X-ray diffraction
The XRD patterns (Figure 1) of the uncarbonated alkali-activated slag specimens are dominated by a mixture of amorphous phases with a broad peak at 29.16°2θ, which can be related to a combination of unreacted slag particles and newly formed C-A-S-H phase47. The main crystalline phase is quartz, which is associated with the aggregate, as well as smaller amounts of K-feldspar. An AFm-phase with a peak at 11.53°2θ, best modeled by hydrotalcite, is also present in small amounts (1%). Note that with the presence of aggregate in the samples, more attention is needed to interpret the XRD data because aggregate particles are typically not homogeneously distributed in the sample at small scale (few grams for XRD measurement). To eliminate the effect of aggregate, we removed the amount of aggregate and then normalized the rest of phases to 100%. In such way, the phase amounts reported in Table 3 should be understood as the paste level (excluding aggregates). The proportion of amorphous phase at paste level, calculated by the internal standard method, is quite similar (97-98%) for all uncarbonated samples with various w/b ratios as shown in Table 3.
The carbonation of the alkali-activated slag specimens results in the formation of carbonate minerals and a reduction of the amorphous content (due to the reaction with CO2). As is often the case for accelerated carbonation experiments6,24,48, the metastable vaterite formed instead of its stable polymorph calcite. The amount of vaterite formed depends on both the water to binder ratio of the mixture and the duration of carbonation as shown in Figure 2 and Table 3. A longer carbonation duration resulted also in a higher vaterite content as observed for AAS 045. The vaterite content after 14 days of carbonation is not much higher than after 7 days. However, this does not mean the carbonation degree of these two carbonation periods are similar because amorphous calcium carbonate is formed from 7 to 14 days of carbonation as proved later by FTIR results. In AAS 035, the carbonation is markedly slower: after 28 days of carbonation only small amounts of vaterite (1.2%) could be observed in the XRD patterns. Overall, the amount of vaterite formed after 28 days of carbonation is the highest in AAS 055 (13.4%), indicating that a higher w/b ratio induces a higher carbonation rate due to a faster diffusion of CO249.
Table 3. Quantitative phase analysis for reference and carbonated AAS specimens at three w/b ratios
Phases
|
AAS 035
|
AAS 045
|
AAS 055
|
|
Ref
|
28D
|
Ref
|
7D
|
14D
|
28D
|
Ref
|
28D
|
Amorphous
|
98.8
|
97.6
|
97.5
|
92.9
|
92.4
|
85.2
|
97.6
|
82.9
|
Hydrotalcite
|
1.2
|
1.2
|
2.5
|
1.2
|
1.3
|
1.2
|
1.2
|
1.2
|
Microcline
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
2.5
|
1.2
|
2.4
|
Vaterite
|
0.0
|
1.2
|
0.0
|
6.0
|
6.3
|
11.1
|
0.0
|
13.4
|
Total
|
100
|
100
|
100
|
100
|
100
|
100
|
100
|
100
|
*Ref: uncarbonated samples
* 7D, 14D, 28D: 7-day, 14-day and 28-day carbonated samples, respectively
Fourier-transform infrared spectroscopy
In uncarbonated samples (Figure 3 (top)), a band at 1643 cm-1 is detected, related to the bending vibration modes of H-OH bonds as chemically bound water in activated slag50. The band ranging from 1500 to 1400 cm-1 relates to the asymmetric stretching mode of C-O bonds of carbonate ions, which could be from the CaCO3 in unreacted slag or the occurrence of natural carbonation during sample preparation as the powdered state is vulnerable to be carbonated. However, the latter may be ignored because samples were controlled to avoid exposing to atmospheric CO2. The asymmetric stretching vibration of Si-O-T bonds (where T is Si or Al) in C-A-S-H is observed at 984 cm-1. This band position is representative for the environment of SiO4 (silicate sites) in the C-A-S-H gels38. The increase in w/b ratio does not significantly influence the AAS structure as similar spectra exhibit for all samples with various w/b ratios. Only a slight difference can be observed in AAS 055, which shows a broader band at 1500-1400 cm-1 and a slightly lower intensity of Si-O-T, probably suggesting a lower degree of geopolymerization in AAS 055.
After accelerated carbonation, in the region from 1500 to 1400 cm-1, the right shoulder at approximately 1400 cm-1 diminishes, and the left shoulder gravitated toward 1500 cm-1 becomes larger. Comparing to the FTIR characteristic frequencies of Ca-carbonates from the report of Andersen51, a broader band is found in this region (1500 to 1400 cm-1), which suggests the presence of amorphous CaCO3. Furthermore, higher wavenumbers in carbonated AAS samples indicate the appearance of vaterite or aragonite. The sharp peak at 875 cm-1 can be assigned to calcite or vaterite as the frequency ranges of both CaCO3 polymorphs overlap with each other. Combining these signals, it is assumed that vaterite is the predominant Ca-carbonate product after 28-day carbonation, which is in line with XRD detection. Furthermore, signals of Na-carbonates are not identified in FTIR spectra, also in agreement with XRD results. In the range between 1200 and 900 cm-1, a remarkable shift of the main Si-O-T vibration band from 954 to around 1027 cm-1 is observed, indicating a higher degree of geopolymerization of silicates after carbonation. Li et al.52 and Zhang et al.21 also reported the peaks at high wavenumbers similar to those detected in the carbonated samples of this study.
Amongst the three carbonated samples, a specific difference can be found at the main Si-O-T bands, which shift toward a higher wavenumber for samples with higher w/b ratios from around 954 cm-1 to 1027, 1035 and 1041 cm-1 corresponding to the w/b ratios of 0.35, 0.45, and 0.55, respectively. This suggests that carbonated samples with higher w/b ratio are more cross-linked, and probably polymerized further under carbonation. A higher decalcification of C-A-S-H gels is also expected in these cases. In addition, the degree of polymerization also depends on the carbonation duration, as shown in the Figure 3 (bottom) by the shifting in the spectra of carbonated AAS 045 after 7, 14, and 28 days of carbonation. The centered peaks of the main Si-O-T bands shift to higher wavenumbers with carbonation duration, which indicates an increase in crosslinks in the gels during carbonation. In consistence with XRD results, the intensity of vaterite peaks at 875 cm-1 also increases, especially in the first 14 days of carbonation, indicating that the carbonation rate is accelerated in this period.
In order to deeply examine the changes in carbonate products and chemical arrangement, several studies21,53 deconvoluted FTIR spectra focusing on the main band of 1300-800 cm-1. They suggested that the appearance of Q1, Q2, Q3, and Q4 sites corresponds to the signals at 865, 976-1000, 1088, and 1136 cm-1, respectively. In this study, it is assumed that AAS samples contain mostly Q1 and Q2, while Q3 and Q4 are preferred in carbonated AAS samples. However, the complex environment around Si centers in AAS structures considerably affects the signals of Si-O vibrations. In that sense, sophisticated NMR technique is more insightful for a better quantification of the C-A-S-H structure compared to the use of FTIR method alone.
Solid-state MAS NMR results
27Al MAS NMR
Figure 4 shows the 27Al MAS NMR spectra of AAS samples before and after carbonation. The spectrum of the raw slag shows a broad resonance ranging from 10 to 80 ppm, demonstrating the high disorder of the slag precursor as also indicated by the typical amorphous hump in the XRD pattern (Figure 1). The centered resonance at around 67 ppm is assigned to the tetrahedral Al environments, which indicates that Al exists in the raw material mainly under four-fold coordination, Al (IV). Upon alkaline activation, the reaction products are recognized by two predominant regions centered at ⁓73 ppm, ⁓10 ppm, and a very minor contribution around 37 ppm, which correspond to the tetrahedral aluminum Al (IV), octahedral aluminum Al (VI), and pentahedral aluminum Al (V) environments, respectively44. Comparing to the spectrum of anhydrous slag, a higher intensity and narrower shape of the Al (IV) resonance is found, suggesting that Al (IV) becomes incorporated in C-A-S-H phase under bridging tetrahedral Al45,54. This Al (IV) region shows two resonances: one at 73 ppm and one at around 68 ppm appearing as a shoulder, which are supposed to be Q2(1Al) and Q3(1Al) sites28,45, respectively. These sites are charge-balanced with H+, Na+ and with more positive charges such as Ca2+ ions. The octahedral Al location with a peak centered at 10 ppm and a small shoulder at 4 ppm is assigned to the layered double hydroxide phases54 (LDHs) with mainly hydrotalcite (Mg-Al), which agrees with the detected signal of hydrotalcite in the XRD results. Especially, these peaks are present in both uncarbonated and carbonated samples, meaning that hydrotalcite is not fully carbonated45,55, which is confirmed more conclusively in the 29Si MAS NMR. In addition, the signal of Al (V), shown by a minor intense peak around 37 ppm is observed, which represents the remnant unreacted slag in the materials55.
Upon 28-day exposure to CO2, the specimens witness a shifting of Al (IV) region from around 73 ppm to 58 ppm, which suggests a higher silicate content than oxhydryl surrounding Al center, leading to a lower electron density and thereby a lower aluminum chemical shift of this region45. Combining with the symmetry of this peak after carbonation, it is indicated that the more cross-linked in the C-A-S-H phases is, the more homogeneous the Al sites in this structure are. A significant reduction of Al (VI) is also observed, while the resonance of Al (V) has mostly disappeared. That means the unreacted slag is transformed into aluminosilicate phases, leading to the dominance of only Al (IV) in C-AAS samples compared to both Al (IV) in C-A-S-H and Al (VI) in alumina-rich phase of AAS ones. This is also consistent with the 29Si MAS NMR results (discussed later) that clearly show a decrease in content of unreacted slag after carbonation. Besides, a very small shoulder at 14 ppm is also detected on C-AAS 035 and C-AAS 045. This peak is supposed to be Ca-Al phase belonging to LDHs types such as stratlingite30. The appearance of these phases may attribute to the reduction in the intensity of hydrotalcite at 10 ppm. However, AFm layers or even hydrotalcite types of LDHs are partially ordered forms in the C-A-S-H interlayers28, which make it difficult to be detected and distinguished clearly by XRD.
The water to binder ratio does not remarkably influence the C-A-S-H structure of reference and carbonated AAS as similar characteristics amongst the samples are observed. However, if a relative comparison between the intensity of Al (IV) and Al (VI) is carried out, Al (VI) seems to be more pronounced with an increase of w/b ratio of uncarbonated AAS, while this is not observed in carbonated AAS. This suggests that the lower w/b ratio can allow the formation of more C-A-S-H phase. However, 27Al MAS NMR should be considered as a qualitative assessment rather than quantitative evaluation56, and a quantitative deconvolution of 29Si MAS NMR will quantify the effects (see further).
23Na MAS NMR
The 23Na MAS NMR spectra of uncarbonated and carbonated AAS samples are shown in Figure 5 by a single broad resonance. The resonance centered at -3.9 ppm indicates Na in a coordination 6-7, which is associated with Na+ in a charge-balancing role in the network of the C-(N)-A-S-H57. Upon 28 days of carbonation, this resonance shifts toward a lower chemical shift at about -6.5 ppm, showing an increase in the Na coordination number58. This is in line with the results of 27Al MAS NMR and 29Si MAS NMR, which show that AAS is more cross-linked after carbonation. The trend of shifting in 23Na resonance is similar to that of Al (IV) in 27Al NMR spectra, which again suggests that the primary role of Na+ is to reduce the negative charge of Al in the Si-O-Al chain59. This is also consistent with XRD results, which show no Na-carbonate precipitation after carbonation. In addition, the similarity amongst the spectrum of samples with various w/b ratios before and after carbonation again illustrates that the role of w/b ratio is less important to the C-A-S-H structure. The only different characteristic that can be observed is that the shoulder around 5 ppm along with the main peak of low w/b ratio sample (AAS 035) appeared with a less coordinated Na (smaller or equal to 5) beside a higher coordinated Na as the main Na state58. This indicates a variation of solvated Na+ in the hydration state in pore water of the specimens44. Therefore, the broader peak toward a higher chemical shift is evident to the low H3O+ in pore solution of AAS 035.
29Si MAS NMR
Figure 6 shows the 29Si MAS NMR spectra of uncarbonated and 28-day carbonated AAS with different w/b ratios. In general, the C-A-S-H structures are not much different either for uncarbonated or carbonated AAS specimens with different w/b ratios, whereas there is a significant evolution of their structure (with the same w/b ratio) due to carbonation. The phases become toward more cross-linked witnessed by a shifting from Q1 and Q2 sites to Q4 species. Especially, the resonance of GBFS is observed in all samples when comparing with the spectrum of slag precursor, suggesting the existence of remnant slag in both uncarbonated and carbonated AASs.
To better understand the structure of samples via their 29Si MAS NMR spectra, each spectrum is deconvoluted, allowing a detailed comparison between component peaks, simulated spectrum and the experimental data as shown in Figure 7. The broad region from approximately -60 to -95 ppm with a maximum at around -75 ppm corresponds to the unreacted slag. In uncarbonated mortars, the peak residing at -74.7 ppm assigns to Q0 species which encompass Si in isolated silica tetrahedral in the products45. The peaks located around -78 ppm is marked as Q1, which represents to the chemical environment of Si at the end of a chain of silicate tetrahedral of C-(A)-S-H60. Herein, Q1 sites are represented by two peaks as Q1a and Q1b at -76.8 and -78.5ppm, respectively, indicating the bonding effect of Q1 units with Ca2+, Na+ or H+ in the environment. This changes the chemical shift of Q1 sites28. Three types of Q2 groups are detected in these alkali activated products including Q2(1Al) sites assigned at around -81 ppm, Q2b and Q2p sites at approximately -84 ppm and -85 ppm, respectively. These Q2 groups are indicated to the middle chain silicates of C-(A)-S-H phases45,54. Interestingly, the signals representative to the high cross-linked sites in gels as Q3 are not well observed although a very small peak of Q3(1Al) at around -88 ppm is still detected. This is not in line with previous studies28,45,54. One explanation may be the lower activator modulus (Ms) used in this current study (Ms = 0.45) as Gao et al.45 reported that an increase of Ms to 1.7 or higher can accelerate the alkali activation resulting in a higher number of Q3 groups. Comparing amongst the three AAS with different w/b ratios, it is found that the difference only comes from the quantity of silicate groups in C-(A)-S-H, implying that the w/b ratio does not define the types of Si groups formed in these phases.
After 28 days of accelerated carbonation, remarkable changes in the structure are noticed as shown in Figure 7. After carbonation, the chain structure of C-(A)-S-H containing predominantly Q1 and Q2 silicate species shifts into a more cross-linked gel, evidenced by the presence of four-connected silicate units (Q4 ) in the deconvoluted spectra of carbonated AAS samples. Similar to AAS mortars, a difference in the spectra of three carbonated AAS corresponding to the three w/b ratios is not clearly observed, implying a less important role of w/b ratio in the formation of C-(A)-S-H structure. In all carbonated mortars, the peaks at approximately -86, -94, -102 ppm are identifiable, assigning for the presence of Q4(4Al), Q4(3Al) and Q4(2Al) units in the gels61, respectively. This observation is consistent with the lower ppm range and high intensity of Al (IV) sites as indicated by 27Al MAS NMR, showing the formation of Al-rich gel.
The relative intensities of Qn species (%) from deconvolution results are reported in Table 4. Interestingly, the unreacted GBFS in carbonated AAS is significantly reduced compared to the reference AAS, from around 55% to 36% in general, meaning that more than one third of unreacted GBFS has been transformed in C-A-S-H phase and/or carbonated products during carbonation. In particular, the Q1 and Q2 units in reference AASs are unidentified in carbonated ones and replaced by plenty of Q4 groups in which Q4(3Al) is dominant (over 40%), indicating the presence of an extremely high cross-linked carbonated aluminosilicate C-A-S-H. This alteration is supposed to be the result of the decalcification of C-A-S-H phase28. Especially, the absence of Q1 and Q2 groups as representative for reacted products of AAS suggests that both existing and new formed C-A-S-H phases in the materials are fully carbonated during 28 days of carbonation. Regarding to the influence of w/b ratios, there is only a slight difference in the relative intensity amongst Al-substituted silicate sites in AAS, and this is also similar in carbonated AAS. The largest difference is observed in samples with the w/b ratio of 0.35, evidenced by a higher percentage of unreacted slag after carbonation. This could be explained by a denser matrix formed before carbonation, which can diminish the diffusion of CO2 into the matrix for carbonation.
Table 4. Deconvolution of 29Si MAS NMR results of uncarbonated and carbonated alkali activated slags
Samples
|
Site type
|
Unreacted GBFS (%)
|
Reaction products (%)
|
Q0
|
Q1a
|
Q1b
|
Q2
(1Al)
|
Q2b
|
Q2p
|
Q3
(1Al)
|
Q4
(4Al)
|
Q4
(3Al)
|
Q4
(2Al)
|
-75
ppm
|
-75
ppm
|
-77
ppm
|
-78
ppm
|
-81
ppm
|
-84
ppm
|
-85
ppm
|
-88
ppm
|
-86
ppm
|
-94
ppm
|
-102
ppm
|
AAS 035
|
54
|
6
|
3
|
10
|
18
|
5
|
2
|
2
|
-
|
-
|
-
|
AAS 045
|
56
|
5
|
3
|
9
|
16
|
5
|
4
|
2
|
-
|
-
|
-
|
AAS 055
|
55
|
6
|
4
|
10
|
13
|
6
|
5
|
1
|
-
|
-
|
-
|
C-AAS 035
|
39
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
9
|
41
|
12
|
C-AAS 045
|
36
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
7
|
44
|
13
|
C-AAS 055
|
36
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
7
|
44
|
13
|
Mechanism of carbonation and its influence on AAS gel structure
The changes in gel structure of AAMs with various w/b ratios exposed to accelerated carbonation (1% CO2, 60% RH) have been thoroughly examined in this study, and it is revealed that calcium carbonate is the dominant crystalline carbonation product, in which vaterite is the main polymorph as observed by XRD. Most of the previous studies16,28,52,62,63 reported the formation of stable calcite as the major stable polymorph. However, those samples were carbonated under different conditions such as high relative humidity (more than 80%), with either too low CO2 concentration of 0.04% (natural carbonation) or high CO2 concentration (5%) but longtime exposure up to 3 years. In addition, early age curing was chosen (3 or 14 days) and especially some studies were performed on AAM powder. Those testing conditions could result in the transformation of CaCO3 polymorphs from the metastable to stable state. In this study, carbonation is performed under intermediate conditions of 1% CO2 and 60% RH on well-cured AAS specimens for a shorter period of 28 days, which could be the reason for the main appearance of vaterite as crystalline carbonate. Also, no sodium crystalline phases is detected. This is also in line with reported data28 as the formation of sodium carbonate is only expected under some specific carbonation conditions. Typically, the Na-rich carbonation product nahcolite can only be observed under a high CO2 concentration of 5% or higher28, which is much higher than the one applied in this study. The evidence of crystalline carbonation products is also indicated by FTIR spectra under characterized wavenumber signals of CO32-, and particularly from 23Na MAS NMR, which provides a strong proof of the non-presence of Na-carbonate precipitates because Na is only in charge-balancing role for C-(N)-A-S-H phase. Additionally, hydrotalcite is detected as a secondary product of AAS, which still remains after carbonation, proven by XRD and 27Al MAS NMR results.
Carbonation also strongly influences the amorphous aluminosilicate structure of AAS. Results of XRD quantification show the decrease of amorphous content during carbonation, resulting from the decalcification of C-(N)-A-S-H to form calcium carbonate precipitates. In particular, the decalcification allows the aluminosilicate gel to become significantly cross-linked. This is evidenced by FTIR and 27Al MAS NMR results. Furthermore, the 29Si MAS NMR spectra with deconvolutions also show mainly Al-substituted Q4 species in aluminosilicate gel instead of Q1 and Q2 in uncarbonated gels, which are consistent with data from previous studies20,46. However, it is worth noting that the content of unreacted slag in AAS specimens (around 55%) is significantly higher than reported data in literature64,65 with approximately 30%. The high amount of unreacted slag can be activated and then carbonated during the carbonation process. As a result, a lower unreacted slag content is observed in carbonated specimens compared to uncarbonated samples.
This study also clarifies the influence of w/b ratio on the evolution of phases and the amorphous gel nanostructure in particular. Overall, the w/b ratio does not influence the type of crystalline reaction products but slightly defines the structure of C-A-S-H, which becomes highly structurally ordered and better resistant to carbonation at a low w/b ratio (e.g. 0.35). Zhang et al.38 and Ismail et al.66 also found a small effect of the w/b ratio on the gel nanostructure of alkali activated materials based on blends of slag and fly ash with the w/b ratio in the range of 0.3-0.6. By using a combination of FTIR, 23Na, 27Al, 29Si MAS NMR, and especially deconvolution of 29Si MAS NMR to assess the gel structures of AASs, this study was able to provide a comprehensive picture on the effect of w/b ratio on the gel structure of AAS, which is typically characterized only by FTIR technique as normally seen in literature38.
Based on the experimental evidences obtained from multiple characterization techniques, it allows us to propose a comprehensive mechanism for a diffusion-driven carbonation process of AAS as follows:
- Diffusion and dissolution of CO2 gas in pore solution of AAS: This step liberates bicarbonate and carbonate ions. However, the former is unstable under the high alkalinity (pH>12.5) in the AAS solution and transforms/converts into carbonate ions .
- Formation of Na-carbonate products: and can react with the abundant Na+ in pore solution to form sodium carbonates including Na2CO3 (natron) and NaHCO3 (nahcolite) precipitates as intermediate products.
In order to verify whether natron and nahcolite are stable under our experimental conditions, we have performed a geochemical modelling to examine the saturation indices of nahcolite, natron and calcite in a 3-component system (Ca, Na, C) in 1 liter of water (20°C). The amounts of Ca and Na are expressed as totals in the system, and C as the partial pressure of CO2 gas. Calculations are done with PHREEQC67 with the CEMDATA18.1 database68 and additional constants for nahcolite and natron obtained from BRGM Thermodem database69. As shown in Figure 8 (top), at 1% CO2, natron tends to be dissolved (negative saturation index) even at a very high Na concentration of 7 mol/l in pore solution. Nahcolite is only stable if the Na concentration is higher than 3 mol/l, which is much larger than the typical Na concentration in pore solution of hardened AAS (1-2 mol/l)70,71. The initial Na concentrations calculated from the mix compositions are 3.5, 2.7, and 2.2 mol/l for AAS with w/b ratios of 0.35, 0.45, and 0.55, respectively. The Na concentration is significantly decreased during polymerization (due to the precipitation of Na containing phases)70 to reach the average Na concentration of 1-2 mol/l.
Formation of Ca-carbonate products: The dissolution of natron and nahcolite in the presence of Ca2+ (from decalcification of C-A-S-H) could induce further carbonation reactions to form a stable precipitate CaCO3 as illustrated in Eqs. (7) and (8). As shown in Figure 8 (top), calcite is oversaturated under our testing conditions. Even with very low Ca concentration of 10-5 mol/l (much lower than the average one of 5´10-4 mol/l71), calcite is still stable regardless of Na concentration (Figure 8 (bottom)). The reactions (7) and (8) also increase the alkalinity in the pore solution with the formation of NaOH. With the high alkalinity and an abundance of unreacted slag in the matrix, the geopolymerization can continue to form additional C-A-S-H phase, which may be then carbonated. In this study, because of the high amount of unreacted slag in AAS, new products (C-A-S-H) could be formed continuously during carbonation, of which the dissolution provides Ca2+ in solution to react with carbonate ions and form calcite, while Na-carbonate products are absent as folows.