Synergistic Use of CO2 Pretreatment and Accelerated Carbonation Curing for Maximum Recycling of Steel Slag

LIU Yaojun, AN Xiaopeng, LIANG Huichao, YU Kexiao, WANG Lan

(China Testing & Certification International Group Co., Ltd, China Building Materials Academy, Beijing 100024, China)

Abstract: Two carbonation approaches are considered for studying the effects on the hardening mechanisms of slurries made of 100 wt% electric arc furnace steel slag (EAF) slag or 80 wt% EAF slag incorporating 20 wt% of Portland cement, which are applied during the hot-stage pretreatment with simulated gas for raw steel slag or the accelerated carbonation curing of slurry. The mechanical strengths, carbonate products, microstructures and CO2 uptakes were quantitatively investigated. Results manifest that accelerated carbonation curing increases the compressive strengths of steel slag slurry, from 17.1 MPa (binder of 80 wt%EAF and 20 wt% cement under standard moisture curing) to 36.0 MPa (binder of 80 wt% EAF and 20 wt%cement under accelerated carbonation curing), with a CO2 uptake of 52%. In contrast, hot-stage carbonation applied during the pretreatment of steel slag increases the compressive strengths to 43.7 MPa (binder of 80 wt%carbonated EAF and 20 wt% cement under accelerated carbonation curing), with a CO2 uptake of 67%. Hotstage carbonation of steel slag is found for particle agglomeration, minerals remodeling and calcite formed, thus causing an activated steel slag with a dense structure and more active components. Accelerated carbonation curing of steel slag slurry paste results in the newly formed amorphous CaCO3, calcite crystalline and silica gels that covered the pores of the matrix, facilitating microstructure densification and strength improvement.Adopting the combinative methods of the hot-stage CO2 pretreatment and accelerated carbonation curing creates a promising high-volume steel slag-based binder with high strengths and CO2 storage.

Key words: high-volume steel slag binders; hot-stage CO2 pretreatment; accelerated carbonation curing;CO2 storage

1 Introduction

Steel slag, an inevitable industrial waste from the steel manufacturing process, covers 15 wt%-35 wt% of crude steel[1]. In China, the annual output of steel slags is over a hundred million tons[2], however the reutilization ratio was below 30 wt%[3]. Steel slags are mainly recycled for cement and concrete[4],road building[5], and bituminous mixture[6], soil amelioration[7]and phosphate fertilizer[8]. However, steel slag utilization may be restricted, primarily because of its slow hydration and undesired deleterious expansion due to mineralogical changes, such as hydration of free CaO and MgO, andα-C2S transformation[9,10].

Carbonation of steel slags is proved as an effective method to resolve these issues, because the reaction rate of steel slags hydration is much slower than that of carbonation in a wet environment, and the carbonation of alkaline oxides improves the mineralogical stability[38]. Meanwhile carbonation may contribute to the CO2storage. Several researchers have investigated the potential application of steel slags for carbon sequestration to produce qualified building materials[3,11-13], by accelerated carbonation curing method. Moreover,in-situCO2pretreatment method not only provides CO2recycling potential, but also reuses of waste heat from exhaust gas to accelerate reactions,which could be as a finishing step in industrial plants[14]. However, there is little progress in the investigation on the binding mechanisms of steel slag simultaneously treated with these two carbonization methods ofin-situCO2pretreatment method and accelerated carbonation curing. That is important to promote the high blended proportion use of steel slag in construction materials.

The objective of this study is to produce highvolume steel slag-based slurries with high strengths through an integrated approach ofin-situCO2pretreatment and accelerated carbonation curing.For comparison, two carbonation approaches are adopted for the slurry specimens of 80 wt% steel slags,namelyin-situCO2pretreatment method (applied during pretreatment of steel slag with simulated gas)and accelerated carbonation curing (applied during curing). Physical, mineralogical, and microstructural performances are investigated and compared, to explore the carbonation mechanisms on the slurry specimens produced by steel slag.

2 Experimental

2.1 Materials

Original electric arc furnace steel slag (EAFs for short): it was obtained from one steel plant in Shandong Province, China. The EAFs (RS for short)were ground and sieved to with a residue on the 80-μm sieve of below 10%, and had particle size distribution of 31.94 and 130.5 μm for a standard of D50 and D90,respectively, shown in Fig.1. Carbonated EAF slag (CS for short): it was obtained by the raw EAF slag heated at 650 ℃ in a 99.9 vol% CO2stream atmosphere at the atmospheric pressure. Reference cement (RC for short)was the 42.5 grade cement produced by Fushun Cement Co. Ltd. in Liaoning Province. Table 1 shows the chemical compositions of the steel slag and reference cement.

Table 1 Chemical composition of steel slag and reference cement

Fig.1 Particle size distribution of original EAF steel slag (RS) and the carbonated EAF slag (CS)

2.2 Preparation of samples

Three different procedures were totally designed(Table 2), for investigating the effects of CO2on the performances of produced EAF slag binders.Hot-stage CO2pretreatment was applied during pretreatment of RS by importing simulated flue gas into the tube reactor (99.9 vol% CO2gas stream, the atmospheric pressure), producing the CS. As indicated in Table 2, “100RS”, “80RS”, “100CS”, “80CS”represented the paste binders composited with 100%RS, 80% RS, 100% CS, 80% CS respectively. “-H”represented the paste binders under standard moisture curing; “-C” represented the paste binders under accelerated carbonation curing. For example, “80RSC” represented the paste binder with 80% RS and 20%RC under accelerated carbonation curing; “80CS-C”represented the paste binder with 80% CS and 20%RC under accelerated carbonation curing. These meant“100RS-H” binder without hot-stage CO2pretreatment or carbonation curing. The blend was obtained by premixing the raw materials in a tumbling mixer. The blend solid powders and water were used to prepare a paste specimen at a water-to-solid ratio of 0.4. The paste binders were then cast into 30 mm cubic moulds,prepared for the mineralogical and microstructural characterization. The pastes were demolded after curing in a standard curing chamber (T= 20 ± 2 ℃,RH≥ 90%) for 24 h; and then paste specimens of “100RSH”, “80RS-H”, “100CS-H”, “80CS-H” transferred back into the standard curing chamber until 28 d curing age, and paste specimens of “100RS-C”, “80RS-C”,“100CS-C”, “80CS-C” were placed immediately into a sealed carbonation chamber (CO2concentration =99.9vol%, constant pressure = 0.1 MPa, temperature=25 ± 2 ℃) for 24 h or 72 h.

Table 2 Carbonation procedures of steel slag-based slurry binders

2.3 Testing and characterization

Compressive strengths of steel slag-based binders with 24 h or 72 h carbonation curing ages were examined. Meanwhile, the compressive strength of binders 100RS-H and 80RS-H at moist curing of 28 d was tested. The carbonation depths of the binders were monitored by the phenolphthalein indicator.

The chemical composition of steel slag and reference cement were studied by X-ray fluorescence(XRF, ARLAdvant’X Intellipower 3600). The carbonation efficiency, formation and decomposition of calcium carbonate of steel slag binders were analyzed by TG-DSC analysis(NETZSCH STA 449 F3) under 100 vol% N2atmosphere at 10 ℃/min from 35 ℃to 1 200 ℃. The minerals of obtained samples were identified by XRD (Bruker D8 Advance) with Cu Kα (λ=0.154 06 nm) ray at the 2θfrom 10° to 70°.The Fourier transform infrared spectra (FTIR) were recorded on infrared spectrometer(Nicolet IS5). The morphology and microstructure of the binders were assessed by SEM coupled with EDS (Hitachi S-4800).The specific surface area and pore size distribution were analyzed by BET analyzer (Micromeritics ASAP2460).The particle size distribution was measured with the Mastersizer 2000 laser granularity meter. The surface atomic concentration and chemical states of the obtained samples were determined by Thermo Fisher K-Alpha X-ray photoelectron spectrometer.

Carbonation efficiency was widely used to estimate the carbonation performance of various solid waste[15]. The theoretical CO2uptake potential in solid waste could be calculated by Eq.(1):

wheremco2,maxis the theoretical CO2uptake potential, in g-CO2/g;mCaO,mSO3, andmMgOare the weight fraction of CaO, SO3and MgO in binders, in g-CaO/g, g-SO3/g and g-MgO/g.mco2.0is the weight fraction of CO2in the slag. The carbonation efficiency is defined by Eq.(2):

3 Results

3.1 Compressive strength

Compressive strengths of the binders are shown in Table 3. The binder 100RS and 80RS apparently gain very low compressive strengths under 28 d of standard moist curing, being 0 and 17.1 MPa respectively. For binder 80RS, the compressive strengths are increased dramatically to 36.0 and 39.6 MPa, under CO2postcuring for 24 and 72 h. Accordingly, the compressive strengths of steel slag binders can be improved by carbonation post-curing.

Table 3 The compressive strengths of steel slag-based slurry binder/MPa

Binders 100CS and 80CS have higher compressive strengths, under either 28 d of standard curing or carbonation curing, compared with that of binders 100RS and 80RS. This can be attributed to pre-carbonation of steel slag. When subjected to carbonation curing for 24 or 72 h, compressive strengths of binders 100CS and 80CS are increased significantly to 13.5 MPa/18.5 MPa and 43.7 MPa/47.5 MPa. These imply that the hot-stage CO2pretreatment of steel slag and CO2post curing could promote the compressive strength development of binders.

3.2 Mineralogical and microstructural characterizations

The RS is found to be rich in elements such as Fe2O3(34.12 wt%), CaO (30.99w t%), SiO2(16.3 wt%), and MgO (7.76 wt%), which were mainly concentrated in C3S (22.45 wt%), C2S (6.42 wt%), the RO phase (24.61 wt%), and calcium ferrite (20.4 wt%)mineral phases, which is shown in Fig.2.

Fig.2 AMICS photos of (a) original EAF steel slag (RS), and (b) the carbonated EAF slag (CS)

The XRD patterns for the three samples are shown in Fig.3(a). The 28 d hydration sample of binder 80RS-H mainly consists of hydration product of calcium silicates. When paste 80RS was carried on the accelerated carbonation curing treatment, diffraction peaks of calcite[16]can be found in the patterns of binder 80RS-C. When the binder 80RS was replaced by the steel slag underin-situCO2pretreatment and the slurry was put into the accelerated carbonation,the peaks of calcite are more prominent in the binder 80CS-C. These results suggest that Ca-bearing minerals were carbonated. These correspond to results of the phenolphthalein indicator testing and thermodynamic calculation[17]. Because calcite exists in crystalline and amorphous forms, the content of CaCO3cannot be obtained by XRD analysis, which is demonstrated by the TG-DSC study.

Fig.3 X-ray diffraction patterns (a), FT-IR spectra (b) and SEM images (c) of the uncarbonated area sample from 80RS-H-28 d, carbonated area samples from 80RS-C-24 h and 80CS-C-24 h

Fig.3(b) shows the FT-IR spectra of the uncarbonated area from 80RS-H-28 d hydration sample, carbonated area samples from 80RS-C-24 h and 80CS-C-24 h. At first, the asymmetric stretching(υ3) vibration peaks of [CO3]2-of these samples[18]are detected at 1 424.25, 1 427.50 and 1448.86 cm-1.The out-of-plane bending (υ4) peaks of C-O bond in calcite[19]are detected at 875.14, 873.19 and 873.52 cm-1. The in-plane bending peaks (υ2) of C-O bond in aragonite and calcite[20]are detected at 714.01 cm-1,711.6 cm-1. It is concluded that carbonation promotes the formation of crystalline and amorphous calcite. The asymmetric stretching peaks (υ3) of Si-O bond of silica gel[21]are detected at 989.94, 1 042.09 and 1 043.34 cm-1, and the out-of-plane bending (υ4) of Si-O bond[22]are detected at 516.10, 568.7 and 576.29 cm-1, which indicating the silica gel formation during carbonation.The stretching vibration of the O-H bond are detected at 3 640-3 645 cm-1, while the vibration of the H2O bond in hydration products were detected at 3 420-3 450 cm-1[23]. When compared with the uncarbonated sample of 80RS-H, bands at 870-880 cm-1decreased,but bands at 510-580, 700-750, 1000-1 100 and 1 400-1 500 cm-1increase for the carbonated samples,which represents that calcium silicates gradually reacte with CO2to form calcites and silica gels during the carbonation.

Fig.3(c) shows the SEM images for the non-carbonated (80RS-H-28 d, c-1) and carbonated(80RS-C-24 h (c-2) and 80CS-C-24 h(c-3) ) steel slag paste samples. In 80RS-H-28 d sample, hydration products with porous microstructure were formed.After accelerated carbonation curing, carbonation products (certain needle-shaped aragonite and a small amount of calcite) can be found at the surface; while they fill up original pores to make the microstructure become denser, for the 80RS-C sample. For 80CSC-24 h sample, an abundance of cube-shaped calcite crystals and a small amount of spherical-like vaterite are formed and aggregated densely. These results are consistent with the results of some researchers[24,25].

In Fig.4, particle size distribution, surface area,average pore size and total pore volume of the original slag, carbonated slag, 80RS-H-28 d, 80RS-C-24 h and 80CS-C-24 h steel slag-based binders are shown.These samples are displaying the unimodal trend, with major grain sizes of 5-200 µm. Carbonation reduced the number of particles with average size and increased that of larger particles (≥150 µm). There can be an existing relationship between carbonation achieved and particle change. 80RS-C and 80CS-C samples have a slightly larger total pore volume and BET surface area than that of 80RS-H. And average pore size of three sample are 17.44, 7.22 and 6.77 nm, respectively.These may be the pore size reduced in the blocks,which are caused by the newly formed calcites and silica gels covering the pores of matrix and precipitates joining together[26,27]. The above findings fully indicate that both carbonation processes can result in the reacted matrix with a lower pore area and lower porosity,therefore promoting a strength improvement.

Fig.4 Particle size distribution and BET analysis of the uncarbonated area sample from 80RS-H-28 d, carbonated area samples from 80RS-C-24 h and 80CS-C-24 h

3.3 CO2 mineralization potential

From the phenolphthalein indicator testing results shown in Fig.5, the cross section of 80RSH sample remained purple because of the nonneutralized pore solution, and that of 80CS-C sample showed no coloration. That denotes 80RS-H has a poor carbonation and 80CS-C has the carbonation with a great extent. The 80RS-C sample appeares a coreshell structure, the purple core is encapsulated around the colorless layer of 5-7 mm thickness; and the core area shows shrinking accompanying the carbonation deepening. According to Thiery[28], phenolphthalein measurement results cannot represent the maximum depth of CO2ingress.

Fig.5 Several cross sections of (a) the 80RS-H-28 d, (b) 80RS-C-24 h and (c) 80CS-C-24 h samples from different depths

The TG-DSC analysis of the uncarbonated area sample from 80RS-H, carbonated area samples from 80RS-C and 80CS-C in 24 h curing age are shown in Fig.6. In the 80RS-H curve, weight loss steps ranging from 25 to 200 ℃, from 420 to 460 ℃ and from 400 to 750 ℃ can be observed, which corresponds to the removal of bound water and physically absorbed water,the decomposition of Ca(OH)2, and the decomposition of calcite[29]. One remarkable weight loss step ranging from 400 to 950 ℃, is observed in 80RS-C and 80CS-C samples, corresponding to the breakdown of amorphous calcite(from 400 to 700 ℃) and crystallized calcite(from 700 to 950 ℃)[30]. The carbonation efficiency(EOC) can be calculated by Eq.(1) and Eq.(2); and the EOCs were 3.3%, 52 % and 67% for 80RS-H, 80RS-C and 80CS-C samples. It can be found that a maximum carbonation efficiency of binders may be achieved by simultaneous application ofin-situCO2pretreatment and accelerated carbonation curing routes.

Fig.6 TG-DSC analysis of the uncarbonated area sample from 80RS-H-28 d, carbonated area samples from 80RS-C-24 h and 80CS-C-24 h

3.4 XPS analysis

XPS analysis results are shown in Fig.7, to demonstrate chemical state of elements in 80RS-H-28 d, 80RS-C-24 h and 80CS-C-24 h test samples.

Fig.7 Ca 2p (a) and Si 2p (b) XPS spectra for the 80RS-H-28 d sample, carbonated area samples from 80RS-C-24 h and 80CS-C-24 h

The Si 2p peaks for 80RS-H-28 d, 80RS-C-24 h and 80CS-C-24 h test samples are respectively located at 99.89, 100.95 and 104.43 eV, which indicates that Si 2p binding energy had increased after carbonation treatments. According to some researcher[31], the loss of non-bridging oxygens (Si-O-Ca moietiesetc) tends to the increase of Si 2p binding energy and related silicate polymerization. Carbonation reaction increases the calcium consumption, and leads to a decrease in Ca/Si ratio and ultimately forms a Q4 silicate[32]and calcite[33]. Researchers[34-36]have found related Si binding energy to that of silicate tetrahedral polymerization. Researchers[37]also have discovered that there was a strong negative correlation between Si binding energy and Ca/Si ratio in C-S-H phase. The Si 2p binding energy peaks of 80RS-C and 80CS-C shift to higher binding energy consistent with the EOC trend, and slightly broader than that of 80RS-H. Peak broadening indicates the silicate structure with a higher disorder.

Ca 2p3/2 and Ca 2p1/2 peaks can be observed in the Ca 2p XPS spectra. The change trend for Ca binding energy peaks is similar to that of Si, in further,the centre of Ca binding energy peak order can be roughly determined as 80CS-C >80RS-C > 80RSH. That indicates that the carbonation reaction leads to the increase of Ca binding energy. It can be clearly discovered that 80CS-C has a much broader Ca 2p binding energy peaks than that of 80RS-C and 80RSH. This may be as a result of a difference in products diversity: monocarbonate, calcium carbonates or Ca and Si-bearing hydrates are all incorporated into the 80CS-C sample.

3.5 Carbonation mechanism

Based on the above researches, hardening mechanisms of steel slag-based slurry binders by accelerated CO2post-curing or simultaneous application of two carbonation treatments can be illustrated in Fig.8. As the EOC increases, the particleparticle interaction increases, some particles even attach themselves to each other and calcium carbonates fill the pores within the matrix, bringing out a denser microstructure. Insoluble silica gels also limit leaching of metal and the proceeding of carbonation process,forming a carbonated product layer at the outer surface of particles. It is assumed that crystalline calcites,amorphous calcite and silica gels with high degree of polymerization formed during the carbonation processes. These are very similar to that function of C-A-S-H in traditional Portland cement hardening,which are responsible for the strength development in steel slag-based slurry binders.

Fig.8 Schematic diagram of hardening mechanisms for different carbonation processes

4 Conclusions

The synergistic use of hot-stage CO2pretreatment and accelerated carbonation curing obtained the maximum CO2storage of 67% based on the binders of 80% steel slags. Carbonation post-curing significantly enhances the compressive strengths of steel slag pastes, from 17.1 MPa of binder 80RS-H to 36.0 MPa of binder 80RS-C. When using both carbonation treatment methods, binder 80CS-C can obtain the highest compressive strengths to 43.7 MPa, which nearly equals to the 28 d compressive strength of ordinary Portland cement, with a CO2uptake of 67%.Hot-stage carbonation of steel slag is found for particle agglomeration, minerals remodeling and calcite formed,thus causing an activated steel slag with a dense structure and more active components. Accelerated carbonation curing of steel slag slurry paste results in the newly formed amorphous CaCO3, calcite crystalline and silica gels that covered the pores of the matrix,which caused microstructure densification and strength improvement. Taking the comprehensive utilization of hot-stage carbonation and accelerated carbonation curing methods together provided one reference on the high-volume reuse of steel slags for preparing the binder or building material product with much more higher value.

Conflict of interest

All authors declare that there are no competing interests.