slag solidified soils

International Journal of Greenhouse Gas Control 91 (2019) 102827

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Quantification and micro-mechanisms of CO2 sequestration in magnesialime-fly ash/slag solidified soils

T

Dongxing Wanga,b, , Jiaye Zhua, Fujin Hea ⁎

a

Key Laboratory of Geotechnical and Structural Engineering Safety of Hubei Province, School of Civil Engineering, Wuhan University, 8 Dong Hu South Road, Wuhan, 430072, China b Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering of the Ministry of Education, Wuhan University, Wuhan, 430072, China

ARTICLE INFO

ABSTRACT

Keywords: Magnesia-lime-fly ash/slag CO2 carbonation Uptake efficiency Thermal analysis Pore structure Micro-mechanism

The combined use of CO2 and industrial by-products offers a novel alternative to traditional Portland cement in soil stabilization, sequestering permanently CO2 emissions and producing low-carbon cementitious materials in an accelerated carbonation environment. This study attempts to propose two approaches to evaluate the CO2 uptake amount of reactive magnesia-lime-fly ash/slag solidified soils, rather than soil improvement proved by previous findings. The CO2 uptake efficiency, pore structure and micro-mechanisms are examined, and the carbon footprint of each designed mixture is evaluated based on life cycle assessment. The key outcomes from accelerated carbonation, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests reveal that: (i) CO2 uptake amount and uptake efficiency estimated by direct weight gain are largely greater than that defined by indirect thermogravimetric analysis, (ii) CO2 uptake amount and uptake efficiency tend to increase with carbonation duration, binder content and mass ratio of magnesia/lime, (iii) accelerated carbonation induces reduced total pore volume, smaller pore size and denser microstructure and reactive magnesia contributes more than lime in filling pore spaces, (iv) magnesium and calcium carbonates are formed in magnesialime-fly ash/slag solidified soils storing permanently CO2, and (v) carbon emissions for magnesia-lime-fly ash/ slag blends are greatly reduced during their whole product life cycle in comparison to Portland cement.

1. Introduction Greenhouse gases from anthropic activities, especially CO2 that puts us at the greatest risk of irreversible changes in comparison with other heat-trapping gases, are the most significant driver of climate change since the mid-20th century (IPCC, 2013). To address actively the climate change problem, an ambitious effort is currently undergoing by all nations to limit expectedly global warming to 1.5 °C according to a special report issued by the IPCC in 2018, and this requires the world to reach "net zero" human-caused CO2 emissions around 2050. To achieve this major target and mitigate the effect of disasters linked to global temperature rise, CO2 emissions from the energy- and industry-related sectors that account for a considerable share of total greenhouse gases emissions must be designedly regulated and reduced. In the construction sector, the cement manufacturing industry, which is extremely energy intensive and an important CO2-emitting source, could even contribute as much as 8% of global CO2 emissions (Andrew, 2018). As the worldwide awareness increases regarding CO2 mitigation and

sustainability, an innovative and effective approach to reduce CO2 emissions is carbon capture, utilization and storage (CCUS) technology (Sanna et al., 2014; Sanna et al., 2016), besides producing low-carbon alternative materials and improving energy efficiencies of cement plants etc. In recent years, an emerging technology – CO2 curing for creation of superior and sustainable building materials has been attracting increasing attention due to its potential of transforming CO2 gas into safe and stable carbonate minerals. The CO2 curing approach combined with CaO- and/or MgO-bearing materials provides a promising possibility for producing high performance cement/concrete and integrating safe and permanent sequestration of CO2. In some previous researches, the technical feasibility and influencing factors of CO2 carbonation curing have been fully discussed in terms of cement-based materials, such as Portland cement (Rostami et al., 2012), waste cement (Fang and Chang, 2015), limestone cement (El-Hassan and Shao, 2015), belite-rich Portland cement (Jang and Lee, 2016), lightweight concrete (Shi and Wu, 2008), precast concrete (Zhang and Shao, 2016), fly ash concrete

⁎ Corresponding author at: Key Laboratory of Geotechnical and Structural Engineering Safety of Hubei Province, School of Civil Engineering, Wuhan University, 8 Dong Hu South Road, Wuhan, 430072, China. E-mail addresses: [email protected] (D. Wang), [email protected] (J. Zhu), [email protected] (F. He).

https://doi.org/10.1016/j.ijggc.2019.102827 Received 16 July 2019; Received in revised form 30 August 2019; Accepted 2 September 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.

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(Khunthongkeaw et al., 2006) and slag-cement concrete (Monkman and Shao, 2010). The carbonation curing of cement-based materials can result in rapid strength gain owning to the formation of carbonate products (Shao and El-Hassan, 2013) and acceleration of hydration reaction of C3S and C2S (Berger et al., 1972; Bukowski and Berger, 1978). In the presence of water, the main calcium silicate phases react with CO2 to form calcium carbonates in the form of calcite and aragonite and calcium silicate hydrate gels (Fabbri et al., 2009). Some factors like CO2 pressure, curing time, relative humidity, temperature and water to cement ratio have been proved to play an important role in the carbonation potential through CO2 absorption (Shi and Wu, 2008; Shi et al., 2012; Zhang et al., 2017). Apart from cement-based materials, the existing CO2 curing technology is also retrofitted into MgObearing cements, which have gotten a lot of attention worldwide since Harrison patented novel reactive MgO cements in 2008 (Harrison, 2008). Afterwards, considerable studies have been devoted to developing MgO-based materials and investigating their carbonation and hydration performance considering the associated factors, such as reactive MgO-Portland cement (Mo and Panesar, 2012; Unluer and AlTabbaa, 2014), MgO-fly ash blend (Wang et al., 2019a), MgO-fly ashPortland cement (Mo et al., 2017), MgO-slag blend (Jin et al., 2015), MgO-slag-Portland cement (Panesar and Mo, 2013), MgO concrete (Dung and Unluer, 2018) and nano-MgO-Portland cement (Yao et al., 2019). As to MgO-bearing materials, the mechanical and microstructural behaviour after exposure to CO2 are considered to be greatly attributed to the presence of magnesium carbonate hydrates, including dypingite (Mg5(CO3)4(OH)2·5H2O), hydromagnesite (Mg5(CO3)4(OH)2·4H2O), artinite (Mg2CO3(OH)2·3H2O) and nesquehonite (MgCO3·3H2O) (Vandeperre and Al-Tabbaa, 2007; Dung and Unluer, 2018). Especially, the dissolved Mg2+ ions tend to inhibit the calcite precipitation in warm environments and be incorporated into the calcite crystal structure, thereby producing Mg-calcite ((Ca, Mg)CO3)) (De Silva et al., 2009; Mo and Panesar, 2012). Nevertheless, the evidence of quantitative assessment of CO2 sequestration has seldom been provided in previous studies, and the implication of carbonation on the microstructure of reactive MgO-bearing systems is not yet well understood. The above-mentioned literatures reveal that CO2 curing is an emerging, efficient and effective technology to improve the mechanical behaviour of CaO- and MgO-bearing cement/concrete with permanent CO2 fixation. However, relatively few studies have provided insight into the implementation of CO2 carbonation in soil stabilization, although the high contents of free CaO and MgO in cementitious materials and formation of Ca- and Mg-carbonates make them potential candidates for carbonated binders to stabilize soils in ground improvement. This interesting concept has already been put into practice and firmly validated by a few scholars, including Yi et al. (2013); Du et al. (2016); Cao et al. (2017); Liu et al. (2017); Hwang et al. (2018) and Wang et al. (2019b, 2019c). Yi et al. (2013) and Liu et al. (2017) found that the main products of carbonated MgO-solidified soils responsible for the strength improvement are the hydrated magnesium carbonates of nesquehonite, hydromagnesite and/or dypingite. The results obtained by Du et al. (2016) and Cao et al. (2017) proved that CO2 carbonation could increase the acid buffer capacity and compressive strength of cement-stabilized Zn and Pb contaminated soils. The atmospheric CO2 curing provided competitive compressive strength of carbonated soils in comparison with cement treatment, but the elevated CO2 concentrations to 50% and 100% would decrease the strength (Hwang et al., 2018). Wang et al. (2019b, 2019c) attempted to introduce industrial by-products into sludge solidification in combination with reactive MgO and CO2 carbonation, although low-calcium class F fly ash did not play an important role in carbonation reaction. These studies suggest that CO2 carbonation implies a great potential for improving quickly soil performance and sequestering safely CO2 emissions. For this reason, the feasibility of solidifying soils by carbonation technology and associated challenges should get more and more attention in future research.

Motivated by developing novel and low-carbon approach for soil stabilization and further understanding the mechanisms behind, this study endeavors to integrate magnesia, lime and industrial by-products as eco-friendly cementitious materials for soil improvement and investigate their contribution to CO2 sequestration and microstructural characteristics. The representative mix designs of magnesia-lime-fly ash/slag are considered, including varying binder amounts and proportions of fly ash/slag, magnesia and lime. The theoretical CO2 uptake capacity of different mix designs is quantified in both cases of ignoring carbonation of MgO contained in fly ash/slag and taking it into account. Two methods of measurement, i.e. direct weight gain by digital balance and indirect weight loss by TGA/DSC technique (thermogravimetric analysis/differential scanning calorimetry), are tentatively proposed to assess the quantity of captured CO2 during carbonation and appraise the CO2 sequestration capacity of the designed materials. CO2 uptake amount and efficiency has historically been difficult to quantify, and up to now only few studies have focused on the balance between CO2 emissions and CO2 uptake in the life cycle. Based on the CO2 uptake quantity estimated by the proposed methods and the CO2 emissions during manufacture of cementitious materials, the net CO2 footprint is comparatively investigated to examine its role in off-setting greenhouse gas emissions associated with cement manufacturing. The microstructural characteristics in terms of pore structure and morphology are identified by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests. The knowledge obtained from this study is essential to further evolve the combined technique of CO2 carbonation, industrial by-products, reactive magnesia and lime for soil improvement and CO2 sequestration. 2. Experimental programs 2.1. Materials The tested soils for CO2 carbonation tests are commercial kaolin clay from certain kaolin Co. Ltd in Hubei province, China. The powdered clay is almost dry and the initial water content is 0.17% measured in a laboratory oven at 40 °C. The specific gravity is 2.75, determined by standard pycnometer method. The liquid limit and plastic limit are respectively 29.63% and 20.15%, and this gives correspondingly a plasticity index of 9.48. It is noteworthy that the above values of physical parameters are determined based on the Chinese standard Test Methods of Soils for Highway Engineering (JTG E40-2007, 2007). The chemical compositions of the chosen binder materials, i.e. slag, fly ash, magnesia and lime, are shown in Table 1. The mineral additives of slag and fly ash are mainly composed of the CaO-MgO-SiO2-Al2O3 glassy phases. The ground granulated blast furnace slag obtained from a new building material company in Hubei province, China contains 42% CaO, and the amount of CaO-SiO2-Al2O3-MgO reaches as high as 93%. Differently, the fly ash sampled from a thermal power plant in Henan province, China includes merely 15.55% CaO, and the percentage of CaO-SiO2-Al2O3-MgO is 90.41. Thus, it can be categorized as a Class C fly ash (high calcium, CaO% > 10%) according to ASTM C618-19 (2019). The reactive MgO is the major composition of magnesia (97.01%), which was produced by an industrial calcination of magnesite (∼800 °C) in a magnesium manufacturer Co. LTD in Shandong Table 1 Chemical composition (% weight) of binder materials. Materials

Slag Fly ash Lime Magnesia

2

Components CaO

SiO2

Al2O3

Fe2O3

MgO

Na2O

Others

42.00 15.55 92.00 0.25

33.00 52.15 0.34 –

12.00 20.11 – –

1.00 3.14 – –

6.00 2.60 1.50 97.01

– 1.94 – 0.01

6.00 – 6.16 2.74

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province, China. It has an iodine absorption value (an index reflects the activity of reactive MgO) of 110. The lime used herein was bought from a certain company in Jiangxi province, China and it has a high purity of 92%.

Table 2 Program of carbonation tests.

2.2. Testing methods The same specimen preparation procedure and testing methods as presented in Wang et al. (2019b, 2019c) were used in this study. The 2 mm-sieved dry kaolin powders, which have been initially pretreated to achieve the designed initial water content of 17%, were afterwards mixed homogeneously with fly ash/slag, magnesia and lime for 5 min through a mechanical mixer. The binder-soil blends were placed in three layers in cylindrical steel molds (39.1-mm inner diameter and 100-mm height), and then compacted by a hydraulic jack to achieve the designed dry density of 1.69 g/cm3 and degree of compaction of 96%. The prepared specimens were cured under relative humidity > 95% and constant temperature of 20 ± 2 °C. A portion of specimens were subjected to CO2 accelerated carbonation tests for 0.5, 1, 3, 6, 12 and 24 h under CO2 pressure of 150 kPa and confining pressure of 300 kPa, which were chosen according to previous studies on carbonation mode reported by Wang et al. (2019b, 2019c). It should be noted that the CO2 accelerated carbonation technique does not work effectively for low gas permeability soils, and this is the main reason why the granular materials such as sand can be incorporated to solve the problem. The schematic illustration of self-developed carbonation apparatus is shown in Fig. 1, where the confining pressure and CO2 pressure are adjustable over a certain range. The carbonation device for soil specimens consists of three parts, i.e. confining pressure loading system, CO2 circulation system and carbonation test platform. The carbonation testing program is reported in Table 2, where the influencing factors such as binder quantity, mass ratio of fly ash/slag: lime: magnesia and carbonation period are incorporated. The amount of fly ash/slag-lime-magnesia blends in percentage is 10, 15, 20 and 25, in terms of the weight of the blends over dry soils. The mass ratio of lime to magnesia is chosen to be 0:4, 1:3 and 4:0, in case that the mass ratio of fly ash/slag to lime-magnesia blends is fixed at 6:4. The carbonation period of 0.5 h is employed all through the carbonation tests, except in the case of investigating the impact of carbonation period on the amount of CO2 uptake. The abbreviations “aPxFyLzMCt” and “aPxSyLzMCt” represent the t-hour carbonated specimens with a% of blends with mass ratio of fly ash/slag:lime:magnesia equal to x:y:z, where P, F, S, L, M and C are the abbreviations of percentage, fly ash, slag, lime, magnesia and carbonation. A range of laboratory tests were implemented on the oven-dried block and powdered samples taken from inside the representative

Binder amount, P (%)

Fly ash/Slag: lime: magnesia, F/S: L: M

Carbonation time, t (h)

Notes

10 15 20 25

6:0:4 6:1:3 6:4:0

0.5 1 3 6 12 24

Confining pressure: 300 kPa CO2 pressure: 150 kPa

specimens, including TGA/DSC (thermogravimetric analysis/differential scanning calorimetry), MIP (mercury intrusion porosimetry) and SEM (scanning electron microscopy). The mass difference was directly measured for specimens before and after the accelerated carbonation process. The mercury intrusion porosimetry tests that characterise the pore structures are conducted on a PoreMaster 33 porosimeter, with pore size measurement range of 0.005–1080 μm and maximal intrusion pressure of 228 MPa. The thermogravimetric tests were performed through a STA 7300 Series instruments, with temperature range of 40–850 °C, heating rate of 10 °C/min, alumina crucibles and nitrogen as the purge gas. The cross-section morphology of block samples was defined by scanning electron microscopy tests on a FEI Quanta 200 machine, and a thin layer of gold was sputter-coated on the fresh surface of tested samples to avoid the charging effect during the SEM testing. 3. Results and discussion 3.1. CO2 uptake capacity defined by theoretical calculation The CO2 uptake capacity represents the maximal amount of CO2 that the alkaline earth metals (i.e. Ca, Mg) in magnesia-lime-fly ash/ slag solidified soil can absorb in theory, assuming that all the contained MgO and CaO are completely carbonated. CO2 is absorbed and chemically combined into carbonate phases, which partially contribute to reducing anthropogenic CO2 emissions. The carbonation process of reactive MgO and CaO is shown in Eqs. (1)–(6), and the final products are generated in the form of calcium carbonates (i.e. calcite, aragonite, vaterite) and hydrated magnesium carbonates (i.e. hydromagnesite Mg5(CO3)4(OH)2·4H2O, dypingite Mg5(CO3)4(OH)2·5H2O, nesquehonite MgCO3·3H2O).

CaO + H2 O

Ca(OH)2 + CO2 MgO + H2 O

(1)

Ca(OH) 2 CaCO3 + H2 O

(3)

Mg(OH)2

Mg(OH) 2 + CO2 + 2H2 O

5Mg(OH)2 + 4CO2 + H2 O

5Mg(OH)2 + 4CO2

(2)

MgCO3 3H2 O

Mg5 (CO3)4 (OH) 2 5H2 O

Mg5 (CO3)4 (OH) 2 4H2 O

(4) (5) (6)

CO2 capacity (g) = (0.88–1.1) × MgO mass (in fly ash, slag, magnesia) (g) + 0.786 × CaO mass (in fly ash, slag, lime) (g) (7) CO2 capacity (g) = (0.88–1.1) × MgO mass (in magnesia) (g) + 0.786 × CaO mass (in fly ash, slag, lime) (g) (8) These equations provide a possibility to predict the CO2 uptake capacity of binding materials and to evaluate their CO2 uptake efficiency under specific carbonation conditions. Based on Eqs. (1)–(6), the theoretical CO2 uptake capacity of per gram MgO and CaO is proved to be 0.88–1.1 g CO2 and 0.786 g CO2, respectively. The upper limit of 1.1 g CO2/g MgO is calculated through Eq. (4), while the lower limit of 0.88 g CO2/g MgO is defined by Eqs. (5) and (6). The CO2 sequestration

Fig. 1. Schematic diagram of carbonation apparatus. 3

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Table 3 CO2 uptake capacity of magnesia-lime-fly ash/slag solidified soils. Binders

CO2 uptake capacity in theory /kg binder

20P6F4L0MC /kg

20P6F1L3MC /kg

20P6F0L4MC /kg

20P6S4L0MC /kg

20P6S1L3MC /kg

20P6S0L4MC /kg

Fly ash /g Slag /g Magnesia /g Lime /g Total /g

145.7-151.6 382.9-396.1 853.7-1067.1 722.9 –

17.5-18.2 – – 57.8 75.3-76.0

17.5-18.2 – 51.2-64.0 14.5 83.2-96.7

17.5-18.2 – 68.3-85.4 – 85.8-103.6

– 46.0-47.5 – 57.8 103.8-105.3

– 46.0-47.5 51.2-64.0 14.5 111.7-126.0

– 46.0-47.5 68.3-85.4 – 114.3-132.9

Fig. 2. CO2 uptake defined by direct weight gain on magnesia-lime-fly ash solidified soils. (a) CO2 uptake amount (b) CO2 uptake efficiency.

capacity of MgO is theoretically greater than that of CaO due to the low molecular weight of MgO assumed to be available for carbonation. Thereafter, per kilogram of fly ash, slag, magnesia and lime chosen in this study accordingly has a CO2 uptake capacity of 145.7–151.6 g, 382.9–396.1 g, 853.7–1067.1 g and 722.9 g, as calculated according to Eq. (7). These data permit to determine the maximal quantity of absorbed CO2 per kilogram of magnesia-lime-fly ash/slag solidified soils, and the obtained results are shown in Table 3. The designed materials are capable of sequestering 75.3–76.0 g, 83.2–96.7 g, 85.8–103.6 g, 103.8–105.3 g, 111.7–126.0 g and 114.3–132.9 g CO2 sequentially for per kilogram of 20P6F4L0MC, 20P6F1L3MC, 20P6F0L4MC, 20P6S4L0MC, 20P6S1L3MC and 20P6S0L4MC. However, it is worthwhile to note that the theoretical prediction of CO2 storage capacity is indeed overestimated to some extent for the designed materials, because in fact only the free-MgO/CaO in fly ash/slag can be the major elements for carbonation.

where %CO2 is the mass percentage of CO2 captured in carbonated specimens, and %CO2th is the theoretical weight percentage of CO2 in carbonated specimens for total available Mg2+ and Ca2+ ions. The CO2 mass defined by approach I (i.e. mass difference in gram between non-carbonated and carbonated samples) can be roughly estimated on the specimens subjected to accelerated carbonation conditions (CO2 partial pressure of 150 kPa, confining pressure of 300 kPa). The actual amount of captured CO2 by 20P6F(S)4L0MC, 20P6F(S) 1L3MC and 20P6F(S)0L4MC is reported in Figs. 2(a) and 3(a), which show the change of CO2 uptake with carbonation duration from 0.5 to 24 h and are affected by the mass ratio of lime to magnesia. It appears that the CO2 uptake amount of solidified soils tends to increase with carbonation duration (except 20P6S4L0MC) and achieves its climax at 24 h. The actual quantity of CO2 uptake at 6 h reaches 4.87 g, 10.22 g, 13.05 g, 6.96 g, 11.66 g and 13.42 g respectively for 20P6F4L0MC, 20P6F1L3MC, 20P6F0L4MC, 20P6S4L0MC, 20P6S1L3MC and 20P6S0L4MC, while the values at 24 h are correspondingly 5.72 g, 12.41 g, 13.33 g, 5.99 g, 13.79 g and 14.66 g. The comparison reveals that, owing to higher content of MgO in slag than in fly ash, the actual CO2 absorption capacity of solidified soils with magnesia-lime-slag blend is proved slightly higher than that with magnesia-lime-fly ash blend. As anticipated, most of the CO2 is sequestered at early ages of 3–6 h. The longer the carbonation duration is, the more the Ca2+ and Mg2+ ions released from magnesia, lime, fly ash and slag are combined into carbonate phases, thus sequestering more CO2. According to Eq. (9), the CO2 uptake efficiency of 20% binder-solidified specimens can be calculated, and its variation with carbonation duration is shown in Figs. 2(b) and 3(b). An identical trend can be found for the evolution of both CO2 uptake amount and CO2 uptake efficiency, i.e. the CO2 uptake efficiency of solidified soils tends to go up with carbonation duration as well (except 20P6S4L0MC). For specimen 20P6S4L0MC, the uptake efficiency increases rapidly with carbonation

3.2. CO2 uptake efficiency defined by direct weight gain The total CO2 uptake capacity of magnesia-lime-fly ash/slag solidified soils is given in Table 3, so the remaining challenge to assess the CO2 uptake efficiency and net life cycle CO2 emissions is how to reasonably determine the actual mass of CO2 absorbed into the designed materials. Two practical and viable approaches for addressing this issue are put forward by this study, that is, measuring directly weight gain by a digital balance with high precision of 0.01 g (approach I) and defining indirectly weight loss by TGA/DSC technique (approach II). As defined by Eq. (9), the CO2 uptake efficiency in percentage is introduced and calculated as the mass fraction of captured CO2 related to the theoretical maximum, i.e. %efficiency = %CO2 captured × 100/%CO2th

(9) 4

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Fig. 3. CO2 uptake defined by direct weight gain on magnesia-lime-slag solidified soils. (a) CO2 uptake amount (b) CO2 uptake efficiency.

duration till the peak of 34.9–35.4% at 3 h, followed by a sharp drop, whereas for 20P6F4L0MC the maximum value of 39.4–39.8% appears at 12 h. That is, in contrast with other formulations, the carbonation efficiency of 20P6F(S)4L0MC is evidently insufficient due to the lower magnesia content. The 20P6F1L3MC, 20P6F0L4MC, 20P6S1L3MC and 20P6S0L4MC have the maximal CO2 uptake efficiency at 24 h, which gives correspondingly higher values of 65.8–76.5%, 66.0–79.7%, 56.1–63.3% and 58.1–67.5%. The higher carbonation degree of 20P6F (S)1L3MC and 20P6F(S)0L4MC is principally because of the incorporation of Mg2+ ions leached from magnesia and formation of various magnesium carbonates, reflecting an excellent capacity of reactive magnesia to bind CO2. It is interesting to find that the CO2 uptake efficiency of magnesialime-fly ash solidified soils is greater than that of magnesia-lime-slag solidified soils owing to a much lower CaO content in fly ash (∼15.55%) than in slag (∼42%), even though the CO2 uptake amount of the former is less than that of the later. The CaO content discussed here is the total content of CaO component in fly ash and slag, which contain a large proportion of free CaO. Especially, the CO2 uptake amount and carbonation efficiency are tightly correlated with the physic-mechanical and microstructural characteristics of solidified soils, and they also have a significant impact on the evaluation of net CO2 emissions (i.e. the amount of CO2 emitted during their manufacture minus the fraction captured by them during carbonation). The relevant probe would be appropriately described in the following sections. The evolution of CO2 uptake amount with magnesia-lime-fly ash/ slag content is illustrated in Fig. 4 for soils solidified with different mass ratios of magnesia to lime (4:0, 3:1 and 0:4). From the obtained results, the CO2 uptake amount appears to be on a steady growth trend as the binder content rises from 10% to 25%, roughly demonstrating an increased quantity of carbonation products on the assumption that all the absorbed CO2 are fully combined with MgO and CaO in the designed blends. Thus, the accelerated carbonation forming calcium and magnesium carbonate phases enhances effectively the CO2 uptake amount of MgO- and CaO-bearing materials since more and more reactive components are involved to react with CO2. The microstructural explanations will be convincingly confirmed by SEM examination discussed as follows (see Figs. 9 and 10). In particular, the hydration of MgO and CaO is likely to cause volumetric expansion to be reckoned with and this would to a certain extent alter the carbonation process.

Fig. 4. Evolution of CO2 uptake amount with binder contents.

The molar volume expansion is about 90% during the hydration of CaO, which is less than the increase for MgO (∼117%) (Chatterji, 1995). The noticeable expansibility is conducive to produce the micro-cracks, which can be distinctly validated by Fig. 5 and accelerates possibly the mass loss of water taken away by CO2 migration. This should be the dominant reason why the amount of absorbed CO2 is somewhat reduced at 25% of binder content (i.e. 25P6F0L4MC, 25P6S4L0MC). The measured values of CO2 uptake are likely to be underestimated, since many factors might affect the unexpected mass loss of specimens, including potential loss of water taken away by CO2 migration and tiny block peeling off during the tests. This is the main reason why the phenomenon of decrease in CO2 uptake amount and uptake efficiency occurred at certain longer carbonation time. However, it should be kept in mind that all the dissolved CO2 in pore solution is supposed to have taken part in the carbonation process of magnesia-lime-fly ash/slag solidified soils and to be transformed into more stable carbonate phases, although the rationality of this hypothesis is an extremely complex issue that does deserve further investigation in future studies. 5

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Fig. 5. Volume expansions of carbonated specimens.

3.3. CO2 uptake efficiency defined by TGA/DSC curves

According to the TG curves in Fig. 6(b), the mass losses within the range of 500–850 °C can be calculated for carbonated specimens, contributing to the determination of total CO2 uptake percentage and CO2 uptake efficiency. As shown in Table 4, the specimens 20P6F0L4MC24h and 20P6S0L4MC24h have respectively the total CO2 uptake of 30.3% and 30.1%, which are much higher than the other specimens and correspond to the CO2 uptake efficiency of 60.6–73.1% and 48.2–55.8%. The analysis agrees well with the above-stated finding that no apparent peaks appear at 400–465 °C and 100–165 °C in the DSC curves. As the content of reactive magnesia decreases, the CO2 uptake amount and CO2 uptake efficiency tend to continuously decrease. It should be closely related to the fact that the replacement of reactive magnesia with lime decreases the source of Mg2+ supplied for the formation of magnesium carbonates, resulting in less total CO2 uptake. It is noteworthy that the phenomenon derived from approach II, i.e. CO2 uptake amount and CO2 uptake efficiency is steadily reduced if the replacement percentage of reactive magnesia by lime goes up, has a good agreement with the conclusion drawn from approach I. The comparison reveals that the CO2 uptake efficiency defined by approach II is obviously smaller than that measured by approach I. Therefore, how to explain the huge divergence between these two approaches and find out the intrinsic reason for that is indeed a troublesome issue, which requires a deeper investigation in the future on the basis of much more experimental results. The mass discrepancy arising from the experimental errors might occur in approach I which is more simple and direct, while only a few dozen milligrams (i.e. 40–60 mg) of dry powders were adopted in TG/DSC tests for approach II, causing possibly some uncertainty in sample representativeness. The multiple measurements by TG/DSC tests can to some extent counteract the uncertainty related to sample representativeness, which might demonstrate superiority in contrast to the approach I. The authors have to admit that the reliability and authenticity of the two approaches proposed in this study need to be further validated, but they do provide two ways to quantitatively asses the approximate amount of captured CO2. Due to its extreme complexity, no standard and generally recognized method has been established until now to evaluate the real quantity of the mineralized CO2 in soils solidified with cementitious binders in accelerated carbonation processes.

The temperature-dependent thermal behaviour of magnesia-lime-fly ash/slag solidified soils subjected to accelerated carbonation facilitates the evaluation of CO2 uptake identified by the mass loss in a specified temperature range. Nevertheless, it is indeed challenging to define the exact mass loss corresponding to each Ca- and Mg-carbonate phase due to the considerable overlap in the thermal decompositions of different phases. On the basis of existing literature (Mo and Panesar, 2012; Mo et al., 2017), the mass loss between 500 °C and 850 °C is proved to be principally related to the decomposition of calcium and magnesium carbonates, taking into account the amorphous crystalline and poorly crystalized carbonates. In consequent, this provides the possibility and feasibility of estimating the total CO2 uptake, and this is called after approach II. Since the tested materials including fly ash, ground granulated blast furnace slag and kaolinite were already calcined at temperature above 1000 °C during their production, the CO2 uptake by mass of initial dry raw materials after ignition of 850 °C is reasonably estimated according to Eq. (10). CO2 uptake = Δmass (500–850 °C)/mass at 850 °C

(10)

The TG and DSC curves of reactive magnesia-lime-fly ash/slag solidified soils after exposure to 24-h accelerated carbonation are shown in Fig. 6. The specimens exhibit evident endothermic peaks at 715–790 °C in Fig. 6(a), which primarily relates to the decomposition of calcium and/or magnesium carbonates, releasing CO2 captured in accelerated carbonation environments. The endothermic peaks detected at 400–465 °C and 100–165 °C correspond to respectively the dehydration of Ca(OH)2 and/or Mg(OH)2 and the dehydroxylation of C-S-H and/or M-S-H gels, indicating that Ca(OH)2 and Mg(OH)2 produced in solidified soils were not fully carbonated. This is consistent with the results presented by Wang et al. (2019a, 2019d), where the formation of C-S-H gels in magneisa-slag-H2O system and M-S-H gels in magnesiafly ash-H2O system has already been validated. It is worth noting that only minor peaks can be observed at the temperature ranges of 400–465 °C and 100–165 °C for specimens 20P6S0L4MC24h and 20P6F0L4MC24h, revealing a significant carbonation of magnesia-fly ash/slag solidified soils without lime.

6

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Fig. 6. TG/DSC curves of carbonated magnesia-lime-fly ash/slag solidified soils. (a) DSC (b) TG. Table 4 Measured CO2 uptake and CO2 uptake efficiency of carbonated magnesia-lime-fly ash/slag solidified soils (weight %). Materials carbonated for 24 h

Measured CO2 uptake (%) CO2 uptake efficiency (II) (%) CO2 uptake efficiency (I) (%)

20P6F4L0MC

20P6F1L3MC

20P6F0L4MC

20P6S4L0MC

20P6S1L3MC

20P6S0L4MC

6.6 18.1 38.6-38.9

16.9 36.4-42.1 65.8-76.5

30.3 60.6-73.1 66.0-79.7

8.1 16.6 29.9-30.3

18.2 30.8-34.5 56.1-63.3

30.1 48.2-55.8 58.1-67.5

3.4. Pore structure identified by MIP The pore structure characteristics (i.e. pore size distribution and porosity) are a great concern for detecting quantitatively the evolution of microstructure and explaining the micro-macro behaviour of carbonated solidified soils. Figs. 7 and 8 show the cumulative pore volume and pore size distribution of selected carbonated specimens identified by mercury intrusion porosimetry (MIP) tests. In Fig. 7 and Table 5, the specimens without reactive magnesia exhibit higher cumulative pore

Fig. 8. Pore size distribution of carbonated magnesia-lime-fly ash/slag solidified soils.

volumes in comparison to the magnesia-bearing specimens, irrespective of the lime replacement percentages of reactive magnesia. With the increasing addition of magnesia, the total intruded Hg volume of carbonated specimens is reduced from 0.206 ml/g for 20P6S4L0MC24h to 0.133 ml/g for 20P6S0L4MC24h and from 0.224 ml/g for 20P6F4L0MC24h to 0.079 ml/g for 20P6F0L4MC24h. This approves that the incorporation of reactive magnesia contributes more to the reduction of total pore volume and densification of microstructure after accelerated carbonation than the lime does. It is likely to be associated

Fig. 7. MIP curves of carbonated magnesia-lime-fly ash/slag solidified soils. 7

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Fig. 9. SEM images of carbonated magnesia-lime-fly ash solidified soils. (a) 20P6F0L4MC0.5h (b) 20P6F1L3MC6h (c) 20P6F4L0MC6h.

with lower amount of portlandite Ca(OH)2 owing to faster hydration of lime than magnesia and the formation of Mg-based carbonated phases filling the inter-particle and inter-aggregate spaces. In terms of total porosity, it is decreased from 13.39% for 20P6S4L0MC24h to 8.53% for

20P6S1L3MC24h and from 17.05% for 20P6F4L0MC24h to 5.95% for 20P6F1L3MC24h. If the mass ratio of magnesia to lime rises from 3:1 to 4:0, the total porosity is found to be slightly increased from 5.95% for 20P6F1L3MC24h to 6.64% for 20P6F0L4MC24h and from 8.53% for 8

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Fig. 10. SEM images of carbonated magnesia-lime-slag solidified soils. (a) 20P6S0L4MC0.5h (b) 20P6S1L3MC24h (c) 20P6S4L0MC24h.

20P6S1L3MC24h to 11.03% for 20P6S0L4MC24h as a result of the volumetric expansion associated with the formation of different hydrated magnesium carbonate species. Besides the reduction in total pore volume, the pore size distribution

of carbonated specimens becomes much finer due to the same replacement percentage of lime by reactive magnesia. The detailed examination of Fig. 8 shows that the carbonated specimens without magnesia (20P6S4L0MC24h and 20P6F4L0MC24h) have a significantly 9

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Table 5 Total pore volume and total porosity of carbonated magnesia-lime-fly ash/slag solidified soils.

Total pore volume (ml/g) Total porosity (%)

20P6F4L0MC24h

20P6F1L3MC24h

20P6F0L4MC24h

20P6S4L0MC24h

20P6S1L3MC24h

20P6S0L4MC24h

0.224 17.05

0.079 5.95

0.079 6.64

0.206 13.39

0.123 8.53

0.133 11.03

higher volume of coarse pores with diameter between 6 μm and 20 μm and a relatively lower volume of fine pores with diameter between 0.01 μm and 0.1 μm. As the magnesia content increases, the sharp peaks located at pore diameters around 10 μm and 0.04 μm are greatly weakened and further shift to smaller pore diameters. Especially for specimens 20P6F1L3MC24h and 20P6F0L4MC24h, no significant peaks can be detected throughout the whole pore diameter range, and this observation is expected to be attributed to the pore-filling of Mgbearing carbonated phases and their positive volumetric expansion squeezing pore spaces. This is to say, the carbonation of reactive magnesia appears to lead to a finer pore size distribution. Furthermore, at pore diameters within the range of 3–8 μm, some important peaks still exist for specimens 20P6S1L3MC24h and 20P6S0L4MC24h, but 20P6F1L3MC24h and 20P6F0L4MC24h have relatively faint peaks and flat pore size distribution as a result of the micro-aggregate effect of fly ash. The above discussion implies without doubt that the lime replacement by reactive magnesia promotes the densification in the microstructure of carbonated magnesia-lime-fly ash/slag solidified soils associated with pore structure refinement due to the formation of magnesium carbonates, which exhibit a higher effectiveness in filling the pore spaces than calcium carbonates.

lime, i.e. 20P6F4L0MC6h in Fig. 9(c), an abundance of CaCO3 with an acicular or needle-like (aragonite), rhomboid (calcite) and round (vaterite) shape is formed and tightly connected after CO2 carbonation for 6 h. Evidently, the fly ash particles are bound by the Ca2+ carbonate products formed nearby, which have three distinct anhydrous polymorphs (calcite, aragonite and vaterite). The incorporation and abundance of Ca2+ ions, which are dominantly leached from the product of lime hydration, affects the crystal morphology of CaCO3 owning to its influence on the crystal nucleation and growth process. Moreover, vaterite is metastable at ambient conditions and highly soluble with respect to calcite and aragonite, and is converted readily to calcite (at lower temperature < 30 °C) and aragonite (at higher temperature ≥ 40 °C) on exposure to water (Singh et al., 2016). This is the main reason why only a small plot of vaterite can be sporadically noticed on the spherical surface of fly ash in Fig. 9(c). Fig. 10 shows the typical SEM images of magnesia-lime-slag solidified soils subjected to CO2 curing and the matrix in the carbonated soils becomes much denser due to the formation of MgCO3 and/or CaCO3 as a result of carbonation. For 20P6S0L4MC0.5h in Fig. 10(a), various Ca2+- and Mg2+-bearing carbonates including calcite and nesquehonite are produced depending on the abundance of Ca2+ and Mg2+ released respectively from slag and magnesia. Different from the spread distribution of nesquehonite in matrix, the scattered rhombohedral calcite are dispersed on the surface of slag blocks, indicating that a small amount of Ca2+ ions are potentially dissolved from the superficial zone of slag and participate in the carbonation reaction. However, the slag is not expected to react much with CO2 due to the low content of Ca2+ ions released from the slag, and thus it has limited contribution to CO2 sequestration at ambient conditions. This phenomenon is similar to the case of fly ash as a result of glassy structure, where no Ca2+ carbonates can be seen although its existence cannot be denied. Owning to the addition of lime (source of dissolved Ca2+) in 20P6S1L3MC24h, magnesian calcite (Ca,Mg)CO3 with round shape can be identified in Fig. 10(b), while an abundance of nesquehonite is easily distinguished and therefore results into a very dense microstructure. The incorporation of Mg2+ into the calcite changes the morphology of CaCO3 crystals and promotes the formation of (Ca,Mg)CO3 during the precipitation process. Calcite normally formed in the CaO-CO2-H2O system is intermingled with hydrated magnesium carbonates in this case and is consequently difficult to be detected clearly. Regardless of the different sources of Ca2+ in 20P6S4L0MC24h, three polymorphs of CaCO3, i.e. calcite, aragonite and vaterite, are all present in Fig. 10(c). It is evident that increasing Ca2+ concentration (lime content) in the pore solution increases the carbonation rate and the amount of Ca2+bearing carbonates, and greater quantity of calcite and aragonite is yielded in Ca2+-rich environments compared to the metastable sphereshaped vaterite. Only a few vaterite that is not yet well crystalized and even amorphous due to its instability emerges on the surface of slag minerals. The development of different CaCO3 phases leads to obstructing the penetration of CO2 gas, and reduces consequently the rate of carbonation reaction and CO2 sequestration. Particularly, the participation degree of slag/fly ash during accelerated carbonation curing depends mainly on the pH level, curing temperature, CO2 partial pressure, moisture state, CO2 exposure time and characteristics of CaOMgO-SiO2-Al2O3 glassy phases (affecting the leaching capability of free Ca2+/Mg2+).

3.5. Mechanism of CO2 uptake confirmed by SEM The morphological characteristics as result of chemical carbonation process are captured to identify the mechanisms of CO2 sequestration of magnesia-lime-fly ash/slag solidified soils. After CO2 gas diffuses into the pore solution of chemically solidified soils and produces carbonate ions, it reacts with Mg2+ and Ca2+ leached from binding materials to precipitate MgCO3 and CaCO3 in different crystal forms. Fig. 9(a)-(c) illustrates the representative SEM images of magnesia-lime-fly ash solidified soils after exposure to the pressurized CO2. As shown in Fig. 9(a), the precipitation of MgCO3 occurs in the form of nesquehonite (MgCO3·3H2O), dypingite (Mg5(CO3)4(OH)2·5H2O) and/or hydromagnesite (Mg5(CO3)4(OH)2·4H2O). The hydrated Mg2+ carbonates are responsible for partially blocking inter-particle pores, which do generate denser microstructure but may obstruct the diffusion pathway of CO2. This finding explains why the amount of CO2 uptake tends to remain constant or decrease with increasing carbonation period. However, it seems difficult to detect the presence of CaCO3 from the selected samples probably due to the poorly crystalized or amorphous form and tiny amount of leached Ca2+ from fly ash within a limited carbonation period, although the tested fly ash contains 15.55% CaO. The fly ash in the system plays a main role in facilitating the CO2 diffusion and providing the nucleation and growth sites for carbonation products. Besides abundant rosette-like dypingite/hydromagnesite and elongated prismatic or rod-like nesquehonite, calcium carbonates in the form of acicular aragonite can be distinctly observed on the pore walls and on the surface of fly ash particles in Fig. 9(b), provided that lime is incorporated to equivalently replace reactive magnesia as for 20P6F1L3MC6h. Especially, longer carbonation time promotes the carbonation degree of solidified soils and ameliorates largely the carbonate crystal growth, when compared to 20P6F0L4MC0.5h. The combined action of well-crystalline Ca2+ and Mg2+ carbonates contributes to the densification of microstructure and thereby improves the physic-mechanical behaviour of solidified soils by pore filling and particle agglomerating. In case magnesia is completely substituted by 10

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of magnesia should exist, taking into account the contribution to mineral sequestration of CO2 and the relevant increase in CO2 emissions for MgO manufacture in comparison to lime and Portland cement. Compared to approach I, a similar variation of CO2 uptake and net CO2 emissions estimated by approach II can be seen in terms of mass ratio of magnesia to lime, but the CO2 uptake values are lower and the net CO2 emissions are greater. Accordingly, this results into a lower ratio of CO2 uptake to CO2 emissions, and it increases from 24.0% of 6F4L0M to 54.1% of 6F0L4M and from 29.5% of 6S4L0M to 53.8% of 6S0L4M when the amount of magnesia used as a lime replacement increases. In other words, the usefulness of approach I defined by direct mass measurement can be confirmed to a certain extent by the approach II based on the thermogravimetric analysis. The proposed approaches, which may be imperfect but valuable, prove that the accelerated carbonation can yield surprisingly environmental benefits by CO2 mineralization in magnesia-lime-fly ash/slag solidified soils. The coupled technology of CO2 accelerated carbonation with industrial byproducts achieves the three-win goal of beneficial reuse of industrial byproducts, permanent fixation of CO2 and solidification of soils.

Table 6 Estimation of net CO2 emissions among different magnesia-lime-fly ash binders (on per tonne of materials). Blends

6F4C 6F4L0M 6F1L3M 6F0L4M

PC used /kg Magnesia used /kg Fly ash used /kg Lime used /kg CO2 emissions due to PC manufacture /kg CO2 emissions due to magnesia manufacture /kg CO2 emissions due to fly ash manufacture /kg CO2 emissions due to lime manufacture /kg Total CO2 emissions /kg Curing conditions (CO2 pressure, time) CO2 uptake (approach I) /kg Net CO2 emissions (I) /kg Ratio of CO2 uptake (I)/CO2 emissions /% CO2 uptake (approach II) /kg Net CO2 emissions (II) /kg Ratio of CO2 uptake (II)/CO2 emissions /%

400 0 600 0 358 0 0 0 358 / / 358 / / 358 /

0 0 0 0 300 400 600 600 600 400 100 0 0 0 0 0 420 560 0 0 0 273.2 68.3 0 273.2 488.3 560 300 + 150 kPa, 24 h 146.7 318.2 341 126.5 170.1 219 53.7 65.2 60.9 65.5 169.3 303.1 208.2 319 256.9 24.0 34.7 54.1

4. Discussions

3.6. Carbon footprint assessment

This study endeavours to demonstrate the potential of magnesialime-fly ash/slag solidified soils in sequestering CO2 emitted from anthropic activities and to investigate the associated microstructure and micromechanisms. The CO2 carbonation process involves primarily the following stages: (i) hydration of pozzolan components and dissolution of Ca(OH)2 and Mg(OH)2, (ii) CO2 dissolution in water and generation of carbonate ions, and (iii) combination of Ca2+/Mg2+ ions with carbonates and precipitation of CaCO3 and MgCO3 phases. Ca2+ originates chiefly from the decalcification of lime, slag and fly ash, while Mg2+ is dominantly provided by magnesia (a small fraction probably from slag and fly ash). Calcium carbonates (calcite, aragonite, vaterite) and magnesium carbonates (nesquehonite, dypingite, hydromagnesite) start to crystalize upon carbonation and precipitate in the pores and at the pore walls when the supersaturated solution is reached. The carbonate precipitates may partially fill and plug the pore channels, reducing the gas permeability and impeding the CO2 diffusion rate. The obstruction of CO2 ingress and formation of dense carbonate layers on the surface of particles will slow down the carbonation rate, and this is the main reason why the CO2 uptake efficiency remains almost constant or declines slightly after 6-h carbonation. As the carbonation reaction proceeds, CO2 can penetrate through the carbonate layer and infiltrate into unreacted minerals, thereby resulting into a slight increase in CO2 uptake efficiency at 24 h. Since the carbonation process is controlled by diffusion of Ca2+, Mg2+ and CO2, the rate and extent of carbonation reaction are influenced by various factors, including carbonation duration, confining pressure, partial pressure, temperature, CO2 concentration, water content and pH of pore solution etc. These process parameters, which are extremely complex and likely to influence the micro-macro behaviour of carbonated solidified soils, could affect the CO2 uptake amount, uptake efficiency and carbonate amount through the dissolution and migration of Ca2+, Mg2+ and CO32−. The formation of Mg(OH)2 and Ca(OH)2 elevates the pH of pore solution accelerating the precipitation of carbonates, but its volumetric expansion is likely to induce the propagation of micro-cracks damaging the soil strength and facilitating the CO2 infiltration. The SEM images reveal that magnesia and lime contribute dominantly to the formation of Ca- and Mg-carbonates and effectively densify the microstructure of matrix with relatively lower pore volume. The carbonation process decreases the cumulative pore volume of solidified soils, and causes a pore size transformation from inter-aggregate pore to inter-particle pore (Wang et al., 2019b). The

To evaluate the CO2 sequestration capacity in magnesia-lime-fly ash/slag solidified soils, it is essential to calculate the net CO2 emissions of magnesia-lime-fly ash/slag blends according to the CO2 emissions of binder blends and the actual CO2 uptake by accelerated carbonation. The obtained results are presented in Tables 6 and 7. During their lifetime, the binding materials naturally capture an important fraction of the CO2 emitted during their manufacture, but the amount of captured CO2 is generally lower than the case of accelerated carbonation. The comparative analysis demonstrates that the CO2 emissions of magnesia-lime-fly ash/slag blends are reduced significantly compared with Portland cement. According to approach I, the net CO2 emissions of 6F4L0M and 6S4L0M decrease respectively from 273.2 kg to 126.5 kg and from 273.2 kg to 115.7 kg, distinctly lower than that of PC-slag/fly ash blends (358 kg). With the increasing replacements of lime with magnesia, the net CO2 emissions appear to be largely increased owning to relatively higher CO2 emissions in the manufacturing process of magnesia. The mass ratio of CO2 uptake to CO2 emissions rises from 53.7% of 6F4L0M to 65.2% of 6F1L0M and from 57.7% of 6S4L0M to 74.3% of 6S1L0M, followed by a drop to 60.9% of 6F0L4M and 68.9% of 6S0L4M. It means that an optimum incorporation amount Table 7 Estimation of net CO2 emissions among different magnesia-lime-slag binders (on per tonne of materials). Blends

6S4C 6S4L0M 6S1L3M 6S0L4M

PC used /kg Magnesia used /kg Slag used /kg Lime used /kg CO2 emissions due to PC manufacture /kg CO2 emissions due to magnesia manufacture /kg CO2 emissions due to slag manufacture /kg CO2 emissions due to lime manufacture /kg Total CO2 emissions /kg Curing conditions (CO2 pressure, time) CO2 uptake (approach I) /kg Net CO2 emissions (I) /kg Ratio of CO2 uptake (I)/CO2 emissions /% CO2 uptake (approach II) /kg Net CO2 emissions (II) /kg Ratio of CO2 uptake (II)/CO2 emissions /%

400 0 600 0 358 0 0 0 358 / / 358 / / 358 /

0 0 0 0 300 400 600 600 600 400 100 0 0 0 0 0 420 560 0 0 0 273.2 68.3 0 273.2 488.3 560 300 + 150 kPa, 24 h 157.5 362.8 385.79 115.7 125.5 174.21 57.7 74.3 68.9 80.7 181.7 301.0 192.5 306.6 259.0 29.5 37.1 53.8

11

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accelerated carbonation of reactive magnesia contributes more to the reduction of total pore volume and the densification of microstructure of carbonated magnesia-lime-fly ash/slag solidified soils, confirming a higher effectiveness of magnesium carbonates in filling the pore spaces than calcium carbonates. The addition of magnesia and lime is expected to provide more Mg2+ and Ca2+ ions for the formation of an interconnected network structure, consisting of densely agglomerated Caand Mg-bearing carbonates. Especially, the phenomenon of localized carbonation can be observed owning to the enrichment of carbonates. The Mg2+-rich environment promotes the possibility of incorporating Mg2+ ions into the calcite lattice to form magnesium calcite (Pokrovsky, 1998). The obtained results indicate that fly ash and slag do contribute little to the generation of the Ca- and Mg-carbonates due to the lack of relatively high hydraulic activity minerals, although they have taken an active part in the carbon capture and storage. Under carbonation condition, CO2 penetrates through the CaO-MgO-SiO2-Al2O3 glassy layers and promotes the leaching of more Ca2+ ions. The combination of Ca2+ with CO32− causes the formation of calcite crystals, which are sporadically spread within matrix, as seen from the SEM images of carbonated magnesia-fly ash/slag solidified soils. The precipitation of hydration and carbonation products on the surface of fly ash and slag particles inhibits further hydration and carbonation, resulting in an inefficiency of fly ash and slag as the source of Ca2+ and limiting the development of microstructural and mechanical behaviour. Fly ash and slag particles do work synergistically in two aspects: (i) the provision of nucleation sites by increasing the surface area to facilitate continuous hydration and carbonation, and (ii) the coupled effect of pore-filling and skeleton-building owning to their micro-aggregated effect. For magnesia-lime-fly ash/slag solidified soils, the quantity of Ca2+ and Mg2+ ions that can be released from slag/fly ash plays an important role in the accelerated carbonation, rather than the total quantity of CaO and MgO contained in slag/fly ash. In other words, the limitations associated with the low dissolution of fly ash and slag glassy phases and the subsequent hydration and carbonation are considered a major challenge to be alleviated in the future study.

cumulative pore volume in contrast to magnesia-bearing specimens. An increase in magnesia content causes sharply weakened peaks at pores around 0.04 and 10 μm and a shift to smaller pores owning to the pore-filling of Mg-bearing carbonated phases and their volumetric expansion squeezing pore spaces. Reactive magnesia contributes more to the reduction of total pore volume and the densification of microstructure than the lime does. (iv) Series of magnesium carbonates and calcium carbonates are produced in carbonated magnesia-lime-fly ash/slag solidified soils, realizing the permanent storage of CO2. The combination of Ca2+ and Mg2+ with carbonates causes reduced porosity and denser microstructure. The fly ash and slag are not expected to react fiercely with CO2 due to the limited content of released Ca2+, but they provide nucleation sites for the carbonate precipitation. The formation of (Ca,Mg)CO3 is promoted in case of the Mg2+ ions incorporated into calcite crystals. (v) Reactive magnesia-lime-fly ash/slag solidified soils have a great potential in permanently sequestering CO2 and efficiently reducing CO2 emissions. Compared with Portland cement, the CO2 emissions for magnesia-lime-fly ash/slag blends can be significantly reduced during their whole product life cycle, and this gives a viable way to rapidly produce cementitious materials for soil solidification. Future study on the application of in-situ carbonation technology is needed in future, which would be of great interest and significance. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant no. 51879202, no. 51609180).

5. Conclusions

References

The CO2 accelerated carbonation coupled with industrial by-products provides an innovative solution to permanently sequester CO2 and stabilize soils. The CO2 uptake efficiency, microstructure and mechanisms are investigated on reactive magnesia-lime-fly ash/slag solidified soils in accelerated carbonation environments. The key conclusions regarding CO2 sequestration efficiency and effect of carbonation on microstructure are drawn as follows.

Andrew, R.M., 2018. Global CO2 emissions from cement production, 1928-2017. Earth Syst. Sci. Data Discuss. 10, 2213–2239. ASTM C618-19, 2019. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. ASTM Int., West Conshohocken. Berger, R.L., Young, J.F., Leung, K., 1972. Acceleration of hydration of calcium silicates by carbon dioxide treatment. Nat. Phys. Sci. 240, 16–18. Bukowski, J.M., Berger, R.L., 1978. Reactivity and strength development of CO2 activated non-hydraulic calcium silicates. Cem. Concr. Res. 9, 57–68. Cao, Z., Zhang, T., Zhang, D., 2017. Effect of Carbonation on the Engineering Properties and Microstructural Characteristics of Cement Solidified Lead-contaminated Soils, Geotechnical Frontiers 2017, March 12-15, Orlando, Florida, USA. Chatterji, S., 1995. Mechanism of expansion of concrete due to the presence of dead burnt CaO and MgO. Cem. Concr. Res. 25, 51–56. De Silva, P., Bucea, L., Sirivivatnanon, V., 2009. Chemical, microstructural and strength development of calcium and magnesium carbonate binders. Cem. Concr. Res. 39, 460–465. Du, Y.J., Wei, M.L., Reddy, K.R., Wu, H.L., 2016. Effect of carbonation on leachability, strength and microstructural characteristics of KMP binder stabilized Zn and Pb contaminated soils. Chemosphere 144, 1033–1042. Dung, N.T., Unluer, C., 2018. Development of MgO concrete with enhanced hydration and carbonation mechanisms. Cem. Concr. Res. 103, 160–169. El-Hassan, H., Shao, Y., 2015. Early carbonation curing of concrete masonry units with portland limestone cement. Cement Concr. Compos. 62, 168–177. Fabbri, A., Corvisier, J., Schubnel, A., Brunet, F., Goffé, B., Rimmele, G., Barlet-Gouédard, V., 2009. Effect of carbonation on the hydro-mechanical properties of Portland cements. Cem. Concr. Res. 39, 1156–1163. Fang, Y., Chang, J., 2015. Microstructure changes of waste hydrated cement paste induced by accelerated carbonation. Constr. Build. Mater. 76, 360–365. Harrison, A.J.W., 2008. Reactive magnesium oxide cements. United States Patent, 7347896, US 11/016,722. Hwang, K.Y., Kim, J.Y., Phan, H.Q.H., Ahn, J.Y., Kim, T.Y., Hwang, I., 2018. Effect of CO2 concentration on strength development and carbonation of a MgO-based binder for treating fine sediment. Environ. Sci. Pollut. R. 25, 22552–22560. IPCC (Intergovernmental Panel on Climate Change), 2013. Climate Change 2013: the Physical Science Basis, Working Group I Contribution to the IPCC Fifth Assessment

(i) Two quantitative approaches, i.e. direct weight gain (I) and indirect TG/DSC analysis (II), are proposed to describe the CO2 uptake amount and uptake efficiency of magnesia-lime-fly ash/slag solidified soils. The CO2 uptake amount and uptake efficiency estimated by approach I are largely higher than that defined by approach II. The reason for the huge discrepancy between CO2 uptakes assessed by the two approaches is a troublesome issue due to its extreme complexity. (ii) The carbonation duration, binder amount and mass ratio of magnesia to lime are proved to affect the CO2 sequestration in magnesia-lime-fly ash/slag solidified soils. The CO2 uptake amount and uptake efficiency appear to be on a growth trend with carbonation duration, binder content and mass ratio of magnesia to lime, since more and more reactive MgO and CaO are involved to react with CO2. The hydration and carbonation of magnesia and lime should be reckoned with due to the potential volumetric expansion. (iii) The specimens without reactive magnesia, which have significantly greater volume of coarse pores (6–20 μm) and relatively lower volume of fine pores (0.01–0.1 μm), exhibit higher 12

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