Preliminary Laboratory-Scale Model Auger Installation and Testing of ...

Geotechnical Testing Journal, Vol. 36, No. 3, 2013 Available online at www.astm.org doi:10.1520/GTJ20120052

Yaolin Yi,1 Martin Liska,2 Akinyemi Akinyugha,3 Cise Unluer,4 and Abir Al-Tabbaa5

Preliminary Laboratory-Scale Model Auger Installation and Testing of Carbonated Soil-MgO Columns

REFERENCE: Yi, Yaolin, Liska, Martin, Akinyugha, Akinyemi, Unluer, Cise, and Al-Tabbaa, Abir, “Preliminary Laboratory-Scale Model Auger Installation and Testing of Carbonated Soil-MgO Columns,” Geotechnical Testing Journal, Vol. 36, No. 3, 2013, pp. 1–10, doi:10.1520/ GTJ20120052. ISSN 0149-6115.

ABSTRACT: This paper presents details of the installation and performance of carbonated soil-MgO columns using a laboratory-scale model auger setup. MgO grout was mixed with the soil using the auger and the columns were then carbonated with gaseous CO2 introduced in two different ways: one using auger mixing and the other through a perforated plastic tube system inserted into the treated column. The performance of the columns in terms of unconfined compressive strength (UCS), stiffness, strain at failure and microstructure (using X-ray diffraction and scanning electron microscopy) showed that the soil-MgO columns were carbonated very quickly (in under 1 h) and yielded relatively high strength values, of 2.4–9.4 MPa, which on average were five times that of corresponding 28-day ambient cured uncarbonated columns. This confirmed, together with observations of dense microstructure and hydrated magnesium carbonates, that a good degree of carbonation had taken place. The results also showed that the carbonation method and period have a significant effect on the resulting performance, with the carbonation through the perforated pipe producing the best results. KEYWORDS: soil stabilisation, soil mixing, MgO, column installation, carbonation, unconfined compressive strength, microstructure

Introduction Portland cement (PC) is the most widely and commonly used binder in soil stabilisation applications (Sherwood 1993; Mosley and Kirsch 2004). However, the significant environmental impacts associated with its production, in terms of high energy consumption and CO2 emissions (0.85 t CO2/t PC), have led to a range of global initiatives to reduce those impacts including the development of alternative cements (WBCSD 2002; IPCC 2004). One such recent development is the family of reactive magnesia (MgO) cements (Harrison 2008) which are PC-MgO blends with a range of potential sustainability and technical benefits depending on their composition ratio and intended applications. Reactive MgO is generally calcinated from magnesite at temperatures of 700 C, much lower than that of PC (1450 C), for which

Manuscript received April 16, 2012; accepted for publication November 28, 2012; published online March 22, 2013. 1 Ph.D. Candidate, Institute of Geotechnical Engineering, Southeast University, Nanjing 210096, China, e-mail: [email protected] 2 Research and Development Manager, David Ball Group, Cambridge CB23 2TQ, UK, e-mail: [email protected] 3 Ph.D. Candidate, Dept. of Engineering, Univ. of Cambridge, Trumpington Rd., Cambridge CB2 1PZ, UK, e-mail: [email protected] 4 Postdoctoral Associate, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States of America, e-mail: [email protected] 5 Reader, Dept. of Engineering, Univ. of Cambridge, Trumpington Rd., Cambridge CB2 1PZ, UK, e-mail: [email protected]

renewable energy sources can be used (Shand 2006). Manufacturing 1t reactive MgO induces 1.4 CO2 emission (Shand 2006), however, most of the CO2 would be reabsorbed through the carbonation of MgO, which would lead to significantly reduced carbon emissions, providing a far more sustainable binder than PC depending on the carbonation degree achieved. Extensive research conducted at the Univ. of Cambridge since 2004 into the fundamental properties of reactive magnesia cements (e.g., Vandeperre and Al-Tabbaa 2007; Vandeperre et al. 2008a,b; Liska et al. 2008, 2012a,b; Liska 2009; Li 2012; Unluer 2012) led to the realisation that the use of MgO alone can have special advantages over the PC-MgO blends in a range of applications as well as sustainability advantages as the use of MgO alone relies on the carbonation of MgO through the sequestration of large quantities of CO2. Much of that work focussed on the applications to porous concrete blocks. In such applications the following hydration and carbonation behaviour of MgO has been observed. In the presence of water, MgO hydrates to form Mg(OH)2, or brucite, as follows MgO þ H2 O ! MgðOHÞ2

(1)

The hydration of MgO results in 2.2 fold solid volume expansion. Brucite has a layered structure with different morphologies depending on its formation conditions and has been found to have a very limited binding ability (Liska 2009; Li 2012; Unluer 2012). However, in the presence of sufficient CO2, and under the appropriate curing conditions, brucite will carbonate

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FIG. 1—Deep soil mixing auger and resulting columns (courtesy of Eco Foundations).

(provided the structure is porous enough for gaseous CO2 to permeate through it) to form one or more of the following hydrated magnesium carbonates MgðOHÞ2 þ CO2 þ 2H2 O ! MgCO3  3H2 O ðnesquehoniteÞ (2) and/or 5MgðOHÞ2 þ4CO2 þH2 O ! Mg5 ðCO3 Þ4 ðOHÞ2 5H2 O ðdypingiteÞ (3) and/or 5MgðOHÞ2 þ 4CO2 ! Mg5 ðCO3 Þ4 ðOHÞ2 4H2 O ðhydromagnesiteÞ (4) The formation of these carbonates is influenced by several factors such as temperature and CO2 partial pressure as well as the composition and characteristics of the original MgO (Davies and Bubela 1973; Ko¨nigsberger et al. 1999; Marini 2007, Xiong and Lord 2008). The carbonation of brucite is also an expansive reaction, resulting in 1.8–3.1 fold solid volume expansion depending on the formed carbonate. Therefore, the solid volume of MgO could increase by 3.8–6.7 times at the end of the carbonation process, which significantly fills available pores. These hydrated magnesium carbonates also form well ramified networks of massive crystals with a very effective binding ability (Liska 2009; Unluer 2012). High levels of carbonation, including full carbonation, were achieved including in full-scale porous blocks trial production (Liska et al. 2012a,b). The application of reactive MgO as a soil stabilisation binder was recently investigated by the authors. In Yi et al. (2012a), the application of MgO in blends with slag was addressed assessing the effectiveness of the reactive MgO as an activator for the ground granulated blast furnace slag and investigating the hydration reactions. The investigation of the use of reactive MgO alone, added in dry form, was initiated by Yi et al. (2012b) in which the carbonation process was investigated using laboratory specimens in a triaxial setup, through which gaseous CO2 was permeated under pressures of 50–200 kPa. The effect of the initial soil water content, CO2 pressure and carbonation period was investigated. The results showed that the strength development rate of the carbonated stabilised soil was proportional to the CO2 pressure applied and also depended on the soil water content. This work

found that this CO2 application process did lead to highly carbonated MgO-stabilised soils, up to 65 %–80 % degree of carbonation through an underestimated observation, which reached, in only a few hours, similar strength values as 28-day corresponding PC-stabilised soils (Yi et al. 2012b). In commercial applications, deep soil mixing has extensively been used to construct soil-binder columns for ground improvement applications to increase the bearing capacity and stability and reduce the settlement of soft ground (Hausman 1990; Bergado et al. 1996; Bruce 2001; Terashi 2003). The most common deep soil mixing tool is the auger system, which introduces and mixes the binder with the soil in situ (Mosley and Kirsch 2004). Examples of auger systems are shown in Fig. 1 together with constructed columns and walls. For deep mixing applications, it is well established that the strength parameters obtained from laboratory treatability studies are usually higher than those obtained in the field, with ratios of up to 5 being reported (Terashi 2005). This is mainly due to the more effective mixing produced under controlled laboratory conditions as well as the natural heterogeneity and mixing conditions in the field. Al-Tabbaa and Evans (1999) indicates that the laboratory-scale auger is an effective tool to model the in situ soil mixing process, although there are also some natural variations which are difficult to model in the laboratory. Recent investigations of the strength of cement-stabilised soils using these laboratory-scale auger also highlighted the higher strength usually obtained with depth, since the mixing at shallow depth is usually less effective (Hernandez-Martinez et al. 2007). Hence, this paper presents details and results of laboratory experiments performed using a laboratory-scale auger to apply MgO and CO2 to the soil to produce carbonated soil-MgO columns and subsequently assess their performance in terms of mechanical performance and microstructure and also compare them with corresponding specimens produced in the triaxial setup and permeated with gaseous CO2, as detailed in Yi et al. (2012b).

Materials and Methods Soil and Binder A sharp sand (from Ridgeons, Cambridge, UK) was used as a model soil, with D50 of 0.8 mm and coefficient of uniformity of

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YI ET AL. ON AUGER INSTALLATION AND TESTING OF SOIL COLUMNS

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FIG. 2—Laboratory-scale auger set-up: (a) grout installation, (b) carbonation method 1 and (c) carbonation method 2.

4.3. The soil was mainly used dry, but was also tested at a water content of 5 % and 10 % by mixing the sand with water in a small concrete mixer. The reactive MgO (from Richard Baker Harrison, Ilford, UK) had the following oxide composition: MgO: 97.2 %, CaO: 1.2 %, SiO2: 1.2 %, Al2O3: 0.2 %, and Fe2O3: 0.2 %. The soils were placed in metal drums, 290 mm in diameter and 400 mm high, using consistent moderate compaction in three equal layers of 100 mm each. The MgO binder was used in grout form, for ease of application, using a water content of 0.8 prepared in a benchtop food mixer. The MgO grout was applied to the soil at a dry binder: dry soil ratio of 10 % and hence the total water content was 8 %, 13 %, and 18 % respectively for the different initial water contents of the soil. The binder was delivered wet rather than dry because the laboratory scale system is designed for wet grout delivery.

holes were positioned between the lowest two layers of cutting blades. A column installation mixing pattern was initially chosen based on previous work using the same experimental apparatus (Osman 2007). The auger penetration and withdrawal (with reversed auger rotation direction) speed was 5.2 mm/s at a rotational speed of 50 rpm, and the grout was pumped at a rate of 7.2 g/s, which delivered a total of 10 % binder grout dosage. The column installation process, shown in Fig. 3, involved injecting the MgO grout into the soil from the start of mixing and for one complete mixing circle (penetration and withdrawal) followed by two more cycles of mixing only, without grout injection, to homogenise the grout with the soil. Two columns were installed in each soil drum for most cases.

Column Carbonation Column Installation A laboratory-scale auger setup, as shown in Figs. 2(a) and 2(b), was used to install the soil-MgO mixed column. The automated system consisted of a vertical track which enabled the lowering and raising of the auger rotation motor, auger and grout injector component. The MgO grout was pumped into the grout injector from a variable speed peristaltic flow pump down the hollow shaft section of the auger. In order to ensure experimental consistency during column installation, a three-switch travel-stop mechanism was available so that the column length, grout introduction rate over the length of the column and mixing cycles could be reproduced accurately and consistently. The auger rotation variable speed motor provided rotation speeds of 5–50 rpm and the penetration variable speed motor was set up to provide penetration speeds of 2–10 mm/s. The auger had four layers of mixing blades with an axis twist of 5 . The shaft diameters of the auger and blades were 20 mm and 100 mm, respectively. The grout release

Two carbonation methods were used in this study. The first was through pumping gaseous CO2 through the auger in the same way as the grout injecting during the column installation. For this method, following the column installation detailed above, the

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FIG. 3—The column installation procedures employed.

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GEOTECHNICAL TESTING JOURNAL TABLE 1—The different columns produced.

Curing Period After Soil Water Carbonation Carbonation Carbonation Stage ID of Mix Content (%) Method Period (min) and Prior to Extrusion N

0

None

/

28 days

C1

0

Auger:

6

7 days

6

7 days

Auger: 3 circles

6

7 days

Auger:

6

1 hour

2

1 hour

10

1 hour

60

1 hour

3 circles C2

5

Auger: 3 circles

C3

10

C4

0

3 circles C5

0

Auger:

in Fig. 2(c). The diameter of the tube was 20 mm, the same as the shaft diameter of the auger, and the tube was perforated as shown in the inset in Fig. 2(c). After the column installation, the tube was inserted into the column center along the hole left by the auger shaft, and the top inlet of the tube was connected to the CO2 canister, and CO2 pumped at 50 kPa pressure into the column for carbonation for 1 h. The columns were then left in their drums to cures until ready for testing. Details of the different columns produced are shown in Table 1 where the first column (N), not subjected to any carbonation, was used as the control. Typical columns during installation, before carbonation and after extruded are shown in Figs. 4(a)–4(c) respectively. Figure 4(a) clearly shows the central hole left behind by the auger and Fig. 4(c) of the extruded N, C1, and C2 columns shows the uniform diameter of 100 mm achieved.

1 circles C6

0

Auger: 5 circles

C7

0

Pipe

auger was cleaned and the grout injector component connected to a CO2 canister [Fig. 2(b)]. The CO2 was then pumped, at a pressure of 150 kPa, through the auger and into the soil and mixed with the soil through full cycles of mixing with the auger. In most cases, 3 cycles of CO2 injection and simultaneous mixing were performed, as well as 1 and 5 cycles for comparison. Each carbonation cycles took 2 min to complete with similar speed or rotation and penetration as the grouting cycle. The second method of carbonation was through the use of a plastic pipe setup, as shown

Test Programme and Procedure Two columns in a drum were tested for most cases, and an extra column in another drum was tested for C1. The columns were extruded from the drums, at the designated period, as indicated in Table 1, and further cured in a container at 98 % RH and 21 C until ready for testing. Each column was then cut using mounted angle grinder to obtain ideally two specimens of 100 mm high each and tested for unconfined compressive strength (UCS) the following day at a constant displacement rate of 1.14 mm/min. Plaster of Paris was used when required to ensure flat-ended specimens. In number of cases, the specimens were not perfect cylinders but were slightly elliptical. Although this was taken into account when calculating the compressive strength, it could result

FIG. 4—Photographs of columns (a) C1 during installation, (b) C2 before carbonation, (c) C1, C2, N and (d) C7 after excavation.

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YI ET AL. ON AUGER INSTALLATION AND TESTING OF SOIL COLUMNS

in higher deviation within the results. Some columns were slightly shorter and the top and bottom parts were of uneven surface. For this reason it was not possible to retrieve two specimens from a single column in those cases. Therefore, only one specimen was obtained and it was denoted as either “top” or “bottom” according to which part of the column the specimen was taken from. Microstructural investigation was performed using X-ray diffraction analysis (XRD) in a powder diffractometer Bruker D8 Advance, Bruker AXS Inc., Madison, WI with a Cu Ka source, to identify the crystalline phases in the stabilised matrix and a scanning electron microscope (SEM) JEOL 820 was employed to acquire highly magnified microimages. All the different columns in Table 1 were tested for UCS and XRD, while only columns N, C4, and C7 were analysed using SEM. Some columns were broken during the extraction or cutting procedure and hence a small number of data points are missing.

Results and Analysis The results of the unconfined compressive strength, elastic stiffness, strain at failure as well as microstructure are presented and compared with the relevant corresponding results obtained from the corresponding triaxial setup experiments subjected to CO2 permeation (Yi et al. 2012b). It should be noted that the triaxial setup sand contained 5 % kaolin and 5 % silica flour, and hence would in general be expected to have lower strength value ranges. Most of the specimens tested had a total water content following the application of the MgO binder of 7.5 %, being the optimum value, where water contents of 5 % and 10 % were also tested. For the triaxial specimens, the MgO binder was mixed in dry. Hence the total water content of the triaxial specimens of 7.5 % is very close to the total content of the lab-auger columns of 8 %, which had an initial water content of 0 % prior to introduction of the MgO grout.

Bulk Density and Unconfined Compressive Strength The bulk densities of the column specimens were all in the same range of 2 g/cm3 610 % with no particular trends between mixes except that columns C7 had the highest densities of around 2.2 g/cm3, which is linked to it high degree of carbonation achieved (as will be discussed later). For comparison, the corresponding benchtop prepared triaxial setup specimens had a density 2.1–2.2 g/cm3 so closer to the top end of those observed with the auger setup. All the UCS results are presented in Table 2. Figure 4(a) is a photograph of column C1 installation in which the injected white MgO grout and the central whole left behind by the auger shaft after column installation can be clearly seen. Figure 4(b) shows a photograph of some of the extracted columns (N, C1, and C2) which shows that uniform diameter columns of 100 mm had formed. The average UCS of the uncarbonated control column, N, of 1.20 MPa with its small standard deviation (SD) shows the relatively high strength achieved by the hydrated MgO and the uniform performance in the two column halves. Corresponding strength values from the triaxial setup specimens were 0.87 MPa

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TABLE 2—The UCS results of the columns where a, b, and c are the labels of the columns, T and B are the top and bottom parts of a column. ID of Mix N C1

C2

C3

ID of Column

UCS Values (MPa)

Average UCS (MPa) and SD (MPa)

aT

1.22

1.20

aB

1.17

(0.04)

aT

4.17

4.99

bT

5.97

(2.81)

bB cT

9.07 4.33

cB

1.41

aT aB

4.23 1.69

bT

4.16

aT aB

3.25 2.08

3.36 (1.45) 3.09 (0.94)

bT

3.94

C4

aT aB

3.58 8.32

5.95 (3.35)

C5

aT aB

2.56 2.23

2.42 (0.19)

C6 C7

bT

2.61

bB

2.28

aT

4.99

3.67

aB

2.35

(1.87)

aT aB

9.77 9.75

9.52 (0.75)

bT

8.42

bB

10.13

(SD 0.19 MPa) i.e., 20 % lower, suggesting that in general the two sets of results are comparable. The results of the three carbonated C1 columns show an overall average strength of 4.99 MPa, which is almost four times higher than the UCS of the control columns; as a result of carbonation. However the standard deviation (SD) value is very high and close inspection of the results shows that the three top halves had values similar to the overall average and the large SD is caused by the performance of the bottom halves, where one could not be tested, one had twice the average value and one had 1/5th of the average value. This suggests consistent mixing and carbonation of the top halves of the columns and extreme lack of consistency in the mixing and carbonation of the bottom halves. Columns C2 and C3, with the two higher water content sands but similar carbonation procedure, showed a similar trend to columns C1 in that the UCS values of the top half of the columns are similar and consistent while the bottom halves had lower strength values. The results show that the average strength value of the top half of the columns decreased with increasing water content, showing that less carbonation was able to take place, with the increased excess water hindering the CO2 from permeating through the column, consistent with the findings from the related study on the triaxial cell setup specimens (Yi et al. 2012b).

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As the work detailed in Yi et al. (2012b) also showed that carbonation of the MgO-stabilised soils in a triaxial cell setup occurred in a very short period of time and with a sizable strength development just after half an hour of carbonation, columns C4 were extruded 1 h after carbonation rather than 7 days as with column C1. While the strength of the top half of the C4 column is slightly lower than the average of the C1 columns, the strength of the bottom half is quite high. This confirms the high rate at which the MgO had carbonated and such a rapid strength development would allow the application of load onto the stabilised soil very shortly after carbonation. Shorter and longer carbonation periods were tested using 1 and 5 carbonation cycles in columns C5 and C6, respectively. The C5 columns yielded around half the strength value of that of the top half of the C4 columns, confirming that a lower carbonation degree had resulted from the reduced carbonation time, consistent with the findings of Yi et al. (2012b) in triaxial cell setup experiments. Having said that, the one cycle of carbonation did yield consistent results throughout both columns with all strength results being very similar. So it appears that the repeated carbonation cycles after the column had achieved relatively high strength after the first carbonation cycle lead to large variability in the results as the mixing becomes more difficult in the stiffer stabilised soil and the column becomes less homogeneous as a result. This was evident in the C6 columns where the auger became stuck in the soil during the fifth carbonation circle of the second C6 column and the column was damaged and was not suitable for testing. The remaining C6 column did show variability between the top and bottom halves and the two additional carbonation cycles did not lead to an increase in strength overall in the column although the strength of the top half was higher than that of the C4 column. This confirms that the rapid strength development interfered with the latter cycles of carbonation. Hence the optimum mixing cycles is best kept to 3 or 4, which is mixing for up to 8 min, and the highest strength achieved on average was 5–6 times higher than the control. In the triaxial setup experiments (Yi et al. 2012b), it was possible to achieve strength values up to 20 times higher than the control when the MgO-stabilised soils were carbonated for 3 h. In order to facilitate longer carbonation periods of the soilMgO columns, to maintain the integrity of the columns and to eliminate any damage to the deep mixing equipment, the pipe system setup was used to pump CO2 through the columns. Columns C7 were carbonated with 50 kPa pressure of gaseous CO2 for 1 h and cured for 1 h before excavation. Figure 4(d) shows the plastic pipe in the middle of the columns as well as the two extruded C7 columns. The strength values of both columns were very consistent throughout with an average UCS value of 9.52 MPa, which was about twice that of the top half of the C4 columns and nearly 10 times that of control columns N. These results are a clear indication of the uniform carbonation throughout the column with the pipe system and suggest that the pipe system is a much more practical and efficient method of uniformly carbonating the MgO stabilised soil columns. The corresponding UCS of the triaxial setup specimens, to columns C1 and C4–C7, was 6.36 MPa (SD 1.36 MPa), which except for columns C7 is higher (a range of 7 % to 160 % higher)

FIG. 5—The correlation with UCS of E50.

suggesting higher degrees of carbonation. The exception is columns C7 which were 50 % higher than the triaxial specimens suggesting that the pipe system which was perforated throughout delivered the CO2 far more homogeneously than the permeation of the CO2 through the triaxial specimens. Additional, Osman (2007) demonstrated that the model auger installed cement-clay columns had much higher UCS variability, SD was up to 60 % of average value, than the mechanically mixed specimens in laboratory. Similarly, the auger mixing also contributed to the higher SD values in Table 2 than those of triaxial setup specimens (Yi et al. 2012b).

Elastic Stiffness and Strain at Failure The elastic stiffness, E50, values and strain at failure plotted against the corresponding UCS values as well as those obtained from the triaxial setup specimens are presented in Figs. 5 and 6 respectively. A good linear correlation is seen between the E50 and UCS of the auger installed columns (Fig. 5), which is E50 ¼ 280UCS. However, the E50 values of the corresponding triaxial setup specimens are much lower and generally fall within a small range of 350–750 MPa, being almost constant with UCS.

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FIG. 6—The correlation with UCS of strain at failure.

YI ET AL. ON AUGER INSTALLATION AND TESTING OF SOIL COLUMNS

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FIG. 7—XRD diffractograms of the columns (a) of centre samples and (b) surface samples. B-Brucite, C-Calcite, Mg-MgO, N-Nesquehonite and Q-Quartz.

In contrast, the strain at failure of the auger-installed columns (Fig. 6) appears generally independent on their UCS values, while those of triaxial setup specimens exhibit a generally increasing trend with increasing UCS. This different behaviour is mainly attributed to the differences in the soils used in the two tests. In this investigation, the pure sharp sand was used as a model soil, and hence the deformation behavior of the carbonated soil-MgO columns is similar to that of the concrete block (Liska 2009), with relatively small strain at failure which is independent on the strength. Although the same sand was used in the triaxial setup specimens (Yi et al. 2012b), there were 5 % kaolin and 5 % silica flour added, and these fine particles significantly changed the deformation and failure behaviors of carbonated MgO-stabilised soil, resulting in relatively lower UCS and higher strain at failure, which is roughly increasing with strength. Additional, the different soil-MgO mixing methods and carbonation methods induced different uniformities of the carbonated specimens are also believed contributed to their elastic stiffness and strain at failure.

Microstructure Representative samples from the centre and outer surface of all the eight different columns were analysed using X-ray diffraction (XRD) and Fig. 7 shows the corresponding diffractograms, including those of the identified compounds. The two XRD diffratograms of the control mix show the highest intensity brucite peaks, consistent with its dominant presence and the lack of carbonation in those samples. The diffractograms of columns C1, C2, and C3 columns, with 0, 5 % and 10 % water content, and all carbonated through three auger circles show that the brucite peaks of C1 column was much weaker than those of C2 and C3 col-

umns. This confirms that the high soil water content of C2 and C3 columns slowed down their carbonation rate and reflected in the reduced UCS values. The quite similar XRD diffractograms of the C1 and C4 columns confirms that they achieved similar carbonation degrees and hence similar strengths were achieved. The C5 columns with the shortest carbonation time of 1 cycle had the highest brucite peaks among the C4-C6 columns. However, the C6 column had a slightly stronger brucite peaks than C4 column, although it was carbonated for a relatively longer period. The XRD of columns C6 and C7 also detected nesquehonite although, there were no hydrate magnesium carbonates detected in the C1-C5 carbonated columns, the brucite peaks of carbonated columns were evidently weaker than those of the N column, indicating some content of the brucite had carbonated. Additionally, the brucite peaks of surface samples [Fig. 7(b)] of C1-C6 columns were a little weaker than those of the centre samples [Fig. 7(a)], indicating the carbonation degree of the surface samples was slightly higher than that of the center samples. This was attributed to the fact that there was some CO2 left in the surrounding soil after the introducing of CO2 procedure and this residual CO2 slightly increased carbonation degree of outside of the column. Typical SEM micrographs of columns N, C4, and C7 are shown in Figs. 8, 9, and 10, respectively. The fused brucite particles with large pores content and weakly interconnected structure were observed in the microimage of the N column (Fig. 8), consistent with the XRD results. Figure 9 shows that the inner microstructure of column C4 was much denser than that of column N, with most of the soil pores filled by the hydrated magnesium carbonates, being mainly of dypingite/hydromagnesite crystals. This explains the higher UCS of C4 columns than that of N columns. However, the brucite can still be seen in Fig. 9, indicating that carbonation was not complete due to the short carbonation time

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(6 min). Since column C7 was carbonated for 1 h, the presence of nesquehonite together with dypingite/hydromagnesite crystals was clearly observed in Fig. 10. This indicates that the carbonation degree of column C7 was higher than that of column C4, consistent with the UCS results.

General Comments FIG. 8—Typical scanning electron micrographs of columns N.

FIG. 9—Typical scanning electron micrographs of columns C4.

FIG. 10—Typical scanning electron micrographs of columns C7.

In both methods of CO2 application investigated here, there is likely to be CO2 seepages, through the auger shaft-soil interface, at the column-native soil interface or through the native soil, especially if it has a higher porosity and gas permeability than the stabilised column. If a dry binder is used, it is possible that the gas permeability of the treated column would be higher than that of the surrounding native soil and hence CO2 escape could be through the treated column itself. In all these cases the CO2 can be contained through a shroud system similar to those used with augers in contaminated land [Fig. 11(a)] of the Keller auger (Keller 2001) or a pipe system [Fig. 11(b)] similar to soil vapour extraction and air sparging systems. While in many cases, it might be advantageous to have such a short time for strength development, sometimes this could be a problem especially if this could lead to the auger equipment being stuck in the ground or damaged. However, much lower CO2 pressures and injection rates can be used to significantly slow down the carbonation rate, so that the strength development rate can be controlled to occur over a required time span as was demonstrated in Yi et al. (2012b). For the pipe setup, again the pipe could be left in the ground as a reinforcement element if required or alternatively it can be removed. The construction of carbonating soil-MgO columns is only preliminary investigated in this work and the results presented are from a limited number of tests in laboratory. There is clearly a need for a much more extensive investigation into this novel method, including field scale trials and a detailed investigation of the controlling variables. Implementation at field scale could be

FIG. 11—Carbonation in-situ through (a) a shroud system (Keller 2001) and (b) pipe system.

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YI ET AL. ON AUGER INSTALLATION AND TESTING OF SOIL COLUMNS

conducted through the existing suitable equipments with some refits if needed. Homogeneity of the application and mixing is clearly of paramount important and the best technique and methodology to achieve this will need to be optimised. The cost and carbon footprint of this method also need to be precisely evaluated through field scale investigation. The applicability of this method for clayey soil, especially soft clay with high water content, has not been investigated to date, and which is important for expanding the potential applications of this method.

Conclusions The installation of carbonated soil-MgO columns using a laboratory-scale model auger setup was successful using both the auger system itself, in the same way that the grout is injected, and a perforated pipe system, installed in the middle of installed columns. It is found that the soil-MgO columns could be carbonated very quickly, in under 1 h, and yielded high unconfined compressive strength values. The resulting strength values were highly dependent on the method and duration of carbonation; the carbonation through the perforated pipe set up was far more successful leading to far more homogeneous carbonation within the columns. The carbonation through the auger system was less predictable and caused problems with more than 3 cycles of CO2 injection and mixing as the column became stiffer. The higher water content columns led to reduced carbonation as it hindered the permeation of the CO2 through the columns. The microstructural analyses confirmed that the MgO carbonation products, nesquehonite and hydromagnesite/dypingite, strongly interconnected the soil particles and filled the pores between them, leading to the significant strength developments of MgO-stabilised soil observed. This work presents promising findings for the in situ application of carbonated MgO as a ground improvement binder.

Acknowledgments The work presented here was performed when the first author was a visiting researcher at the University of Cambridge in the academic year 2009–2010. The assistance of the laboratory technicians with the work is appreciated. The funding from CSC, NSSF (51279032) and MOST (2012BAJ01B02-01) of China is gratefully acknowledged.

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