freeze-thaw cycles on the strength and physical properties of cement-stabilised soil containing recycled bassanite and coal ash. Cold Regions Science and Technology, 2014, 106–107: 36–45

[11] Fakhimi A, Gharahbagh E A. Discrete element modeling of the influence of void size and distribution on the mechanical behavior of rock. In: Proceedings of the 3rd CANUS Rock Mechanics Symposium, 2009, Toronto

[12] Dewanckele J, De Kock T, Boone M A, Cnudde V, Brabant L, Boone M N, Fronteau G, Van Hoorebeke L, Jacobs P. 4D imaging and quantification of pore structure modifications inside natural building stones by means of high resolution X-ray CT. Science of the Total Environment, 2012, 416: 436–448

[13] Masad E, Saadeh S, Al-Rousan T, Garboczi E, Little D. Computations of particle surface characteristics using optical and X-ray CT images. Computational Materials Science, 2005, 34(4): 406–424

[14] Jerram D A, Mock A, Davis G R, Field M, Brown R J. 3D crystal size distributions: a case study on quantifying olivine populations in kimberlites. Lithos, 2009, 112: 223–235

[15] Otani J, Mukunoki T, Kikuchi Y. Visualization for engineering property of in-situ light weight soild with air foams. Soils and Foundations. Japanese Geotechnical Society, 2002, 42(3): 93–105

[16] Daly M, Leonard F, Sharples J K, Sherry A H. Advanced assessment of ductile tearing in nuclear reactor pressure vessel steel using X-ray tomography. In: Proceedings of the 13th International Conference on Fracture. Beijing, China, 2013

[17] Singh K, Niven R K, Senden T J, Turner M L, Sheppard A P, Middleton J P, Knackstedt M A. Remobilization of residual non-aqueous phase liquid in porous media by freeze-thaw cycles. Environmental Science & Technology, 2011, 45(8): 3473–3478

[18] Mukunoki T, Mikami K. Study on mechanism of two-phase flow in porous media using X-ray CT Image Analysis. In: Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, 2013, Paris

[19] Peth S, Nellesen J, Fischer G, Horn R. Non-invasive 3D analysis of local soil deformation under mechanical and hydraulic stresses by micro CT and digital image correlation. Soil & Tillage Research, 2010, 111(1): 3–18

[20] Promentilla M A B, Sugiyama T. X-ray microtomography of mortars exposed to freezing-thawing action. Journal of Advanced Concrete Technology, 2010, 8(2): 97–111

[21] Taylor H F, O’Sullivan C, Sim W W. A new method to identify void constrictions in micro-CT images of sand. Computers and Geotechnics, 2015, 69: 279–290

[22] Mukunoki T, Miyata Y, Mikami K, Shiota E. X-ray CT analysis of pore structure in sand. Solid Earth, 2016, 7(3): 929–942

[23] Otani J, Mukunoki T, Obara Y. Application of X-ray CT method for characterization of failure in soils. Soils and Foundations. Japanese Geotechnical Soctiey, 2000, 40(2): 113–120

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature