Accelerated carbonation of steel slag monoliths at low CO2 pressure – microstructure and strength development

Journal of CO₂ Utilization 36 (2020) 124–134

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Accelerated carbonation of steel slag monoliths at low CO2 pressure – microstructure and strength development

T

P. Nielsena,*, M.A. Booneb, L. Horckmansa, R. Snellingsa, M. Quaghebeura a b

Sustainable Materials Management, VITO, Mol, 2400, Belgium XRE nv, Gent, Belgium

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbonation Stainless steel slag Microstructure Compressive strength

Several studies have focused on the parameters that influence the CO2-uptake of steel slags during mineral carbonation, however, less is known about the parameters that affect the strength development of the carbonated products. In this study monoliths with a compressive strength of 30–50 MPa were made by mineral carbonation of compacted stainless steel slag in an autoclave under mild operating conditions (i.e. 10–60 °C, 1.5 bar P total, and 5–100 % CO2). The influence of moisture content, CO2 concentration, temperature and exposure time on CO2 uptake and strength development were evaluated. In general a good correlation was observed between CO2 uptake and compressive strength of the monoliths. At different temperatures however the compressive strength as a function of CO2 uptake differed. The highest CO2 uptake was realized for carbonation at 60°, the resulting compressive strength however is lower than for experiments carried out at lower temperature. The microstructure indicated a difference in carbonate precipitation between the samples carbonated at different temperatures. In the samples carbonated at 10 and 20 °C the carbonate crystals mainly precipitated on the slag particle surfaces and at particle contacts, strengthening the particle packing of the compacts. At 60 °C carbonate crystals did initially not precipitate on particle surfaces, but most likely at the fluid-gas interfaces, which resulted in a different microstructure and lower compressive strengths for an equivalent CO2 uptake.

1. Introduction

for reducing the amount of CO2 emitted to the atmosphere. The costs for some of these mineral carbonation processes have been evaluated and are generally considered high [3]. However, by using mineral carbonation as a means to produce marketable construction products [4–9], the costs for mineral carbonation can be compensated for and an economic profit can be made [e.g. 4]. In addition, the environmental benefit of using these slags to produce construction materials is high, as large reductions of net CO2 emisions can be realized. The compressive strength of carbonated monoliths depends on their initial particle packing density (porosity), the compaction pressure applied and the formation of carbonates that act as binder and fill intergranular pores [10]. The compressive strength enhancement during carbonation is attributed to a microstructural densification associated with pore refinement and porosity reduction due to the precipitation of carbonates [11]. For compacts made with similar starting material (mineralogy and particle size distribution), and shaped using the same compaction pressure, there is generally a very good correlation between CO2-uptake and strength development [10]. However, the CO2-uptake or degree of carbonation is not the only factor controlling the strength

Steel slag is the term used for the by-product generated during steel production, by melting ore or metal scrap in a basic oxygen furnace or electric arc furnace, respectively. The chemistry of the slag depends on the raw materials used for the specific furnace and the mineralogy is determined by the chemistry and cooling rate of the slag. Steel slags are generally rich in calcium and magnesium, which make them suitable for mineral carbonation. Mineral carbonation is based on natural weathering reactions of Caand Mg-silicate minerals with atmospheric CO2, which results in the precipitation of carbonates. Steel slags with high amounts of Ca (and Mg) are an attractive feedstock for mineral carbonation because their chemical composition is similar to that of silicate minerals undergoing natural weathering. Among all industrial residues and by-products, steel slags offer a number of advantages as a feedstock for CO2 sequestration such as wide availability, low cost and high reactivity [1,2]. In most studies mineral carbonation has been studied to evaluate the potential of industrial waste materials to capture and sequester CO2

⁎

Corresponding author. E-mail address: [email protected] (P. Nielsen).

https://doi.org/10.1016/j.jcou.2019.10.022 Received 20 July 2019; Received in revised form 17 October 2019; Accepted 30 October 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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of the compacts. De Silva et al. [12] studying the carbonation of calcium hydroxide compacts concluded that it was not the amount of Ca (OH)2 converted, and thus the amount of carbonates formed, that was the main factor controlling the strength of the binder, but the morphology or crystalline state of the CaCO3 binder. These authors concluded that well-developed crystalline structures and crystal habits of carbonate provide better binder performances, and thus compact strengths, than amorphous-appearing products with no defined morphology [12]. In this study monoliths of 30–50 MPa strength were made by mineral carbonation of stainless steel slag under mild operating conditions (i.e. 10–60 °C, 1.5 bar Ptotal, 5–100% CO2) using an autoclave. The main objective of this work was to explore the factors that affect the compressive strength development of the carbonated compacts. Fig. 1. particle size distribution of the 1/1 mix stainless steel slag sample based on a combination of sieve results and laser diffraction (particles < 1 mm).

2. Materials and methods 2.1. Materials

refractive index of 1.4500 with an absorption of 0.1 were chosen. The particle size distribution of the particles larger than 1 mm was established by sieving with a Retsch AS 200 digital sieve shaker. The particle size distribution of the stainless steel slag mix used in this study is given in Fig. 1.

The material used in this study is a fine grained mineral fraction that is produced during demetalisation of stainless steel slag. During this demetalisation process slag fractions with different particle size distributions are produced. For this study two of these mineral fractions were mixed in a 1/1 ratio, i.e. a sand fraction (d10 = 185 μm, d50 = 500 μm, d90 = 1300 μm) and a finer fraction (d10 = 11 μm, d50 = 240 μm, d90 = 1000 μm). The particle size distribution of the mix was d10 = 95 μm, d50 = 410 μm, d90 = 1000 μm.

2.2.3. Accelerated carbonation process The carbonation process developed to produce the carbonated materials contains three main steps: (1) pretreatment of the slags, (2) shaping in a mould by hydraulic compaction, and (3) CO2 curing in an autoclave. A schematic overview of the process is given in Fig. 2. A detailed description of the process is provided in supporting information. The degree of carbonation which includes the contribution of carbonates already present in the starting material due to natural carbonation was determined by TC analysis. The carbonation extent is given in the form of a CO2 uptake, expressed as the mass of CO2 (kg) sequestered by the slag per mass of CO2-free slag (tonne). It must be noted that the initial slag used here was already partly carbonated (26 kg CO2/tonne slag), due to contact with atmospheric CO2, before the accelerated carbonation experiments were carried out.

2.2. Methods 2.2.1. Compositional analysis The chemical composition of the slags was determined by EDXRF under He-atmosphere on powders and beads (made by melting the sample with Li2B4O7 at a 1/10 ratio (sample/Li2B4O7)). For Na and Mg there is an uncertainty of 30%. The results for sulphur are indicative due to possible overlap with Pb. The resuls are given in Table 1. The total carbon (TC) content of the carbonated samples was analysed by combustion of the sample and infrared detection of CO2. 2.2.2. Particle size distribution The particle size distribution was determined by laser diffraction using a Malvern Mastersizer X laser diffraction particle size analyzer. A polydisperse mode of analysis with air as dispersant and a particle

2.2.4. Porosity measurements of the carbonated compacts High pressure (200 MPa) mercury porosimetry measurements were carried out with a Pascal 240 instrument from Thermo Scientific. The following calculation parameters were used: Hg surface tension of 0.480 N/m, Hg contact angle of 140°, Hg temperature of 23 °C and Hg density of 13.5389 g/cm³.

Table 1 Chemical composition of the slags based on XRF data.

bead CaO SiO2 MgO Al2O3 MnO TiO2 Fe2O3 Cr2O3 Na2O K2O P2O5 LOI Powder Cr Mo Ni V Zn S

sand fraction

finer fraction

1 /1 mix

mass.% mass.% mass.% mass.% mass.% mass.% mass. % mass. % mass. % mass. % mass. % mass. %

44.5 29.7 10.9 3.88 1.44 1 2.32 3.92 < 0.70 < 0.15 < 0.60 0.96

47.5 26.5 10.5 3.03 0.76 1.03 0.7 2.11 < 0.70 < 0.15 < 0.60 6.29

47.2 27.6 10.8 3.41 1.09 1 1.5 2.92 < 0.70 < 0.15 < 0.60 2.97

mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg

17000 94 200 340 9 1600

11000 47 95 250 23 3500

10500 62 120 230 15 3400

2.2.5. Mineralogical analysis of the carbonated compacts The mineralogical composition of the slags and carbonated products was analysed with an X’Pert PRO XRD diffractometer from Philips with a Cu tube (CuKα source with λ = 0.15418 nm). Mesaurements were carried out from 2° to 120° (2θ) with a step size of 0.04° and a dwell time of 4 s. The resulting diffractograms were analysed using the X’Pert HighScore Plus version 4.0 software package of PANalytical. Thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) analyses were carried out to obtain additional information on the carbonation products. TGA/DTG analyses were performed using a NETSCH-STA 449 C, Jupiter instrument. About 50 mg of powder was placed in an alumina crucible and heated at a uniform rate of 10 °C/min from ambient temperature to 1050 °C, in a nitrogen flow of 50 ml/min. For one of the samples an additional TGA/MS measurement was carried out with a TGA instrument (NETSCH – STA 449 F3 Jupiter) coupled to a Mass spectrometer (MS) (NETSCH - QMS 403 D Aeolus) to better constrain the decarbonation temperatures. 125

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Fig. 2. Schematic overview of the carbonation process.

2.2.6. Microstructural analysis of the carbonated compacts Micro-structures were analysed by a Scanning Electron Microscope (SEM) FEI NOVA NANOSEM 450 with EDX analyser BRUKER QUANTAX 200 with SDD detector. 2.2.7. Compressive strength analysis of the carbonated compacts The samples were loaded at a rate of 1 mm/min using a 100 kN compression testing machine. Compressive strengths of the compacts were analysed using standard procedures developed for concrete paving blocks (NBN EN 772-1:2011). 3. Results and discussion 3.1. Characteristics of the stainless steel slags (compacts) The chemical composition of the stainless steel slags is given in Table 1 and the mineralogical composition is shown in Fig. 3.

Fig. 4. CO2 uptake of carbonated compacts as a function of time and moisture content. Carbonation was carried out at room temperature (approx. 20 °C) without temperature control, at a pressure of 1.5 bar and a CO2 content of approx. 17% in a climate chamber.

3.2. Strength development of carbonated compacts

compacts with 4 and 10% moisture content (MC) increased in a comparable way (Fig. 4), after 8 h however, the CO2-uptake slowed down significantly for the compacts with a moisture content of 4 % (Fig. 4). In the compacts with 10% MC the CO2-uptake remained high during the first 16 h. The compressive strength of the compacts correlates well with the measured CO2 uptake (Fig. 5).

3.2.1. Influence of moisture content To test the influence of moisture content on strength development, compacts with an average size of 61 × 61 × 33 mm were made containing either approximately 4% or 10% demineralised water. These compacts were placed in a ventilated compression chamber (climate chamber). An overpressure of 0.5 bar was applied by adding a gas mixture of 50% CO2 and 50% N2, resulting in a total pressure of 1.5 bar in the compression chamber with approximately 17% CO2 (pCO2 = approx. 0.25 bar). After designated time intervals the CO2-uptake of the compacts was measured as a function of time (Fig. 4). Initially, the CO2-uptake of the

3.2.2. Influence of temperature To determine the influence of temperature on the CO2-uptake and compressive strength development during low pressure carbonation, experiments were conducted in an autoclave at fixed temperatures of Fig. 3. Mineralogical composition (XRD data using a Cu Kα source). C = calcite (CaCO3), G = gehlenite (Ca2Al(AlSiO7), A = akermanite (Ca2Mg(Si2O7)), M = merwinite (Ca3Mg (SiO4)2), B = bredigite (Ca7Mg(SiO4)4), P = portlandite (Ca (OH)2), Pe = periclase (MgO), L = free lime (CaO), Cu = cuspidine D = donathite ((Fe,Mg)(Cr,Fe)2O4), (Ca4(Si2O7)(F,OH)2), O = Calcio-olivine (γ-Ca2SiO4).

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Fig. 5. CO2-uptake (based on TC measurements) and compressive strength of the carbonated compacts as a function of moisture content. Fig. 8. CO2-uptake as a function of CO2 concentration in the autoclave during carbonation at 20 °C and a total pressure of 1.5 bar.

CO2 uptakes. Especially, the initial CO2 uptake was significantly higher for the experiments carried out at 60 °C, with most of the CO2 uptake taking place within the first 2 h (Fig. 6). At lower temperatures the CO2 uptake is initially slower. A higher compressive strength is, however, reached for a similar CO2 uptake. This indicates that the initial fast CO2 uptake (observed for the high temperature experiments) contributes less to compressive strength (expressed as compressive strength/CO2uptake) than the slower CO2 uptake. For a fixed CO2 uptake (e.g. 100 kg CO2 uptake/ton slag), the compressive strength differs according to the temperature at which the CO2 uptake occurred. The highest compressive strength is recorded for the experiment carried out at the lowest temperature, provided that the carbonation reactions are allowed to proceed for sufficient time to reach the same CO2 uptake and that this CO2 uptake can be realised at this lower temperature. Fig. 6. CO2-uptake as a function of temperature and carbonation time.

3.2.3. Influence of CO2 concentration The influence of the CO2 concentration was examined by carbonating compacts (made using the same raw materials and compaction methods) in the autoclave at different CO2 concentration levels. Higher CO2 concentrations resulted in higher initial reaction rates (Fig. 8). At lower CO2 concentrations however the same conversion rate may be attained after longer time periods, and in the long run may even surpass the carbonate conversion rate obtained at higher CO2 concentrations (Fig. 8). A very good correlation between CO2-uptake and strength development is observed for all the compacts (Fig. 9). The final CO2 uptake and compressive strength of the compacts carbonated at lower CO2 concentrations surpassed that of the compacts carbonated at 100% CO2 concentration (Fig. 9). 3.2.4. Mineralogy of the carbonates formed The only carbonate minerals formed during carbonation according to the XRD data are calcite and Mg-calcite. The first mineral to carbonate is portlandite (Fig. 10). Note that during carbonation with 5% CO2 the main portlandite peak disappeared completely after 16 h, which is not the case for carbonation with 33% CO2. During the later stages of the carbonation reaction also a clear reduction in merwinite and bredigite content is observed (Fig. 10). The carbonation of merwinite and bredigite lead to the formation of calcite and Mg-calcite (Fig. 11). TGA/DTG and TGA/MS were performed to obtain additional information on the carbonation products. Different weight loss ranges of calcium carbonate observed in TGA measurements have been attributed to crystallinity, polymorphism and substitution. Thiery et al. [13] proposed that three decomposition temperatures of CaCO3 at 550−680 °C, 680−780 °C and 780−990 °C exist, which correspond

Fig. 7. CO2 uptake and compressive strength as a function of carbonation temperature. (Compaction force 150 kgf/cm², 1.5 bar total pressure).

10 °C, 20 °C and 60 °C. The resulting CO2 uptakes as a function of time are shown in Fig. 6. In Fig. 7 the CO2 uptake and resulting compressive strengths are shown. Note that for the different temperatures there is a good correlation between CO2-uptake and compressive strength (R² = 0.98 for the experiments at 10 °C and 20 °C, and R² = 0.72 for the experiments at 60 °C, Fig. 7). Higher temperatures resulted in higher 127

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differentiated in the XRD pattern (see supplementary information). The endothermic peak at the lower calcination temperature is higher for the experiments carried out with 5% CO2. The experiments carried out with 5% CO2 indicate that increasing the carbonation duration from 16 to 120 h results in increased CaCO3 contents but the ratio of the two endothermic peaks remain more or less the same (Fig. 12). Changing the temperature from 20 °C to 60 °C also seems to have no significant effect on the crystallinity of the produced carbonates (Fig. 12). This is in agreement with the results of others who have observed that the crystallinity of carbonates formed during accelerated carbonation is not dependent on carbonation duration, but more likely related to the concentration of CO2 [14,15].

3.2.5. Effect of temperature on microstructure The microstructure of the compacts was examined by SEM after carbonation at 1.5 bar pressure with 33% CO2 at 20 °C (Fig. 14) and 60 °C (Fig. 15). Note that carbonation at 20 °C resulted in a good densification of the microstructure between the slag particles (Fig. 14A). Large pores are still present in these compacts (Fig. 14B). In the compacts carbonated at 60 °C large pores are relatively scarce (Fig. 15A). The matrix between the slag particles, however, is microporous (Fig. 15B) and based on BSE-SEM images less dense than the matrix in the compacts carbonated at 20 °C. The morphology of the calcite crystals in the low temperature experiments can be seen to be mainly rhombic calcite about 0.1 to 0.5 μm in diameter. Based on the XRD data calcite of a similar composition as in the low temperature (10 and 20 °C) experiments were formed at 60 °C (Fig. 10). The TGA data also indicate that the calcites formed at 20 °C and 60 °C have similar composition and crystallinity (Fig. 12).

Fig. 9. Compressive strength as a function of CO2 concentration in the autoclave during carbonation at 20 °C and a total pressure of 1.5 bar.

repectively to amorphous carbonate, vaterite and aragonite, and well crystallized calcite. Other authors have indicated that poorly crystalline and well crystalline CaCO3 are expected to decompose at temperatures of around respectively, 520−720 °C and 720°−950 °C [14,15]. TGA-DTG plots of the original slag and compacts carbonated at different CO2 contents and temperatures are shown in Fig. 12. Based on the DTG data the weight loss can be divided into 3 regions: (1) dehydration in the temperature range 50−250 °C; (2) dehydration of Mg (OH)2 and Ca(OH)2 in the temperature range 300−450 °C; and (3) calcination in the temperature range of 350−800 °C with two clear endothermic peaks: one around 620−650 °C and one around 680−740 °C. These endothermic peaks are related to the CO2 release (Fig. 13) of two carbonate phases. In the XRD patterns no polymorphs of calcite were detected, nor were any aragonite or vaterite observed in the SEM images. The presence of magnesian calcite was however observed in the XRD patterns. Calcination of magnesite (MgCO3) results in an endothermic peak around 590−650 °C [16]. The carbonate phase responsible for the endothermic peak at 620−650 °C, however, is due to the calcination of calcite, as can be seen from in-situ high temperature XRD measurements (see supplementary information). The two observed endothermic peaks in the TGA-DTG plots both correspond to the calcination of calcite, the two endothermic peaks cannot be

3.2.6. Effect of CO2-concentration on microstructure The microstructure of compacts carbonated at 1.5 bar pressure with different CO2 concentrations (5% and 100%) were also compared. The compacts carbonated with 5% CO2 during 120 h showed a good densification of the matrix (Fig. 16A). In addition to the densification of the matrix, the pores are locally lined with a dense carbonate layer (Fig. 16B). The compacts carbonated with 100% CO2 display a more patchy distribution of the carbonates. A very good carbonation is observed in the fines around larger grains. Although carbonation was allowed to proceed for 96 h no carbonate layers or only very thin carbonate layers were observed to line larger pores (Fig. 17A & B).

Fig. 10. XRD of compacts carbonated at different temperatures and CO2 concentrations. 128

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Fig. 11. Comparison XRD of original slag and carbonated compact to illustrate formation of Mg-calcite (arrows).

3.2.7. Pore structure of the carbonated compacts The pore structure of the carbonated compacts was also analysed by mercury intrusion porosimetry (MIP). In the SEM images pores are visible as black areas. SEM images are especially useful to visualize the distribution of the larger pores of more than a few μm in diameter. It is not possible to estimate the total pore volume if small pores are present. With mercury porosimetry the pore size distribution and total pore volume of a sample is calculated from mercury intrusion pressure data. The intrusion pressures record the largest connection (pore throat or pore channel) from the sample surface towards a pore, but not the actual inner size of the pores. Large pores with a small pore throat opening are filled at higher pressures, and thus detected as smaller pores than they actually are. As a result MIP overestimates the volume of the smallest pores [17,18], and will show smaller pore sizes than observed by SEM. The pore size distribution of 4 samples carbonated at different temperatures and CO2 concentrations as determined by MIP are given in Fig. 18. Subsamples taken from the core and rim of the compacts displayed similar maximum pore throat sizes. The cumulative pore volume however can vary significantly (data not shown). Other characteristics of these 4 samples are given in Table 2. The largest pore throat diameters (up to approximately 100 μm) were recorded for the sample treated with 33% CO2 at 20 °C during 16 h. The pore throat diameters of the sample treated with 33% CO2 at 60 °C during 16 h is generally < 10 μm. The small pores recorded for the sample carbonated at 60 °C is consistent with the SEM observations. The sample treated with 100% CO2 at 20 °C during 96 h is also characterized by relatively large pore throats (largest approximately 50 μm). This is also consistent with the SEM data, where large pores are observed with little or no carbonate lining. The sample treated with 5% CO2 at 20 °C during 120 h has very few large pore throats (similar to the largest MIP pores observed for carbonation with 100% CO2). Most pore throats are, however, < 10 μm, similar to the sample treated with 33% CO2 at 60 °C. Based on the SEM images we know that the sample contains large pores but these are generally lined with a relatively thick carbonate layer, which may narrow the pore throats opening towards the surface of the sample.

Fig. 12. TG-DTG analysis of compacts carbonated at different CO2 and temperature conditions.

MS: CO2 MS: H2O

4. Discussion 4.1. Mineral carbonation Carbonation of stainless steel slags is a liquid-phase reaction of carbonate ions (CO32-) and cations (Ca2+ and Mg2+) to produce carbonates. The reaction consists of three main steps: (1) the absorption/

Fig. 13. DTG-MS data showing CO2 and H2O release. The CO2 release corresponds to the two endothermic peaks observed in the DTG data.

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Fig. 14. (A) BSE-SEM photograph of compact carbonated at 20 °C with 33% CO2 during 16 h. Note densely carbonated areas around and between slag particles and presence of larger pores; (B) Higher magnification photomicrograph showing carbonate precipitates between slag particles and one of the larger pores (p).

Fig. 15. (A) BSE-SEM photograph of compact carbonated at 60 °C with 33% CO2 during 16 h. Note the microporous nature of the matrix and the absence of large intergranular pores. (B) Higher magnification photomicrograph showing the microporous nature of the matrix between the larger light coloured slag particles.

(3) Carbonation results primarily in the conversion of portlandite (Ca(OH)2) to calcite (CaCO3) according to reaction (3):

dissolution of CO2 in an aqueous solution to form carbonic acid; (2) the dissolution of calcium and magnesium from the slag, and (3) the precipitation of calcium and calcium-magnesium carbonates. The dissolution step of Ca (and Mg) from the slag is generally considered the ratedetermining step of the process [19–22]. (1) The gas/liquid transfer is a classic absorption process enhanced in this case by chemical reaction due to the basicity of the stainless steel slags. Above pH 10 the following reactions are important: CO2 + OH− ⇌ HCO3− (slow) HCO3−

−

+ OH

⇌ CO3

2−

+ H2O (fast)

Ca+2 + 2OH- + CO2(aq) + ↔ CaCO3(s) + H2O

(3)

The rate of this process is physically controlled by the diffusion of carbon dioxide into the pore vapour space and chemically by the availability of dissolved calcium and hydroxide ions in the pore water [26]. As portlandite is depleted, calcium ions are provided by the decalcification or dissolution of calcium silicates or calcium-magnesium silicates such as merwinite, according to reaction (4).

(1) (2)

Ca3Mg(SiO4)2 + (y + z)H2O → Ca(3-y)Mg(1-z)(SiO4)2 +yCa(OH)2 +zMg(OH) 2 (4)

CO2 absorbtion is controlled by parameters such as the gas/liquid exchange (interface) and the concentrations of CO2 in the gas and liquid phase [23], as well as on pH (see reaction 1 and 2 above) and temperature. (2) Dissolution rates of Ca and Mg from (hydr)oxides and silicates depends on the L/S ratio, composition of the fluid, pH, temperature, and CO2 pressure as well as on the crystallinity and Ca/Si ratio of the silicates [e.g. [24] [25],].

4.1.1. The influence of moisture content on CO2 uptake and strength development The CO2 uptake of the compacts with 4% MC was lower than that of the compacts with 10% MC. For the compacts with a moisture content of 4% insufficient water was present to support the carbonation process Fig. 16. (A) BSE-SEM photograph of compact carbonated at 20 °C with 5% CO2 during 120 h. Note dense areas due to carbonation around many slag particles (arrows) and presence of larger pores (black); (B) Detail of white rectangle showing larger pores (p) lined with calcite crystals grown together to form a high density carbonate precipitate (c).

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Fig. 17. (A) BSE-SEM photograph of compact carbonated at 20 °C with 100% CO2 for 96 h. Carbonation seems to occur mainly around larger slag particles that seem to be largely coated with fine slag particles, that are largely carbonated. Note large pores (p) between the latter particles. (B) Detail of white rectangle.

correlation is, however, temperature dependent (Fig. 7). The optimal water content is a function of the particle size and compaction pressure applied to the compacts [27]. 4.1.2. Influence of temperature on CO2 uptake and strength development In this study the highest CO2 uptake was recorded for the carbonation experiments carried out at 60 °C. The highest CO2 uptake, however, did in this case not lead to the highest compressive strength. A good correlation between CO2 uptake and compressive strength is observed, however, the correlation is temperature dependent (Fig. 7). The temperature during carbonation influences: (1) the dissolution rate of slag constituents, (2) the solubility of CO2, (3) the nucleation and growth rate of the carbonation products, and (4) the morphology and mineralogy of the carbonates. The XRD data indicate that calcite (and Mg-calcite) were the principal crystalline carbonates formed in all carbonation experiments. The TGA-DTG data also indicate the presence of two principal carbonate phases characterized by endothermic peaks at about 640 °C and 740 °C. A change in the carbonation temperature from 20 °C to 60 °C in the experiments carried out with 33% CO2 did not seem to influence the ratio of these two endothermic peaks. Thus the mineralogy of the carbonates cannot explain the differences in strength between compacts carbonated at different temperatures. Increasing the temperature from 25 °C to 90 °C improves the dissolution rate of slag constituents, such as Ca2SiO4, CaSiO3 and Ca2FeAlO5, considerably [28,29]. Increasing the temperature also results in a decreasing solubility of the carbonates because of their inverse solubility and thus promotes carbonate formation. However, increasing the temperature from 25 to 90 °C decreases the solubility of CO2 in water, which can have a negative impact on the carbonation reaction [30]. Hence increasing the temperature leads to opposite effects on two main steps of the carbonation reaction: (1) leaching of Ca2+ and Mg2+ ions from the slag particles and (2) dissolution of CO2 in water (Fig. 19). The CO2 uptake of slags increases with increasing temperature up to 60 °C at atmospheric pressure [31]. Above 60 °C the CO2 uptake has been observed to decrease [31,32], most likely due to the decreased solubility of CO2 in water at elevated temperatures (see Fig. 19). According to Lekakh et al. [33] the initial stage of the dissolution

Fig. 18. Pore size distribution of compacts subjected to different carbonation conditions determined by MIP.

to the same degree as 10% moisture. The temperature increase due to the exothermic nature of the carbonation reaction (a temperature increase from 20 to 33 °C was registered) may cause evaporation and result in the drying out of the compact. In addition not all the moisture present in the compact may be available for the carbonation reactions. Some water can be consumed by hydration reactions or become absorbed to amorphous silica, a reaction product formed during carbonation of silicates. As a result this water is no longer available for the carbonation reactions. This may explain why carbonation was less intense, i.e. less CO2 uptake was recorded for the compacts with 4% MC, compared to the compacts with 10% MC, although some residual water (approx. 2%) was still present in the compacts after carbonation. If too much water is applied the pores of the compact become blocked by water and CO2 diffusion is hindered resulting in less CO2 dissolution and carbonate precipitation. Therefore, the moisture content influences the amount of carbonate that can be precipitated. As the amount of carbonate (and thus CO2 uptake) shows a good correlation with the compressive strength of the compacts, the amount of carbonate precipitated largely determines the strength of the compact. This

Table 2 Some characteristics of the compacts carbonated under different conditions (temperature, CO2 content, duration, relative humidity (RH)). Carbonation conditions

Product characteristics

temp. (°C)

CO2 content (%)

duration (hours)

RH (%)

dry matter after carb. (%)

CO2 uptake (%)

compressive strength (Mpa)

permeability (MD)

MIP – largest pore throat (μm)

20 20 20 60

5 33 100 33

120 16 96 16

∼ ∼ ∼ ∼

94.6 96.5 97.6 98.4

9.1 7 9.2 9.1

47 ± 5 38 ± 6 31 ± 2 33

240 1500 1200 230

∼ ∼ ∼ ∼

95 85 70 50

131

10 100 50 10

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travel. Subsequent diffusion through this porous structure becomes the rate limiting step for the dissolution reaction. Each reaction mechanism has a different sensitivity to the solution temperature. In the case studied by Lekakh et al. [33] increasing the temperature of the aqueous solution enhanced the Ca leaching rate during the initial reaction period (of approximately 1 h). The leaching rate during the remaining period, when the dissolution rate is diffusion controlled, did not differ significantly between 20 and 60 °C. This was attributed to the observation that the solution temperature affects the rate of the chemical reaction more than it does the rate of diffusion [33]. The fact that the leaching rate in the second period, when leaching is diffusion controlled, does not differ significantly between 20 and 60 °C may explain the near parallel trends observed for the correlation between CO2 uptake and compressive strength in Fig. 7. As all the data points in Fig. 7 are the result of 4–120 hours of carbonation, the release rate of Ca to the pore solution at the different temperatures was probably comparable and the strength increase/ CO2 uptake was also comparable during this time period. The morphology of the carbonates and the compact microstructure were examined by SEM and complemented with pore size distribution data obtained by MIP. When comparing the morphology of the compacts carbonated at 1.5 bar with 33% CO2 at 20 and 60 °C, it is clear that carbonation at 20 °C resulted in more dense carbonates that mainly precipitated on the surface of the slag particles. Larger pores are still present in these compacts as can be seen by SEM images and the MIP

Fig. 19. Solubility of CO2 gas as a function of pressure and temperature. Data points are from compilations by [34,35].

reaction of the slag particles is controlled by a chemical reaction whereby Ca2+ ions are dissolved in the water. As the dissolution reaction proceeds a porous surface structure developes on the dissolving slag particles resulting in a tortuous path for the dissolved ions to

Fig. 20. (A) Carbonation at 10 °C and low CO2 pressure: the low temperature enhances the dissolution of CO2 in the pore fluid, but decreases the dissolution of Ca from the slag particles. As a result carbonate precipitation takes place at the slag particle surface where dissolution occurs, which is generally considerd the rate limiting step for mineral carbonation; (B) Carbonation at 60 °C and low CO2 pressure: the high temperature and low CO2 pressure results in low dissolution of CO2, but the high temperature increases the dissolution of Ca2+ from the slag particles. As a result the fluid contains more Ca2+ than at lower temperatures and calcite saturation occurs at the water/gas interface where CO2 dissolves into the pore water.

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temperature experiments a high initial carbonation rate of the compacts resulted in lower final compressive strengths. This lower compressive strength is most likely the result of the patchy distribution of the carbonates (Fig. 17). The reason for this patchy distribution of the carbonates is not clear, but is likely linked to the distribution of water in the compact.

data. The compacts carbonated at 60 °C on the other hand contain very few large pores (in the order of 100 μm diameter or more), and the matrix is microporous. These observations are interpreted to result from the fast initial dissolution rate at 60 °C, which resulted in carbonates being formed all over the compact, filling both smaller and larger pores. In contrast carbonation carried out at lower temperatures (at 10 or 20 °C) has resulted in a denser mass of carbonates precipitated on and between slag particles, while larger pores (> 100μm) remained open. These cements are likely to strengthen the compacts. The dense carbonate cements are also likely to give the compacts a higher strength compared to the porous microstructure observed for the samples carbonated at 60 °C. At 60 °C the Ca2+ ion saturation in the pore fluids was initially (first hour) likely higher than in the experiments conducted at lower temperature due to the enhanced dissolution of the Ca-(Mg)silicates. The concentration of dissolved CO2 in the pore fluids on the other hand was lower at 60 °C, than in the experiments conducted at 10 and 20 °C (Fig. 19). This had an effect on where carbonates precipitated, i.e. nucleation of calcite on the slag particle surfaces (at 10 and 20 °C) or nucleation at the liquid-gas interface (at 60 °C), due to much higher saturation degrees in the pore fluid. At low saturation indexes (< 1) the nucleation of calcite is favoured on the surface of slag particles that releases elements required for its precipitation [36] or on the surface of minerals belonging to the same orthorhombic crystal structure [37]. Only at saturation indexes greater than 1 can carbonates nucleate spontaneously in the absence of growth substrates [38]. Calcite saturation in the pore fluids is also reached faster at higher temperature because of the inverse solubility of calcite. Based on the microstructure observations the following carbonation mechanism is proposed:

5. Conclusions In this study carbonation of compacts of stainless steel slag was carried out at low pressures (1.5 bar), with CO2 concentrations from 5% to 100% and at temperatures between 10 and 60 °C. For most variables (moisture content, CO2 content, duration) a good correlation was found between the CO2 uptake and the compressive strength. For the experiments carried out at different temperatures the correlation however also depended on the temperature of the reaction. In these experiments carbonation carried out at 60 °C resulted in the highest CO2 uptake rate and some of the highest total CO2-uptakes measured (95 g CO2/kg slag after 4 h and 106 g CO2/kg slag after 48 h). These high total CO2 uptakes did, however, not result in higher strengths than experiments carried out a lower temperatures. At 20 °C carbonation experiments reached the same CO2 uptake (i.e. 106 g/CO2) after 120 h, the strength reached was 47 MPa compared to 39 MPa for the experiment at 60 °C. At 10 °C a strength of 45 MPa was reached with a total CO2 uptake of only 82 g CO2/kg slag after 72 h. The different compressive strengths could be explained by the microstructure of the compacts. In the compacts carbonated at 60 °C carbonates are omnipresent but form a less dense microstructure. The largest pores observed in the compacts carbonated at 60 °C are signifcantly smaller than the largest pores observed for compacts carbonated at lower temperature, i.e. about 10 μm versus 100 μm for compacts carbonated at 20 °C (based on MIP data). Carbonation at lower temperatures did not affect the larger pores to the same degree as carbonation at higher temperatures but resulted in a better densification of the matrix between the slag particles, leading to a higher compressive strength. When using 100% CO2 the initial CO2 conversion rate was also high. As the carbonation reaction is an exothermic reaction, a high carbonation rate may also lead to higher temperatures. The microstructure of the compacts carbonated in a 100% CO2 atmosphere at 20 °C, however, did not display the same features as the compacts carbonated in a 33% CO2 atmosphere at a temperature of 60 °C. In the high CO2 environment the carbonation seems to be more patchy, with densely carbonated areas and poorly carbonated and highly porous areas, which may explain the lower compressive strengths of the latter compacts. When high strength compacts are desired, it is important to meet the experimental conditions that favour the precipitation of calcium carbonate at the grain contacts of the slag particles to enhance the compressive strength of the compacts, more so than finding the optimum conditions for a maximum CO2-uptake.

(1) At 60 °C, when the dissolution of calcium from the slag is initially high and the dissolution of CO2 (gas) low, the pore fluid becomes enriched in dissolved Ca2+ (Fig. 20A). In this case dissolved CO2 can immediately precipitate as CaCO3 at the gas-water interface because of the high dissolved calcium concentration in the fluid. (2) At low temperatures (10–20 °C), when CO2 gas easily dissolves in the pore fluid, but the dissolution of Ca-silicate minerals is very slow, the dissolved CO2 concentrations in the pore fluid are relatively high and the concentration of dissolved Ca2+ ions relatively low. In this case CaCO3 is more likely to precipitate on the surface of the slag particles where Ca is released to the solution (Fig. 20B). Precipitation is in this case driven by calcite saturation near the surface of the calcium rich slag particles. (3) During the second stage of the carbonation process when dissolution of Ca from the silicates slows down and becomes diffusion controlled, the carbonation behavior at the different temperatures is more comparable, resulting in similar trends between CO2 uptake and compressive strength (Fig. 7). 4.1.3. Influence of CO2 concentration on strength development The CO2 concentrations at which the experiments ware carried out did not have a significant effect on the correlation between the CO2uptake and compressive strength (Fig. 9). All the data points for the experiments carried out at different CO2 concentrations (5-17-100%) and a temperature of 20 °C occur along the same line (with the exception of two outliers) indicating that for a certain CO2-uptake a similar compressive strength is obtained for experiments carried out at different CO2 concentrations. The CO2 concentration does however influence the carbonation rate (Fig. 8) and the final strength that can be obtained. A high initial carbonation rate, as observed for the highest CO2 concentration (100%), was detrimental for the strength that could be gained at the end of the experiment (Fig. 9). After 100 h of carbonation the strength of the compacts carbonated at 5% and 17% CO2 surpassed the compressive strength of the compacts carbonated at 100% CO2 (Fig. 9). The carbonate minerals formed, i.e. calcite and Mgcalcite were identical for all CO2 concentration levels. As with the high

Declaration of Competing Interest The authors declarethere are no competing interests. Acknowledgements We thank Bo Peeraer, Raymond Kemps, Myrjam Mertens, AnneMarie De Wilde, Dirk Vanhoyweghen for their assistance in carrying out all lab analyses. We also thank the anonymous reviewers for their valuable input. References [1] W.J.J. Huijgen, Carbon Dioxide Sequestration by Mineral Carbonation, PhD Thesis Energy research Centre of the Netherlands, The Netherlands, 2007.

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