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or post-curing
Journal of CO₂ Utilization 36 (2020) 135–144
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Production of artificial aggregates from steel-making slag: Influences of accelerated carbonation during granulation and/or post-curing
T
Yi Jiang, Tung-Chai Ling* Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, Hunan, China
ARTICLE INFO
ABSTRACT
Keywords: Steel slag Granulation Accelerated carbonation Artificial aggregates
Accelerated carbonation is envisaged to be a significant curing method for the development of cement-free artificial aggregates. In this paper, two accelerated carbonation approaches, applied during granulation and post-curing respectively, are investigated for their influences on the properties of the aggregates made by 100 % basic oxygen furnace slag (BOFS). The result shows that carbonation during post-curing significantly enhances the overall properties of the granulated BOFS, such as increasing the strength by 220 %. In contrast, synchronized carbonation applied simultaneously during the granulation process yields considerable property degradation, though a CO2 uptake of ∼16 % can be achieved. It contradicts with the common understanding that carbonation is beneficial to the strength enhancement. This is because synchronized carbonation of BOFS is found to release extensive heat and vaporize a great amount of bridging water for agglomeration, thus causing a very loose structure. In general, synchronized and post-granulation carbonation result in two notably different influences on the properties of BOFS aggregates. The integrated approach of granulation and post-granulation carbonation creates a promising cement-free alternative to the local natural aggregates; while the synchronized carbonation shows significant potential for the production of lightweight aggregates.
1. Introduction Finding the potential economic value in the approach of carbon dioxide (CO2) mineralization and utilization has been an interesting topic and expedited extensive researches. Application of accelerated carbonation on alkaline residues is reckoned as an effective approach for CO2 mineralization and utilization. Steel slag is an alkaline residue, generated from steel-making industry; it accounts for about 25 % of the total residue production in China’s iron and steel-making field, with an annual production of 101 million tons [1]. Basic oxygen furnace slag (BOFS) is generated in BOF process and accounts for about 70 % of the annual steel slag output [2]. Critical barriers such as the low hydration reactivity and significant free calcium oxide (f-CaO) content are preventing BOFS from being valorized in cement-based construction materials. However, based on the high CO2 reactivity of BOFS from the high CaO content [3,4], extensive studies of direct carbonation on BOFS were carried out under different processing conditions. For examples, van Zomeren et al. [5] applied carbonation on 2–3.3 mm BOFS at 90 °C for 70 h and recorded a CO2 uptake of 1.5 % (CO2/steel slag) at the CO2 concentration of 20 %. By decreasing the particle size of BOFS to below 38 μm, Huijgen et al. [6] found that under the condition of 19 bar CO2
⁎
pressure and 100 °C a maximum carbonation degree of 74 % was achieved after 30 min. Pan et al. [7] used 8 μm BOFS to remove the CO2 in the exhaust gas (CO2 concentration = 30 %) through high gravity carbonation in slurry state. With the optimization of processing parameters, a CO2 removal efficiency of 97.3 % and estimated CO2 capture capacity of 165 kg per day could be achieved. To find the commercially viable end-use products for the mineralized CO2, part of the recent attentions have been paid to the cold bonding granulation for producing the alternatives to natural aggregates that are in shortage. This is closely associated with the tightened environmental policies in China. Carbonation of the low-hydraulic BOFS opens a possible pathway of utilizing the CO2-activated BOFS for manufacturing cold-bonded aggregates as demonstrated by Eqs. (1) and (2) [8,9] :
C3 S+ (3-x)CO2 +yH2 O C x SHy + (3-x)CaCO3
(1)
C2 S+ (2-x)CO2 +yH2 O C x SHy + (2-x)CaCO3
(2)
Previous attempts to prepare artificial aggregates with various wastes such as fly ash [10], municipal solid waste incineration (MSWI) fly ash [11], marine sediments [12] and quarry dust [13], etc. claimed
Corresponding author. E-mail address: [email protected] (T.-C. Ling).
https://doi.org/10.1016/j.jcou.2019.11.009 Received 2 June 2019; Received in revised form 17 October 2019; Accepted 7 November 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 36 (2020) 135–144
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Fig. 1. As-received BOFS powder.
that a percentage of up to 30 % Portland cement was imperative to bind the fines and attain adequate strength. However, aother investigation on the strength profile of CO2-cured cold-bonded aggregates indicated the feasibility of reducing cement dosage by carbonation curing [14]. Therefore, the main focus of this work is to produce cement-free BOFS artificial aggregates for use in concrete through an integrated approach of accelerated carbonation and cold bonding granulation. For comparison, two accelerated carbonation approaches are employed namely synchronized carbonation (applied simultaneously during granulation) and post carbonation (applied during post-curing). Physical, mechanical, mineralogical, and micro-structural properties are specifically evaluated and compared in order to investigate the differences of the two carbonation approaches on the granulation process and properties of the end products.
Fig. 3. X-ray diffraction (XRD) pattern of the BOFS powder.
mineral phases are portlandite, larnite, srebrodolskite and wustite, etc. (see Fig. 3). 2.2. Production and carbonation of granulated BOFS aggregates Granulation was conducted in a customized pan granulator. The granulator is 700 mm in diameter and 200 mm in collar height (Fig. 4a), with a scraper stuck to the bottom surface and a feeding hopper connected at the glass cover (Fig. 4b). The revolution speed for granulating was set at 15 rpm with a tilting angle of 45°. BOFS powder was used as the only feed material and an appropriate amount of water was sprayed onto the dry powders to wet the particles and form nuclei. Subsequently, growth of the nuclei took place by coalescence or layering processes, during which well-formed agglomerates collided and formed a single agglomerate or fine grains adhered to the surface of agglomerates [15]. Synchronized carbonation was applied during granulation process by introducing 99.9 % CO2 stream into the granulator at the atmospheric pressure. Post carbonation was conducted after granulation in a controlled environment (CO2 concentration = 20 %, relative humidity = 65 %, temperature = 20 °C). In order to investigate the influences of CO2 on the BOFS granulation and the properties of produced aggregates, a total of three different carbonation regimes were designed in this study. As indicated in Table 1, CC designates carbonation applied both during and after granulation; AC indicates carbonation applied only after granulation as a post-curing treatment and AA refers to the control samples that were granulated and cured under ambient air (without carbonation). It should be noted that duration for post carbonation was determined as 4 days based on the trails and the results of previous experimental studies [16,17] which indicated that the CO2 uptake reached a plateau after 4 days. After carbonation, the aggregates were further cured in natural air condition (relative humidity = 50 %, temperature = 20 °C) to 7, 14 and 28 days before testing. The total liquid to solid (L/S) ratio for AA and AC group was 0.16. However, in the case of CC, extensive heat and vaporized water were released during CO2-granulation process and therefore a double amount of water was required (L/S ratio = 0.32) for the nuclei to grow. Corresponding to the increase in water content, the duration of the granulation for CC was also doubled, from 16 min to 32 min.
2. Materials and methods 2.1. Basic oxygen furnace slag (BOFS) Mechanically ground BOFS obtained from a commercial corporation located in Guangxi Province, China, was used in this study (see Fig. 1). The true density of as-received BOFS as measured by a Micromeritics AccuPyc II 1340 instrument is 3.4 g/cm3. The particle size distribution tested by Mastersizer 2000, Malvern Panalytical is shown in Fig. 2, indicating a mean particle size of 6 μm. The chemical composition was analyzed by X-ray fluorescence (XRF) on a Axios, Malvern Panalytical and the mineralogical composition was analyzed by X-ray diffraction (XRD) on a Bruker D8 Advance instrument. The XRD data were collected with CuKα radiation operating at 40 kV, 40 mA between a 2θ from 5° to70° in 0.02° steps and 0.2 s per step. The results show that the BOFS is rich in CaO (∼42.1 %), Fe2O3(∼21.7 %), SiO2(∼16.5 %), MgO (∼6.7 %), MnO (∼4.8 %) and Al2O3 (∼3.5 %), etc. and the major
2.3. Testing of granulated BOFS aggregates To understand the engineering properties of the produced BOFS aggregates, the particle size distribution, loose bulk density, 24 h water
Fig. 2. Particle size distribution of raw BOFS powder. 136
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Fig. 4. (a) granulation chamber and (b) sketch map of the granulation system. Table 1 Carbonation regimes.
CO2 (wt. %) =
Notation
Carbonation regime
CC AC AA
synchronized and post carbonation post carbonation /
CO2 uptake(wt. %) = Ca (%)=
MCaO (g/mol) mCO2 /m105°C 100 × × × 100 ( mCO2/m105°C) MCO2 (g/mol) CaOtotal (wt. %)
3. Results and discussion
(3)
mH2O × MCaO MH2O × m105°C
(6)
Where ΔmCO2 is the mass loss due to the CaCO3 decomposition (600−800 °C), m105°C is the dry weight of the sample measured at 105 °C, MCaO is the molecular weight of CaO (i.e., 56 g/mol), MCO2 is the molecular weight of CO2 (i.e., 44 g/mol), and CaOtotal is the weight fraction of CaO in the fresh BOFS (i.e., 42.09).
3.1. Physical properties
where is the aggregate strength (MPa), P is the load at failure (N) and d is the mean diameter (mm) derived by taking three axial measurements. For each group, ten aggregate particles were tested and the average strength was reported. The alteration of mineralogical compositions after curing was determined by XRD with a Bruke D8 advance instrument. The f-CaO content of the samples was determined using the combination of the ethylene glycol method (GB176-2008) and thermogravimetric analysis (TGA). The former was used to determine the total free calcium content while the latter was used for the deduction of calcium from calcium hydroxide. Eq. (4) was followed for the quantification:
f -CaO(wt.%) = f -CaOtotal (wt.%)
1
CO2carbonated (wt. %) CO2initial (wt. %) × 100 100 CO2carbonated (wt. %)
(5)
(7)
absorption, aggregate strength, mineralogical compositions, porosity, microhardness, and CO2 uptake/calcium conversion were investigated. Particle size distributions of the aggregates were obtained through sieving test with square-opening sieves. Density and water absorption were measured in accordance with ASTM C29 and C127 respectively. Aggregate strength on individual granulated BOFS was tested according to Eq. (3) [18] with a California Bearing Ratio tester:
2.8P/ d2
mCO2 × 100 m105°C
The particle size distribution (PSD) of the BOFS aggregates obtained under different carbonation regimes are shown in Fig. 5. It was found that these aggregates obtained from pan granulation were continuously graded with the particle sizes ranging from 1.18–19 mm. In spite of some variations, the PSD of AA and AC aggregates were comparable and the grading curves were very close to that of size number 67 coarse aggregates as prescribed in ASTM C33 for use in concrete. However, the difficulty of granulation process under a CO2 atmosphere as reported for CC group resulted in significantly smaller particles, with 86 wt.%
(4)
Where f-CaOtotal is the result of ethylene glycol method, ΔmH2O is the mass loss due to the Ca(OH)2 decomposition in BOFS samples, MH2O is the molecular weight of H2O (i.e., 18 g/mol). A phenolphthalein indicator was used to monitor the carbonation depth. Morphology of the aggregates was obtained by a JSM-6490LV scanning electron microscope (SEM). The porosity was analyzed based on back-scattered electron (BSE) images. Microhardness was measured with a HVS-1000Z microhardness tester on polished samples. A load of 1.961 N was applied with a dwell time of 10 s. CO2 weight, CO2 uptake and calcium conversion were quantified by thermogravimetric analysis (TGA) using a Rigaku TG-DTA 8121H as per Eqs. (5), (6) and (7) [7,19,20]. The aggregate particles with the sizes of 10 mm ( ± 2 mm) were chosen and about 15 mg of the homogeneously ground sample was weighed and heated from room temperature to 1000 °C at a heating rate of 10 °C/min. Nitrogen atmosphere was provided with a flow rate of 30 mL/min.
Fig. 5. Particle size distribution of the final aggregates. 137
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Fig. 6. (a) physical appearance, (b) loose bulk density, (c) water absorption and (d) aggregate strength of the final aggregates at different ages.
aggregates finer than 9.5 mm compared with only about 50 % in AA and AC groups. Concerning the appearance, AA and AC aggregates were dark gray and presented some evident humps (Fig. 6a). These humps were unevenly distributed nuclei due to the insufficient layering in agglomeration process. In comparison, CC aggregate demonstrated a more spherical surface based on the prolonged granulation duration and thus the sufficient layering. According to the Fig. 6b, the loose bulk density of AA and AC aggregates were relatively similar, ranging from 1250 kg/m3 to 1280 kg/m3 while the density of CC was remarkably lower, about 900 kg/m3, which fulfilled the requirement set for lightweight aggregate as per BS EN 13055-1 (1200 kg/m3). In terms of water absorption, AA and AC aggregates showed similar values, which were about 11−12 wt.% (Fig. 6c). Whereas, CC showed a significantly higher capacity to absorb water, reaching 27 wt.%. No significant change of both the density and water absorption with curing time for all curing types of BOFS aggregates was found. Aggregate strength at different ages are presented in Fig. 6d. AC had the highest strength among the three groups regardless of age, reaching an optimal average value of 5.23 MPa at 14 days. By applying 4 days of carbonation curing after granulation, the aggregate strength increased to 320 % in comparison with AA. Surprisingly, CC aggregate gained the lowest strength, which was only 0.55 MPa at 14 days. The strength development of CC almost stagnated, showing no increase with age. In contrast, AA and AC aggregates gained a 34 % and 52 % increase in strength respectively from 7 days to 14 days. The huge difference between AC and CC was obviously caused by the CO2-granulation, which impacted the micro-structural development. The physical properties of AA, AC and CC aggregates are also compared with other artificial aggregates in previous studies (see Fig. 7a). Gesoğlu et al. [21] used 10 % cement and 90 % ground granulated blast furnace slag to fabricate aggregates and obtained the strength of up to 10 MPa (zone 1-Fig. 7a). Suresh and Karthikeyan [22] produced aggregates with 100 % class-C fly ash by granulation and chemical curing (zone 2-Fig. 7a). They obtained aggregates with the water absorption of 3.90 % and the aggregate strength of 4.72 MPa after curing in 12 M sodium hydroxide solution for 28 days. González-
Corrochano et al. [23] manufactured artificial lightweight aggregates by sintering washing aggregate sludge, fly ash and motor oil at 1150 °C. The produced aggregates obtained the strength of about 8 MPa with the corresponding water absorption of 14–17% (zone 3-Fig. 7a). Comparing AC aggregate produced in this study with the above-referenced aggregates that were activated either by alkali, sintering or cement, the utilization of BOFS and post carbonation could be a cost-effective and eco-friendly approach to produce aggregates. However, life cycle cost analysis (LCCA) and life cycle assessment (LCA) are needed for further verification. Recently, Shi et al. [14] also introduced post carbonation for aggregate production (zone 4-Fig. 7a). It was reported that the aggregates produced by cement and waste concrete powder showed comparable properties to the AA aggregate in this study. Gunning et al. [16] (zone 5-Fig. 7a) used a number of wastes such as paper ash, wood ash, cement kiln dust, etc. to produce aggregates with both synchronized and post carbonation. Nevertheless, the strength of the produced aggregates were less than 0.2 MPa, significantly lower than the CC aggregate. By comparison, BOFS can be regarded as a highly effective raw material for aggregate production with the activation of CO2. From Fig. 7a, no significant correlation between aggregate strength and water absorption was observed. However, the bulk density presented a negative linear correlation with water absorption as shown in Fig. 7b. As reflected by the adjusted R square of 0.99, this liner correlation fitted better for the aggregates in this study and in Suresh and Karthikeyan’s work [22]. 3.2. Mineralogical compositions Mineralogical alterations of BOFS aggregates exposed to different carbonation regimes are presented in Fig. 8. By comparing the XRD pattern of AA-28d with as-received BOFS, it was found that no obvious peak transformation took place after hydration for 28 days due to the low hydration reactivity, apart from a slight increase in peak 4 (calcite). In contrast, when synchronized or post carbonation was applied (i.e. CC-0d or AC-28d), portlandite and calcium silicate were found to be greatly consumed accompanied by the sharp increase of calcite peaks. 138
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Fig. 7. Comparison of the AA, AC, CC and other artificial aggregates reported in the literature (a) aggregate strength and (b) bulk density against water absorption [10,14,16,21–23,44].
aggregate undergone only post carbonation with a limited carbonation front. f-CaO is the critical phase that affects the volume stability of steel slag products. It was determined that the f-CaO content in as-received BOFS powder was only 1.44 %, most of which were already converted to Ca(OH)2 as confirmed by the XRD pattern. After granulation and carbonation, the f-CaO content of the aggregates reduced to an extremely low level (< 0.5 %), which fully eliminated the volume instability [27]. 3.3. Microstructural characterization A phenolphthalein indicator was used to monitor the carbonation depths of the aggregates. As indicated in Fig. 9, the whole cross section of CC aggregate showed no coloration because of the neutralization of pore solution, denoting the carbonation of great extent, whereas the AA aggregate remained purple as a result of poor carbonation. Concerning the AC aggregate, a colorless layer of 1−2 mm thickness was clearly observed encapsulating a purple core, thus forming a core-shell structure. However, it should be noted that phenolphthalein measurement did not represent the depth of maximum ingress of CO2 and therefore the core may also be partially carbonated [28]. Fig. 10 shows the SEM images of AA, AC and CC aggregates. Typical hydration products such as fibrous calcium silicate hydrate (C-S-H) and hexagonal-plate monosulfoaluminate hydrate or monocarboaluminate hydrate (AFm) were observed in AA-28d (Fig. 10a). In the core region of AC-28d (Fig. 10b), some fibrous C-S-H and large hexagonal-prism portlandite (CH) could also be observed, similar with AA due to the
Fig. 8. XRD patterns of as-received BOFS, AA, AC and CC aggregates.
The peaks overlapped at the two theta of around 42° corresponded to larnite and wustite; after synchronized carbonation (i.e. CC-0d), larnite was consumed while the wustite remained inert, leading to the decreased intensity of the peak. With the proceeding of post carbonation (i.e. CC-28d), the intensity of wustite peaks, especially at 42°, showed an obvious increase. This is attributed to the partial dissolution and reaction of srebrodolskite (Ca2Fe2O5) to form iron oxide and calcium carbonate as supported by Bodor et al. [24] and Berryman et al. [25], even though srebrodolskite is less vulnerable to carbonation compared with larnite, portlandite, etc. Given the complexity of the diffraction patterns, positive identification of all phases is not always possible. It was believed that the RO phase, a solid solution of CaO–FeO–MnO–MgO [26], was also recognized at the two theta of 42° and 61° and the calcium carbonate indicated by the calcite peaks may be incorporated with magnesium, forming (Mg0.03Ca0.97)(CO3). There were only slight differences observed in the patterns of AC and CC aggregates despite the fact that mineralogy in the core region of CC aggregate may also be altered, which was different from the AC
Fig. 9. Cross sections of the aggregates. 139
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Fig. 10. SEM images of AA, AC and CC aggregates.
hydration of BOFS. However, the shell of AC was different; rhombohedral calcite, as well as some acicular clusters could be easily found (see Fig. 10c), which was believed to be aragonite by EDS (Fig. 10d) and morphology [29,30]. Though aragonite is generally recognized to be meta-stable relatively to calcite, preferential aragonite formation can be found with the presence of Mg [31]. The formation of aragonite in steel slag was confirmed by Salman et al. [30] at either elevated temperatures or pressures. However, the formation of aragonite was not identified in XRD patterns, which may be attributed to its low content
or overlapped peaks with calcite. Unlike the AA-28d and AC-28d aggregates in which dense and compact structures were formed by the hydration products due to the lack of available space, CC-28d-core showed extensive early-age hydration products with clear morphology (see Fig. 10e). This may be attributed to the higher porosity and thus higher surface area for hydration products to grow. Upon carbonation, both aragonite and calcite could be found in CC-28d as shown in Fig. 10e. The diameter of acicular aragonite and the side length of rhombohedral calcite were in the range of 0.5–1 μm. 140
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Fig. 11. Image analysis of the artificial aggregates.
the pores distinguished as black pixels were obtained. An overflow method proposed by Wong et al. [34] was adopted for thresholding. The critical threshold corresponded to the inflection of the cumulative grayscale curve determined from the intersection between two linear segments and a factor (i.e. 0.9) was multiplied to avoid porosity overestimation. Examples of the original BSE images, cumulative grayscale curves and thresholded images are shown in Fig. 11. The area fraction of black pixels in the thresholded image is equivalent to the porosity of the aggregate [34] and the average porosity and standard deviation are obtained after processing 15 images for each shell or core part. However, it is noteworthy that the porosity processed in this study is macroporosity i.e. 0.19 μm < pore diameter < 182 μm due to the resolution limitation. Final results are seen in Table 2; the porosities of AAshell, AC-shell and CC-shell were 12.2 %, 11.6 % and 26.3 %, while those of the corresponding core parts were 11.6 %, 7.9 % and 48.0 % respectively. The shell and core porosity of AA aggregate were consistent since almost only the hydration of BOFS occurred in the aggregate, barely influenced by carbonation. By applying post carbonation, it is interesting to find that shell porosity of AC remained
Table 2 The porosity of different aggregates. porosity shell AA-28d AC-28d CC-28d
core
12.2 (4.0) 11.6 (3.1) 26.3 (3.7)
＊
11.6 (4.3) 7.9 (3.3) 48.0 (4.2)
*standard deviation is shown in brackets.
Image analysis for aggregate porosity is performed on BSE images. Compared with other structure characterizing techniques such as X-ray tomography in which elaborate three-dimensional model can be obtained [32,33], this method allows a better convenience in operation for estimating the porosity while taking the samples for other microstructural analyses. In this method, original BSE images were first collected at 500x magnification and digitised to 1280*1024 pixels at a pixel spacing of 0.19 μm. A threshold was applied on each image and 141
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Fig. 12. Microhardness measurement pattern of (a) shell and (b) core.
comparable with the AA aggregate but the core porosity exhibited a decrease. As for the CC aggregate, dramatically higher porosity for both the shell and core were noticed. Apart from the porosity analysis, the Vickers microhardness of the aggregates was measured in order to characterize the differences between the core and shell matrices of the aggregates (see Fig. 12). Each microhardness value shown in Table 3 were averaged based on at least 36 (4*9) indentations and the standard deviations were shown in brackets. For AA aggregate, a relatively homogeneous development of strength was observed since the microhardness values of shell and core were similar, reaching 83.4 and 80.2 respectively. While AC aggregate obtained significantly improved microhardness values of 108.3 on the core and 111.9 on the shell. CC aggregate showed the lowest microhardness i.e. 78.0 on the shell and 56.3 on the core. It was found that the data in terms of porosity and microhardness were inconsistent given the fact that the two factors are linearly related [35]. In this regard, the reason for the inconsistency is discussed in section 3.5. 3.4. CO2 uptake Apart from the influences of synchronized and post carbonation on the physical, mechanical, mineralogical and micro-structural properties of BOFS aggregates, comparison of the two carbonation approaches on CO2 uptake was also assessed by TGA. It is known that the major differences between synchronized carbonation and post carbonation are temperature, duration and CO2 concentration. Synchronized carbonation was conducted in a gradually self-heating condition at ∼100 % CO2 concentration for about 30 min while the post carbonation was kept constant at 20 °C with 20 % CO2 concentration for 4 days. Temperature variation influences the solubility of CO2, rate of calcium dissolution, nucleation, and growth rate of reaction products [36], while extended duration on CO2 exposure and higher CO2 concentration allow a greater conversion of reactive minerals to calcium carbonate [36,37]. A previous investigation on steel slag carbonation [6] suggested that elevated temperature up to 200 °C favored the carbonation kinetics. In addition, synchronized carbonation was conducted from the fine powders while post carbonation was conducted to
Fig. 13. Thermogravimetric analysis of AA, AC and CC aggregates.
aggregates in larger sizes. TGA results of the aggregates with different carbonation regimes are plotted in Fig. 13. It includes differential thermogravimetric (DTG) curves of AA-28d, AC-28d, CC-28d and CC0d. The peaks observed in Zone 1 (30−300 °C) were mainly derived from the decomposition of hydration products such as C-S-H and AFm while the peak in Zone 2 (400−500 °C) denoted the decomposition of portlandite. Carbonation degree was characterized by the mass loss in Zone 3 (600−800 °C) due to the decomposition of calcium carbonates [38]. It can be seen that, except for the as-received BOFS, AA-28d possessed the lowest peak in Zone 3 and the corresponding CO2 weight was about 3.87 % with the CO2 uptake of 3.78 % (see Table 4). Through the application of synchronized carbonation (i.e. CC-0d), the CO2 uptake was significantly increased to 7.72 %. However, the AC-28d and CC-28d can sequestrate more CO2 with recorded values of 10.03 % and 15.70 %
Table 3 Microhardness values for AA, AC and CC samples. Microhardness value shell AA-28d AC-28d CC-28d
core
83.4 (18.6) 111.9 (18.7) 78.0 (23.4)
＊
80.2 (26.8) 108.3 (34.7) 56.3 (13.6)
*standard deviation is shown in brackets. 142
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CaCO3 and C-S-H were about 1.79 GPa and 0.45-0.83 GPa respectively [41,42]. In comparison with AA and AC, CC aggregate was more porous based on the up to fourfold porosity; the core part was particularly identified as the main weakness on the basis of the highest porosity (48.0 %) and lowest microhardness (56.3). However, it is unlikely attributed to the carbonation of C-S-H that would lead to porosity augment considering the experimental observations that i) the CC-28d-shell and CC-28d-core had similar CO2 uptake (14.84 % versus 15.95 %, see Table 4) but significantly different porosity (26.3 % versus 48.0 %), and ii) the CC aggregate clearly had more hydration products as seen in DTG curves when compared with AA and AC. As mentioned in section 2.2, the physical interaction among heat released, water and tumbling process during the CO2-granulation was believed to be the main reason for having higher porosity in CC aggregate; and it was specifically caused by the fierce carbonation kinetics during granulation, which released extensive heat. The heat released vaporized a huge amount of bridging water used to temporarily bind the BOFS particles, causing extensive unfilled voids. The extensive voids, especially these in the core part, were not eliminated in consolidation and growth period and therefore caused the loose structure [15]. The results were supported by previous studies where CO2 was also introduced in granulation process and it produced aggregates with extremely low strength [16,43]. In short, this phenomenon occurred in the poorly wetting system accompanied by the exothermic reaction (i.e. accelerated carbonation). However, it’s worth noting that based on the bulk density of the aggregates produced [16], CO2-granulation technique was found potentially promising in manufacturing lightweight aggregates.
Table 4 CO2 weight, CO2 uptake and calcium conversion. Samples
CO2 weight (%)
CO2 uptake (%)
Calcium conversion (%)
Raw BOFS AA-28d CC-0d AC-28d
0.24 3.87 7.39 9.33 10.11 7.10 13.78 13.13 13.96 1.08
– 3.78 7.72 10.03 10.99 7.38 15.70 14.84 15.95 0.85
0.73 12.17 24.11 31.13 34.00 23.10 48.32 45.70 49.08 3.29
CC-28d AC-0.5h
overall shell core overall shell core
respectively. Portlandite was fully reacted thus no clear peaks corresponding to portlandite decomposition can be observed. The corresponding overall calcium conversions for AA-28d, CC-0d, AC-28d and CC-28d were 12.17 %, 24.11 %, 31.13 % and 48.32 % respectively. Considering that duration for synchronized carbonation was about 30 min, the CO2 weight, uptake and Ca-conversion of AC after 30 min of post carbonation was also listed in Table 4 for comparison. It could be concluded that synchronized carbonation was more efficient in carbonating the BOFS. As compared with the current study, it is interesting to find that Gunning et al. [39] reported the CO2 uptake of 1.9%–11.1% by their artificial aggregates made from some incineration residues, which was about 4 % lower than the CC aggregate described herein. Apart from the carbonation on aggregates, Salman et al. [30] examined the CO2 sequestration capacity of dry-mixed steel slag paste (4*4*4 cm3) with the aim of fabricating high-value building materials. Under the condition of 22 °C, 5 % CO2 concentration and 80 % relative humidity, only 4.26 % CO2 was sequestrated after 21 days. By setting a more aggressive carbonation condition (i.e. 80 °C, 100 % CO2 concentration, 8 bar CO2 partial pressure), the CO2 uptake was dramatically increased to 8.06 % at 16 h. These products suggested lower CO2 sequestration/ calcium conversion levels or lower carbonation efficiency than those under aqueous carbonation, where the reactant particles could be fully exposed [6,7,40]. However, the highlight of accelerated carbonation on construction products in this study was the additional strength gain instead of the CO2 uptake.
4. Conclusions In this paper, basic oxygen furnace slag (BOFS) powder was adopted to prepare artificial aggregates by a wet granulation technique. Two accelerated carbonation approaches applied during granulation and curing were investigated for their influences on the characteristics of the produced BOFS aggregates. It is found that the aggregates showed high reactivity to CO2 and therefore the physicochemical properties and microstructure were significantly altered.
• Post
3.5. Discussion Accelerated carbonation altered the microstructure of AC and CC aggregates by precipitating calcium carbonate. It is known that the solid volume increases by 11–14% upon the carbonation of CH, thus leading to a reduction in porosity; while the solid volume decreases upon the carbonation of C-S-H, resulting in porosity augment [38]. Given the thermodynamic perspective that all CH should carbonate before C-S-H [38], lack of CH peak in the DTG curve of AC-28d-shell (see Fig. 13) implied the exhaustion of CH and the further carbonation of C-S-H; thus it offset the beneficial effect on porosity from CH carbonation and led to the similar porosity of AC-28d-shell with that of AA-28d-core/shell (∼12 %). In contrast, the protruding CH peak in the DTG curve of both AC-4d-core and AC-28d-core (see Fig. 13) certainly indicated that the CH was not fully depleted by carbonation and thus it contributed to decreasing the porosity of AC-core to ∼8 %. In addition, knowing the porosity of AC-shell was similar with AA-core/shell, it was interesting to find that the microhardness value of AC-shell was significantly higher than AA-core/shell (111.9 versus 80.2/83.4). This can be obviously attributed to the mineralogical differences that the former was dominated by calcium carbonates while the latter mainly consisted of hydration products, since the nano-indentation hardness values for
•
carbonation is an effective approach for performance enhancement. The produced BOFS aggregates treated by post carbonation (AC) can significantly increase the strength by 220 % in contrast with their uncarbonated counterparts (AA), reaching an optimal value of 5.24 MPa. A double-layer structure was clearly observed based on phenolphthalein test, with the CO2 uptake of 10.99 % and 7.38 % in the shell and core respectively. Besides, an overall 40 % higher microhardness value was recorded due to the formation of carbonate precipitates. Synchronized carbonation during granulation is inappropriate for obtaining high-strength aggregates yet it is promising for the production of lightweight aggregates. By the combination of synchronized and post carbonation, the produced aggregates (CC) was found to be ∼350 kg/m3 lower in loose bulk density and 25 % higher in water absorption, reaching an average aggregate strength of ∼0.55 MPa. In addition, the aggregates obtained an CO2 uptake of 15.70 % and an up to 48.05 % porosity in the core. The defect was primarily attributed to the extensive heat released during CO2granulation process that vaporized a great amount of bridging water and thus left unfilled voids.
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.
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Acknowledgments The research funding from the Hunan Province Key Research Project (2017WK2090) and the NSFC International (Regional) Cooperation and Exchange Program (51750110506 & 5181101350) are gratefully acknowledged.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Production of artificial aggregates from steel-making slag: Influences of accelerated carbonation during granulation and/or post-curing
T
Yi Jiang, Tung-Chai Ling* Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, Hunan, China
ARTICLE INFO
ABSTRACT
Keywords: Steel slag Granulation Accelerated carbonation Artificial aggregates
Accelerated carbonation is envisaged to be a significant curing method for the development of cement-free artificial aggregates. In this paper, two accelerated carbonation approaches, applied during granulation and post-curing respectively, are investigated for their influences on the properties of the aggregates made by 100 % basic oxygen furnace slag (BOFS). The result shows that carbonation during post-curing significantly enhances the overall properties of the granulated BOFS, such as increasing the strength by 220 %. In contrast, synchronized carbonation applied simultaneously during the granulation process yields considerable property degradation, though a CO2 uptake of ∼16 % can be achieved. It contradicts with the common understanding that carbonation is beneficial to the strength enhancement. This is because synchronized carbonation of BOFS is found to release extensive heat and vaporize a great amount of bridging water for agglomeration, thus causing a very loose structure. In general, synchronized and post-granulation carbonation result in two notably different influences on the properties of BOFS aggregates. The integrated approach of granulation and post-granulation carbonation creates a promising cement-free alternative to the local natural aggregates; while the synchronized carbonation shows significant potential for the production of lightweight aggregates.
1. Introduction Finding the potential economic value in the approach of carbon dioxide (CO2) mineralization and utilization has been an interesting topic and expedited extensive researches. Application of accelerated carbonation on alkaline residues is reckoned as an effective approach for CO2 mineralization and utilization. Steel slag is an alkaline residue, generated from steel-making industry; it accounts for about 25 % of the total residue production in China’s iron and steel-making field, with an annual production of 101 million tons [1]. Basic oxygen furnace slag (BOFS) is generated in BOF process and accounts for about 70 % of the annual steel slag output [2]. Critical barriers such as the low hydration reactivity and significant free calcium oxide (f-CaO) content are preventing BOFS from being valorized in cement-based construction materials. However, based on the high CO2 reactivity of BOFS from the high CaO content [3,4], extensive studies of direct carbonation on BOFS were carried out under different processing conditions. For examples, van Zomeren et al. [5] applied carbonation on 2–3.3 mm BOFS at 90 °C for 70 h and recorded a CO2 uptake of 1.5 % (CO2/steel slag) at the CO2 concentration of 20 %. By decreasing the particle size of BOFS to below 38 μm, Huijgen et al. [6] found that under the condition of 19 bar CO2
⁎
pressure and 100 °C a maximum carbonation degree of 74 % was achieved after 30 min. Pan et al. [7] used 8 μm BOFS to remove the CO2 in the exhaust gas (CO2 concentration = 30 %) through high gravity carbonation in slurry state. With the optimization of processing parameters, a CO2 removal efficiency of 97.3 % and estimated CO2 capture capacity of 165 kg per day could be achieved. To find the commercially viable end-use products for the mineralized CO2, part of the recent attentions have been paid to the cold bonding granulation for producing the alternatives to natural aggregates that are in shortage. This is closely associated with the tightened environmental policies in China. Carbonation of the low-hydraulic BOFS opens a possible pathway of utilizing the CO2-activated BOFS for manufacturing cold-bonded aggregates as demonstrated by Eqs. (1) and (2) [8,9] :
C3 S+ (3-x)CO2 +yH2 O C x SHy + (3-x)CaCO3
(1)
C2 S+ (2-x)CO2 +yH2 O C x SHy + (2-x)CaCO3
(2)
Previous attempts to prepare artificial aggregates with various wastes such as fly ash [10], municipal solid waste incineration (MSWI) fly ash [11], marine sediments [12] and quarry dust [13], etc. claimed
Corresponding author. E-mail address: [email protected] (T.-C. Ling).
https://doi.org/10.1016/j.jcou.2019.11.009 Received 2 June 2019; Received in revised form 17 October 2019; Accepted 7 November 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. As-received BOFS powder.
that a percentage of up to 30 % Portland cement was imperative to bind the fines and attain adequate strength. However, aother investigation on the strength profile of CO2-cured cold-bonded aggregates indicated the feasibility of reducing cement dosage by carbonation curing [14]. Therefore, the main focus of this work is to produce cement-free BOFS artificial aggregates for use in concrete through an integrated approach of accelerated carbonation and cold bonding granulation. For comparison, two accelerated carbonation approaches are employed namely synchronized carbonation (applied simultaneously during granulation) and post carbonation (applied during post-curing). Physical, mechanical, mineralogical, and micro-structural properties are specifically evaluated and compared in order to investigate the differences of the two carbonation approaches on the granulation process and properties of the end products.
Fig. 3. X-ray diffraction (XRD) pattern of the BOFS powder.
mineral phases are portlandite, larnite, srebrodolskite and wustite, etc. (see Fig. 3). 2.2. Production and carbonation of granulated BOFS aggregates Granulation was conducted in a customized pan granulator. The granulator is 700 mm in diameter and 200 mm in collar height (Fig. 4a), with a scraper stuck to the bottom surface and a feeding hopper connected at the glass cover (Fig. 4b). The revolution speed for granulating was set at 15 rpm with a tilting angle of 45°. BOFS powder was used as the only feed material and an appropriate amount of water was sprayed onto the dry powders to wet the particles and form nuclei. Subsequently, growth of the nuclei took place by coalescence or layering processes, during which well-formed agglomerates collided and formed a single agglomerate or fine grains adhered to the surface of agglomerates [15]. Synchronized carbonation was applied during granulation process by introducing 99.9 % CO2 stream into the granulator at the atmospheric pressure. Post carbonation was conducted after granulation in a controlled environment (CO2 concentration = 20 %, relative humidity = 65 %, temperature = 20 °C). In order to investigate the influences of CO2 on the BOFS granulation and the properties of produced aggregates, a total of three different carbonation regimes were designed in this study. As indicated in Table 1, CC designates carbonation applied both during and after granulation; AC indicates carbonation applied only after granulation as a post-curing treatment and AA refers to the control samples that were granulated and cured under ambient air (without carbonation). It should be noted that duration for post carbonation was determined as 4 days based on the trails and the results of previous experimental studies [16,17] which indicated that the CO2 uptake reached a plateau after 4 days. After carbonation, the aggregates were further cured in natural air condition (relative humidity = 50 %, temperature = 20 °C) to 7, 14 and 28 days before testing. The total liquid to solid (L/S) ratio for AA and AC group was 0.16. However, in the case of CC, extensive heat and vaporized water were released during CO2-granulation process and therefore a double amount of water was required (L/S ratio = 0.32) for the nuclei to grow. Corresponding to the increase in water content, the duration of the granulation for CC was also doubled, from 16 min to 32 min.
2. Materials and methods 2.1. Basic oxygen furnace slag (BOFS) Mechanically ground BOFS obtained from a commercial corporation located in Guangxi Province, China, was used in this study (see Fig. 1). The true density of as-received BOFS as measured by a Micromeritics AccuPyc II 1340 instrument is 3.4 g/cm3. The particle size distribution tested by Mastersizer 2000, Malvern Panalytical is shown in Fig. 2, indicating a mean particle size of 6 μm. The chemical composition was analyzed by X-ray fluorescence (XRF) on a Axios, Malvern Panalytical and the mineralogical composition was analyzed by X-ray diffraction (XRD) on a Bruker D8 Advance instrument. The XRD data were collected with CuKα radiation operating at 40 kV, 40 mA between a 2θ from 5° to70° in 0.02° steps and 0.2 s per step. The results show that the BOFS is rich in CaO (∼42.1 %), Fe2O3(∼21.7 %), SiO2(∼16.5 %), MgO (∼6.7 %), MnO (∼4.8 %) and Al2O3 (∼3.5 %), etc. and the major
2.3. Testing of granulated BOFS aggregates To understand the engineering properties of the produced BOFS aggregates, the particle size distribution, loose bulk density, 24 h water
Fig. 2. Particle size distribution of raw BOFS powder. 136
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Fig. 4. (a) granulation chamber and (b) sketch map of the granulation system. Table 1 Carbonation regimes.
CO2 (wt. %) =
Notation
Carbonation regime
CC AC AA
synchronized and post carbonation post carbonation /
CO2 uptake(wt. %) = Ca (%)=
MCaO (g/mol) mCO2 /m105°C 100 × × × 100 ( mCO2/m105°C) MCO2 (g/mol) CaOtotal (wt. %)
3. Results and discussion
(3)
mH2O × MCaO MH2O × m105°C
(6)
Where ΔmCO2 is the mass loss due to the CaCO3 decomposition (600−800 °C), m105°C is the dry weight of the sample measured at 105 °C, MCaO is the molecular weight of CaO (i.e., 56 g/mol), MCO2 is the molecular weight of CO2 (i.e., 44 g/mol), and CaOtotal is the weight fraction of CaO in the fresh BOFS (i.e., 42.09).
3.1. Physical properties
where is the aggregate strength (MPa), P is the load at failure (N) and d is the mean diameter (mm) derived by taking three axial measurements. For each group, ten aggregate particles were tested and the average strength was reported. The alteration of mineralogical compositions after curing was determined by XRD with a Bruke D8 advance instrument. The f-CaO content of the samples was determined using the combination of the ethylene glycol method (GB176-2008) and thermogravimetric analysis (TGA). The former was used to determine the total free calcium content while the latter was used for the deduction of calcium from calcium hydroxide. Eq. (4) was followed for the quantification:
f -CaO(wt.%) = f -CaOtotal (wt.%)
1
CO2carbonated (wt. %) CO2initial (wt. %) × 100 100 CO2carbonated (wt. %)
(5)
(7)
absorption, aggregate strength, mineralogical compositions, porosity, microhardness, and CO2 uptake/calcium conversion were investigated. Particle size distributions of the aggregates were obtained through sieving test with square-opening sieves. Density and water absorption were measured in accordance with ASTM C29 and C127 respectively. Aggregate strength on individual granulated BOFS was tested according to Eq. (3) [18] with a California Bearing Ratio tester:
2.8P/ d2
mCO2 × 100 m105°C
The particle size distribution (PSD) of the BOFS aggregates obtained under different carbonation regimes are shown in Fig. 5. It was found that these aggregates obtained from pan granulation were continuously graded with the particle sizes ranging from 1.18–19 mm. In spite of some variations, the PSD of AA and AC aggregates were comparable and the grading curves were very close to that of size number 67 coarse aggregates as prescribed in ASTM C33 for use in concrete. However, the difficulty of granulation process under a CO2 atmosphere as reported for CC group resulted in significantly smaller particles, with 86 wt.%
(4)
Where f-CaOtotal is the result of ethylene glycol method, ΔmH2O is the mass loss due to the Ca(OH)2 decomposition in BOFS samples, MH2O is the molecular weight of H2O (i.e., 18 g/mol). A phenolphthalein indicator was used to monitor the carbonation depth. Morphology of the aggregates was obtained by a JSM-6490LV scanning electron microscope (SEM). The porosity was analyzed based on back-scattered electron (BSE) images. Microhardness was measured with a HVS-1000Z microhardness tester on polished samples. A load of 1.961 N was applied with a dwell time of 10 s. CO2 weight, CO2 uptake and calcium conversion were quantified by thermogravimetric analysis (TGA) using a Rigaku TG-DTA 8121H as per Eqs. (5), (6) and (7) [7,19,20]. The aggregate particles with the sizes of 10 mm ( ± 2 mm) were chosen and about 15 mg of the homogeneously ground sample was weighed and heated from room temperature to 1000 °C at a heating rate of 10 °C/min. Nitrogen atmosphere was provided with a flow rate of 30 mL/min.
Fig. 5. Particle size distribution of the final aggregates. 137
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Fig. 6. (a) physical appearance, (b) loose bulk density, (c) water absorption and (d) aggregate strength of the final aggregates at different ages.
aggregates finer than 9.5 mm compared with only about 50 % in AA and AC groups. Concerning the appearance, AA and AC aggregates were dark gray and presented some evident humps (Fig. 6a). These humps were unevenly distributed nuclei due to the insufficient layering in agglomeration process. In comparison, CC aggregate demonstrated a more spherical surface based on the prolonged granulation duration and thus the sufficient layering. According to the Fig. 6b, the loose bulk density of AA and AC aggregates were relatively similar, ranging from 1250 kg/m3 to 1280 kg/m3 while the density of CC was remarkably lower, about 900 kg/m3, which fulfilled the requirement set for lightweight aggregate as per BS EN 13055-1 (1200 kg/m3). In terms of water absorption, AA and AC aggregates showed similar values, which were about 11−12 wt.% (Fig. 6c). Whereas, CC showed a significantly higher capacity to absorb water, reaching 27 wt.%. No significant change of both the density and water absorption with curing time for all curing types of BOFS aggregates was found. Aggregate strength at different ages are presented in Fig. 6d. AC had the highest strength among the three groups regardless of age, reaching an optimal average value of 5.23 MPa at 14 days. By applying 4 days of carbonation curing after granulation, the aggregate strength increased to 320 % in comparison with AA. Surprisingly, CC aggregate gained the lowest strength, which was only 0.55 MPa at 14 days. The strength development of CC almost stagnated, showing no increase with age. In contrast, AA and AC aggregates gained a 34 % and 52 % increase in strength respectively from 7 days to 14 days. The huge difference between AC and CC was obviously caused by the CO2-granulation, which impacted the micro-structural development. The physical properties of AA, AC and CC aggregates are also compared with other artificial aggregates in previous studies (see Fig. 7a). Gesoğlu et al. [21] used 10 % cement and 90 % ground granulated blast furnace slag to fabricate aggregates and obtained the strength of up to 10 MPa (zone 1-Fig. 7a). Suresh and Karthikeyan [22] produced aggregates with 100 % class-C fly ash by granulation and chemical curing (zone 2-Fig. 7a). They obtained aggregates with the water absorption of 3.90 % and the aggregate strength of 4.72 MPa after curing in 12 M sodium hydroxide solution for 28 days. González-
Corrochano et al. [23] manufactured artificial lightweight aggregates by sintering washing aggregate sludge, fly ash and motor oil at 1150 °C. The produced aggregates obtained the strength of about 8 MPa with the corresponding water absorption of 14–17% (zone 3-Fig. 7a). Comparing AC aggregate produced in this study with the above-referenced aggregates that were activated either by alkali, sintering or cement, the utilization of BOFS and post carbonation could be a cost-effective and eco-friendly approach to produce aggregates. However, life cycle cost analysis (LCCA) and life cycle assessment (LCA) are needed for further verification. Recently, Shi et al. [14] also introduced post carbonation for aggregate production (zone 4-Fig. 7a). It was reported that the aggregates produced by cement and waste concrete powder showed comparable properties to the AA aggregate in this study. Gunning et al. [16] (zone 5-Fig. 7a) used a number of wastes such as paper ash, wood ash, cement kiln dust, etc. to produce aggregates with both synchronized and post carbonation. Nevertheless, the strength of the produced aggregates were less than 0.2 MPa, significantly lower than the CC aggregate. By comparison, BOFS can be regarded as a highly effective raw material for aggregate production with the activation of CO2. From Fig. 7a, no significant correlation between aggregate strength and water absorption was observed. However, the bulk density presented a negative linear correlation with water absorption as shown in Fig. 7b. As reflected by the adjusted R square of 0.99, this liner correlation fitted better for the aggregates in this study and in Suresh and Karthikeyan’s work [22]. 3.2. Mineralogical compositions Mineralogical alterations of BOFS aggregates exposed to different carbonation regimes are presented in Fig. 8. By comparing the XRD pattern of AA-28d with as-received BOFS, it was found that no obvious peak transformation took place after hydration for 28 days due to the low hydration reactivity, apart from a slight increase in peak 4 (calcite). In contrast, when synchronized or post carbonation was applied (i.e. CC-0d or AC-28d), portlandite and calcium silicate were found to be greatly consumed accompanied by the sharp increase of calcite peaks. 138
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Fig. 7. Comparison of the AA, AC, CC and other artificial aggregates reported in the literature (a) aggregate strength and (b) bulk density against water absorption [10,14,16,21–23,44].
aggregate undergone only post carbonation with a limited carbonation front. f-CaO is the critical phase that affects the volume stability of steel slag products. It was determined that the f-CaO content in as-received BOFS powder was only 1.44 %, most of which were already converted to Ca(OH)2 as confirmed by the XRD pattern. After granulation and carbonation, the f-CaO content of the aggregates reduced to an extremely low level (< 0.5 %), which fully eliminated the volume instability [27]. 3.3. Microstructural characterization A phenolphthalein indicator was used to monitor the carbonation depths of the aggregates. As indicated in Fig. 9, the whole cross section of CC aggregate showed no coloration because of the neutralization of pore solution, denoting the carbonation of great extent, whereas the AA aggregate remained purple as a result of poor carbonation. Concerning the AC aggregate, a colorless layer of 1−2 mm thickness was clearly observed encapsulating a purple core, thus forming a core-shell structure. However, it should be noted that phenolphthalein measurement did not represent the depth of maximum ingress of CO2 and therefore the core may also be partially carbonated [28]. Fig. 10 shows the SEM images of AA, AC and CC aggregates. Typical hydration products such as fibrous calcium silicate hydrate (C-S-H) and hexagonal-plate monosulfoaluminate hydrate or monocarboaluminate hydrate (AFm) were observed in AA-28d (Fig. 10a). In the core region of AC-28d (Fig. 10b), some fibrous C-S-H and large hexagonal-prism portlandite (CH) could also be observed, similar with AA due to the
Fig. 8. XRD patterns of as-received BOFS, AA, AC and CC aggregates.
The peaks overlapped at the two theta of around 42° corresponded to larnite and wustite; after synchronized carbonation (i.e. CC-0d), larnite was consumed while the wustite remained inert, leading to the decreased intensity of the peak. With the proceeding of post carbonation (i.e. CC-28d), the intensity of wustite peaks, especially at 42°, showed an obvious increase. This is attributed to the partial dissolution and reaction of srebrodolskite (Ca2Fe2O5) to form iron oxide and calcium carbonate as supported by Bodor et al. [24] and Berryman et al. [25], even though srebrodolskite is less vulnerable to carbonation compared with larnite, portlandite, etc. Given the complexity of the diffraction patterns, positive identification of all phases is not always possible. It was believed that the RO phase, a solid solution of CaO–FeO–MnO–MgO [26], was also recognized at the two theta of 42° and 61° and the calcium carbonate indicated by the calcite peaks may be incorporated with magnesium, forming (Mg0.03Ca0.97)(CO3). There were only slight differences observed in the patterns of AC and CC aggregates despite the fact that mineralogy in the core region of CC aggregate may also be altered, which was different from the AC
Fig. 9. Cross sections of the aggregates. 139
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Fig. 10. SEM images of AA, AC and CC aggregates.
hydration of BOFS. However, the shell of AC was different; rhombohedral calcite, as well as some acicular clusters could be easily found (see Fig. 10c), which was believed to be aragonite by EDS (Fig. 10d) and morphology [29,30]. Though aragonite is generally recognized to be meta-stable relatively to calcite, preferential aragonite formation can be found with the presence of Mg [31]. The formation of aragonite in steel slag was confirmed by Salman et al. [30] at either elevated temperatures or pressures. However, the formation of aragonite was not identified in XRD patterns, which may be attributed to its low content
or overlapped peaks with calcite. Unlike the AA-28d and AC-28d aggregates in which dense and compact structures were formed by the hydration products due to the lack of available space, CC-28d-core showed extensive early-age hydration products with clear morphology (see Fig. 10e). This may be attributed to the higher porosity and thus higher surface area for hydration products to grow. Upon carbonation, both aragonite and calcite could be found in CC-28d as shown in Fig. 10e. The diameter of acicular aragonite and the side length of rhombohedral calcite were in the range of 0.5–1 μm. 140
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Fig. 11. Image analysis of the artificial aggregates.
the pores distinguished as black pixels were obtained. An overflow method proposed by Wong et al. [34] was adopted for thresholding. The critical threshold corresponded to the inflection of the cumulative grayscale curve determined from the intersection between two linear segments and a factor (i.e. 0.9) was multiplied to avoid porosity overestimation. Examples of the original BSE images, cumulative grayscale curves and thresholded images are shown in Fig. 11. The area fraction of black pixels in the thresholded image is equivalent to the porosity of the aggregate [34] and the average porosity and standard deviation are obtained after processing 15 images for each shell or core part. However, it is noteworthy that the porosity processed in this study is macroporosity i.e. 0.19 μm < pore diameter < 182 μm due to the resolution limitation. Final results are seen in Table 2; the porosities of AAshell, AC-shell and CC-shell were 12.2 %, 11.6 % and 26.3 %, while those of the corresponding core parts were 11.6 %, 7.9 % and 48.0 % respectively. The shell and core porosity of AA aggregate were consistent since almost only the hydration of BOFS occurred in the aggregate, barely influenced by carbonation. By applying post carbonation, it is interesting to find that shell porosity of AC remained
Table 2 The porosity of different aggregates. porosity shell AA-28d AC-28d CC-28d
core
12.2 (4.0) 11.6 (3.1) 26.3 (3.7)
＊
11.6 (4.3) 7.9 (3.3) 48.0 (4.2)
*standard deviation is shown in brackets.
Image analysis for aggregate porosity is performed on BSE images. Compared with other structure characterizing techniques such as X-ray tomography in which elaborate three-dimensional model can be obtained [32,33], this method allows a better convenience in operation for estimating the porosity while taking the samples for other microstructural analyses. In this method, original BSE images were first collected at 500x magnification and digitised to 1280*1024 pixels at a pixel spacing of 0.19 μm. A threshold was applied on each image and 141
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Fig. 12. Microhardness measurement pattern of (a) shell and (b) core.
comparable with the AA aggregate but the core porosity exhibited a decrease. As for the CC aggregate, dramatically higher porosity for both the shell and core were noticed. Apart from the porosity analysis, the Vickers microhardness of the aggregates was measured in order to characterize the differences between the core and shell matrices of the aggregates (see Fig. 12). Each microhardness value shown in Table 3 were averaged based on at least 36 (4*9) indentations and the standard deviations were shown in brackets. For AA aggregate, a relatively homogeneous development of strength was observed since the microhardness values of shell and core were similar, reaching 83.4 and 80.2 respectively. While AC aggregate obtained significantly improved microhardness values of 108.3 on the core and 111.9 on the shell. CC aggregate showed the lowest microhardness i.e. 78.0 on the shell and 56.3 on the core. It was found that the data in terms of porosity and microhardness were inconsistent given the fact that the two factors are linearly related [35]. In this regard, the reason for the inconsistency is discussed in section 3.5. 3.4. CO2 uptake Apart from the influences of synchronized and post carbonation on the physical, mechanical, mineralogical and micro-structural properties of BOFS aggregates, comparison of the two carbonation approaches on CO2 uptake was also assessed by TGA. It is known that the major differences between synchronized carbonation and post carbonation are temperature, duration and CO2 concentration. Synchronized carbonation was conducted in a gradually self-heating condition at ∼100 % CO2 concentration for about 30 min while the post carbonation was kept constant at 20 °C with 20 % CO2 concentration for 4 days. Temperature variation influences the solubility of CO2, rate of calcium dissolution, nucleation, and growth rate of reaction products [36], while extended duration on CO2 exposure and higher CO2 concentration allow a greater conversion of reactive minerals to calcium carbonate [36,37]. A previous investigation on steel slag carbonation [6] suggested that elevated temperature up to 200 °C favored the carbonation kinetics. In addition, synchronized carbonation was conducted from the fine powders while post carbonation was conducted to
Fig. 13. Thermogravimetric analysis of AA, AC and CC aggregates.
aggregates in larger sizes. TGA results of the aggregates with different carbonation regimes are plotted in Fig. 13. It includes differential thermogravimetric (DTG) curves of AA-28d, AC-28d, CC-28d and CC0d. The peaks observed in Zone 1 (30−300 °C) were mainly derived from the decomposition of hydration products such as C-S-H and AFm while the peak in Zone 2 (400−500 °C) denoted the decomposition of portlandite. Carbonation degree was characterized by the mass loss in Zone 3 (600−800 °C) due to the decomposition of calcium carbonates [38]. It can be seen that, except for the as-received BOFS, AA-28d possessed the lowest peak in Zone 3 and the corresponding CO2 weight was about 3.87 % with the CO2 uptake of 3.78 % (see Table 4). Through the application of synchronized carbonation (i.e. CC-0d), the CO2 uptake was significantly increased to 7.72 %. However, the AC-28d and CC-28d can sequestrate more CO2 with recorded values of 10.03 % and 15.70 %
Table 3 Microhardness values for AA, AC and CC samples. Microhardness value shell AA-28d AC-28d CC-28d
core
83.4 (18.6) 111.9 (18.7) 78.0 (23.4)
＊
80.2 (26.8) 108.3 (34.7) 56.3 (13.6)
*standard deviation is shown in brackets. 142
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CaCO3 and C-S-H were about 1.79 GPa and 0.45-0.83 GPa respectively [41,42]. In comparison with AA and AC, CC aggregate was more porous based on the up to fourfold porosity; the core part was particularly identified as the main weakness on the basis of the highest porosity (48.0 %) and lowest microhardness (56.3). However, it is unlikely attributed to the carbonation of C-S-H that would lead to porosity augment considering the experimental observations that i) the CC-28d-shell and CC-28d-core had similar CO2 uptake (14.84 % versus 15.95 %, see Table 4) but significantly different porosity (26.3 % versus 48.0 %), and ii) the CC aggregate clearly had more hydration products as seen in DTG curves when compared with AA and AC. As mentioned in section 2.2, the physical interaction among heat released, water and tumbling process during the CO2-granulation was believed to be the main reason for having higher porosity in CC aggregate; and it was specifically caused by the fierce carbonation kinetics during granulation, which released extensive heat. The heat released vaporized a huge amount of bridging water used to temporarily bind the BOFS particles, causing extensive unfilled voids. The extensive voids, especially these in the core part, were not eliminated in consolidation and growth period and therefore caused the loose structure [15]. The results were supported by previous studies where CO2 was also introduced in granulation process and it produced aggregates with extremely low strength [16,43]. In short, this phenomenon occurred in the poorly wetting system accompanied by the exothermic reaction (i.e. accelerated carbonation). However, it’s worth noting that based on the bulk density of the aggregates produced [16], CO2-granulation technique was found potentially promising in manufacturing lightweight aggregates.
Table 4 CO2 weight, CO2 uptake and calcium conversion. Samples
CO2 weight (%)
CO2 uptake (%)
Calcium conversion (%)
Raw BOFS AA-28d CC-0d AC-28d
0.24 3.87 7.39 9.33 10.11 7.10 13.78 13.13 13.96 1.08
– 3.78 7.72 10.03 10.99 7.38 15.70 14.84 15.95 0.85
0.73 12.17 24.11 31.13 34.00 23.10 48.32 45.70 49.08 3.29
CC-28d AC-0.5h
overall shell core overall shell core
respectively. Portlandite was fully reacted thus no clear peaks corresponding to portlandite decomposition can be observed. The corresponding overall calcium conversions for AA-28d, CC-0d, AC-28d and CC-28d were 12.17 %, 24.11 %, 31.13 % and 48.32 % respectively. Considering that duration for synchronized carbonation was about 30 min, the CO2 weight, uptake and Ca-conversion of AC after 30 min of post carbonation was also listed in Table 4 for comparison. It could be concluded that synchronized carbonation was more efficient in carbonating the BOFS. As compared with the current study, it is interesting to find that Gunning et al. [39] reported the CO2 uptake of 1.9%–11.1% by their artificial aggregates made from some incineration residues, which was about 4 % lower than the CC aggregate described herein. Apart from the carbonation on aggregates, Salman et al. [30] examined the CO2 sequestration capacity of dry-mixed steel slag paste (4*4*4 cm3) with the aim of fabricating high-value building materials. Under the condition of 22 °C, 5 % CO2 concentration and 80 % relative humidity, only 4.26 % CO2 was sequestrated after 21 days. By setting a more aggressive carbonation condition (i.e. 80 °C, 100 % CO2 concentration, 8 bar CO2 partial pressure), the CO2 uptake was dramatically increased to 8.06 % at 16 h. These products suggested lower CO2 sequestration/ calcium conversion levels or lower carbonation efficiency than those under aqueous carbonation, where the reactant particles could be fully exposed [6,7,40]. However, the highlight of accelerated carbonation on construction products in this study was the additional strength gain instead of the CO2 uptake.
4. Conclusions In this paper, basic oxygen furnace slag (BOFS) powder was adopted to prepare artificial aggregates by a wet granulation technique. Two accelerated carbonation approaches applied during granulation and curing were investigated for their influences on the characteristics of the produced BOFS aggregates. It is found that the aggregates showed high reactivity to CO2 and therefore the physicochemical properties and microstructure were significantly altered.
• Post
3.5. Discussion Accelerated carbonation altered the microstructure of AC and CC aggregates by precipitating calcium carbonate. It is known that the solid volume increases by 11–14% upon the carbonation of CH, thus leading to a reduction in porosity; while the solid volume decreases upon the carbonation of C-S-H, resulting in porosity augment [38]. Given the thermodynamic perspective that all CH should carbonate before C-S-H [38], lack of CH peak in the DTG curve of AC-28d-shell (see Fig. 13) implied the exhaustion of CH and the further carbonation of C-S-H; thus it offset the beneficial effect on porosity from CH carbonation and led to the similar porosity of AC-28d-shell with that of AA-28d-core/shell (∼12 %). In contrast, the protruding CH peak in the DTG curve of both AC-4d-core and AC-28d-core (see Fig. 13) certainly indicated that the CH was not fully depleted by carbonation and thus it contributed to decreasing the porosity of AC-core to ∼8 %. In addition, knowing the porosity of AC-shell was similar with AA-core/shell, it was interesting to find that the microhardness value of AC-shell was significantly higher than AA-core/shell (111.9 versus 80.2/83.4). This can be obviously attributed to the mineralogical differences that the former was dominated by calcium carbonates while the latter mainly consisted of hydration products, since the nano-indentation hardness values for
•
carbonation is an effective approach for performance enhancement. The produced BOFS aggregates treated by post carbonation (AC) can significantly increase the strength by 220 % in contrast with their uncarbonated counterparts (AA), reaching an optimal value of 5.24 MPa. A double-layer structure was clearly observed based on phenolphthalein test, with the CO2 uptake of 10.99 % and 7.38 % in the shell and core respectively. Besides, an overall 40 % higher microhardness value was recorded due to the formation of carbonate precipitates. Synchronized carbonation during granulation is inappropriate for obtaining high-strength aggregates yet it is promising for the production of lightweight aggregates. By the combination of synchronized and post carbonation, the produced aggregates (CC) was found to be ∼350 kg/m3 lower in loose bulk density and 25 % higher in water absorption, reaching an average aggregate strength of ∼0.55 MPa. In addition, the aggregates obtained an CO2 uptake of 15.70 % and an up to 48.05 % porosity in the core. The defect was primarily attributed to the extensive heat released during CO2granulation process that vaporized a great amount of bridging water and thus left unfilled voids.
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.
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Acknowledgments The research funding from the Hunan Province Key Research Project (2017WK2090) and the NSFC International (Regional) Cooperation and Exchange Program (51750110506 & 5181101350) are gratefully acknowledged.
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