Spatially resolved nanostructural analysis of disordered phases in carbonated alkali-activated slag

Domain: physics
Published: 2025-12-11
Editorial outcome: minor_revision

Abstract
Alkali-activated slag (AAS) is a promising low-CO2 alternative cement consisting of several disordered phases of similar composition. Although their local atomic arrangements are known to influence macroscopic behavior, determination of structural changes in response to external stimuli remains a challenge. Here, X-ray diffraction-computed tomography (XRD-CT), pair distribution function-CT (PDF-CT), and nanoprobe X-ray fluorescence (nano-XRF) have been used to uncover how an increase of magnesium in AAS affects the atomic structure and spatial arrangement of phases after aggressive carbonation (100% dry CO2), conditions experienced in applications such as oil and gas wells and geological storage of CO2. From PDF-CT it is found that a higher magnesium content decreases the average nanoscale crystallite size of disordered calcium carbonate. At the same time, higher magnesium content is correlated with a less decalcified C-(N)-A-S-H gel, as determined via analysis of Ca-Si atom-atom correlations from PDF-CT and Ca/Si ratios from nano-XRF. Finally, nano-XRF reveals that the disordered (i.e., amorphous) calcium carbonate is stabilized by the presence of silicates.

Introduction
Cement production accounts for 5–8% of global anthropogenic CO2 emissions1,2. As such, alkali-activated materials (AAMs), including those manufactured using blast furnace slag, coal-derived fly ash, and calcined clays such as metakaolin, are being developed for the partial3–5 or full6–8 substitution of commonly-used ordinary Portland cement (OPC). AAMs can exhibit comparable mechanical properties to OPC9 and have seen real-world deployment10, whilst reducing CO2 emissions by as much as 70% or more11–13. Alkali-activated slag (AAS) is produced by mixing slag, a glassy calcium-magnesium aluminosilicate phase, with an alkaline solution such as sodium hydroxide or sodium silicate, forming a mechanically hard amorphous binding phase known as sodium-containing calcium-aluminosilicate-hydrate (C-(N)-A-S-H) gel. The composition of the gel, secondary phases, and pore solution in AAS are affected by both initial slag composition14–16 and choice of alkaline activator17–19.

A major source of OPC and AAM concrete degradation is CO2 exposure, via acidification of the pore solution20. In OPC concrete, the presence of portlandite (Ca(OH)2) buffers the pore solution at >12.5, which initially prevents depassivation and corrosion of reinforcing steel, if present21. Concurrent with the loss of portlandite, the strength-giving calcium-silicate-hydrate (C-S-H) gel decalcifies and CaCO3 phases (such as calcite, vaterite, and aragonite) precipitate22,23. However, after portlandite is consumed, additional CO2 causes a reduction in pore solution pH, which may instigate steel corrosion for reinforced concrete, leading to a loss of tensile strength and formation of expansive iron oxide phases24,25. Carbonation resistance is of particular concern for industrial applications where concrete is subjected to elevated concentrations of CO2 in geological conditions, for example, oil, gas, and geothermal wells and the geological storage of CO2 26–29. Nevertheless, there are certain applications where extensive carbonation is beneficial for a cement-based material, such as carbonate cement (i.e., carbonation of pseudo-wollastonite and rankinite)30 and carbonation of waste cement as a means of reducing CO2 emissions across the lifecycle of OPC concrete31.

Carbonation resistance of OPC is predominantly controlled by two attributes: low permeability and, if present, portlandite (Ca(OH)2) buffering the pore solution. The chemical composition of C-S-H gel has also been found to influence carbonation behavior, where higher Ca/Si ratio synthetic C-S-H gels have slower rates of gel decomposition32. Similarly, for AAS, it is known that permeability and gel composition affect carbonation behavior, where C-(N)-A-S-H gel with a high Ca/Si ratio has been linked with a lower extent of CO2-induced gel decalcification33,34. Moreover, high alkali dosage and increased silicate modulus of AAS have been observed to improve carbonation resistance, where a high alkali dosage leads to faster gel formation and increases pore solution alkalinity35,36. Multiple studies have shown improved carbonation resistance of AAS with higher Mg content of slag37–41. For example, Bernal et al. and Khan et al. linked improved carbonation resistance to the presence of a hydrotalcite-like layered double hydroxide (LDH) phase (with composition Mg1-xAlx(OH)2(An-)x/n·mH2O, where x = 0.17–0.33 and An− is an anion42) which sequesters carbonate anions within its layered structure37,38,43. Similar behavior in OPC-slag blended systems has been reported, where the hydrotalcite-like LDH phase acts as a sorbent for carbonate anions44. Our previous research39–41 found that a high Mg AAS facilitates the formation of amorphous calcium carbonate (ACC) during accelerated carbonation, where we hypothesized that the capability of Mg to stabilize ACC45,46 leads to improved carbonation resistance in AAS. It has been shown that Mg stabilization of ACC is due to the higher solvation energy of Mg2+ compared with Ca2+47, making it harder for dehydration to occur for transformation into anhydrous crystalline CaCO3 45. With respect to AAS, since ACC is less thermodynamically stable than crystalline CaCO3 48, the higher solubility product of ACC increases Ca2+ concentration in the pore solution, which lowers the driving force for C-(N)-A-S-H gel decomposition39. Because of this difference in ACC stability according to the availability of Mg, we have observed that high Mg AAS undergoes less pore structural change upon accelerated carbonation41.

Relatively few investigations on the carbonation behavior of OPC and AAS report the presence of ACC, which is likely attributed to the multiphase nature of these materials, including one or more amorphous phases, making the identification of individual amorphous phases difficult. However, as indicated by our previous research and alluded to in other literature49, this amorphous phase may play an active role in the carbonation mechanism, particularly in magnesium-containing systems (i.e., high Mg slag). One technique that can probe amorphous structures is pair distribution function (PDF) analysis. When carried out using a small beam size at a synchrotron source (<10×10μm2), the technique can be combined with computed tomography50, similar to the more established X-ray diffraction computed tomography (XRD-CT) technique51–53. This approach, known as PDF-CT, can spatially resolve the local atomic structures contained within heterogeneous multiphase materials and has recently been used to study catalyst and battery materials54–57. Due to the need for high-Q scattering data, this technique requires high-energy X-rays, while a small beam size (microns) is needed to achieve adequate spatial resolution.

Here, we employ XRD-CT, PDF-CT and nanoprobe X-ray fluorescence (nano-XRF) to investigate the different phases present in the heterogeneous microstructure of early-age sodium silicate-activated slag and the impact of aggressive CO2 exposure on these phases. Specifically, we study the local atomic structure of uncarbonated AAS with high Mg content slag, then investigate the effects of low and high Mg content slag on the local atomic structure of the AAS after exposure to 100% dry CO2, together with the nanoscale chemical composition. Powders have been used to limit the effects of transport-dependent processes during CO2 exposure and, therefore, isolate how slag chemistry affects CO2-induced chemical changes. PDF-CT is used to resolve structural differences between amorphous C-(N)-A-S-H gel and amorphous unreacted slag in the AAS pastes, and to determine the influence of Mg content on the extent of C-(N)-A-S-H gel and slag decalcification after CO2 exposure. Furthermore, this technique is employed to quantify the nanoscale crystallite size of regions identified as calcium carbonate, revealing the impact of Mg on the formation of ACC. These PDF-CT findings are complemented by nano-XRF, which allows for 2D spatial mapping of phases, with finer resolution (~40 nm) than that of PDF-CT.

Results
While both low and high-Mg-content AAS after carbonation were studied using PDF-CT and XRD-CT analysis, only the high-Mg-content slag was studied using these techniques prior to carbonation. Isothermal conduction calorimetry (ICC) data given in Supplementary Fig. 1 show similar cumulative heat after 24 h of curing, indicating a similar extent of reaction (i.e., extent of slag dissolution and amount of C-(N)-A-S-H gel precipitation). The water content of the low and high Mg AAS pastes is visible in the thermogravimetric analysis (TGA) data reported in Supplementary Fig. 2, where the low Mg AAS paste possesses a slightly higher amount of surface and weakly bound water visible via more weight loss between ambient temperature and 105 °C. Additional insights on AAS paste properties prior to CO2 exposure are given in the Supplementary Information, based on existing literature58–61 and our recent work39–41.

Alkaliactivatedslag
A tomographic slice of high Mg AAS paste is displayed in Fig. 1a, which shows the cross-section region subsequently chosen for XRD-CT and PDF-CT analysis. This region contains mostly large solid regions of AAS rather than loose powder particles, and is seen to consist of angular, partially-reacted slag particles surrounded by the gel matrix. Such solid regions are particularly evident in Fig. 1b (XRD-CT and PDF-CT have voxel dimensions of 2×2×10 µm3, with 10 µm aligned along the length of the capillary), where the diffraction intensity at Q = 2.09 Å−1 is mapped, which corresponds to the main peak observed for C-(N)-A-S-H gel33. Since the intensity of this peak varies across the cross-section, the average X-ray scattering pattern has been computed and is shown in Fig. 1c, along with diffraction patterns for two individual voxels of different peak intensities from Fig. 1b. In each case, the diffraction pattern shows diffuse scattering with amorphous C-(N)-A-S-H gel as the primary component. A small peak is observable at Q = 0.8 Å−1 from the individual voxels, which is attributed to an LDH secondary phase15. Due to the amorphous nature of the slag and C-(N)-A-S-H gel, phase identification has been conducted in real space. Figure 1d shows the average PDF across the whole sample in addition to PDFs from the two individual voxels noted previously. The individual voxels show similar peak positions and intensities to the average, although the ‘+’ voxel appears to have some longer-range order. The average scan corresponds well with the local structure of AAS of the same age and composition determined from bulk PDF measurement39. The peaks at 1.63 Å and 2.37 Å correspond to tetrahedrally bonded Si and AlIV atoms (labeled T) with oxygen, and Ca–O bonds, respectively62. The peak at 3.15 Å represents the correlation (i.e., distance) between two T atoms (T–T correlation) in addition to a correlation between calcium and tetrahedral atoms (Ca–T), while the peak at 3.65 Å predominantly contains contributions from a Ca atom and a tetrahedrally bonded atom (Ca–T) along with those from two calcium atoms (Ca–Ca)63,64. Small contributions to AAS PDFs that are typically observed in the literature, for example, a small peak at 2.0 Å from Mg–O bonds and a shoulder or peak at 2.6 Å from O–O interactions are absent here14,39. This is attributed to the relatively low Qmax value used for the Fourier transformation (data were collected to 20 Å−1 but processed to 16 Å−1) that was necessary to obtain sufficient quality real space data. For the two individual voxels, clear differences are observed in their PDFs, in particular the relative intensities of peaks at close range (r = 1–4 Å) as well as peak positions and intensities for longer range correlations, demonstrating spatial variance in atomic structure not observable with XRD-CT.

In order to identify the phase distribution of slag and C-(N)-A-S-H gel, it is useful to consider the relative PDF peak intensity at r = 3.65 Å for each voxel, since this is known to noticeably increase as C-(N)-A-S-H gel forms64,65. This relative peak intensity can be approximated by the G(r = 3.65 Å)/G(r = 1.63 Å) ratio from PDF, i.e., the intensity at r = 3.65 Å divided by the intensity at r = 1.63 Å. Figure 2a shows the histogram of values for this ratio, along with a reference value of 0.53 obtained for the same slag14 and a reference value of 1.1 for C-(N)-A-S-H gel, obtained by subtraction of slag contribution to PDF in an AAS sample63. As the majority of voxels fall within these two limits, this indicates that many voxels contain a mixture of phases. Clearly, within the resolution of the voxel, there is heterogeneity of AAS, with slag and C-(N)-A-S-H gel intermixed. This is consistent with electron microscopy studies of AAS, where slag and C-(N)-A-S-H gel are also observed to intermix within 10 μm17,18,66.

To determine the spatial distribution of slag- and gel-rich regions within the AAS microstructure, the voxels that have a ratio between 0.2–0.55 and a ratio between 1–2 (shaded red and blue in Fig. 2a) were assigned as voxels containing predominantly slag and C-(N)-A-S-H gel, respectively. A minimum cutoff of 0.2 for slag and a maximum cutoff of 2 for C-(N)-A-S-H gel were used to keep ratios to realistic values. The PDF of a voxel with a ratio <0.2 is provided in Supplementary Fig. 3a, where the local atomic structure is observed to be distinctly different from that of slag. By contrast, Supplementary Fig. 3b, c confirms that an individual voxel assigned as slag or C-(N)-A-S-H gel shows similar local atomic structure to the average of the respective phase. Individual voxels appear to have greater long-range order, which is attributed to regions of (nano) crystallinity. The higher degree of long-range ordering in C-(N)-A-S-H gel compared to slag indicates the C-(N)-A-S-H gel has a greater degree of nanocrystallinity, with ordering up to 30 Å observed, in agreement with a previous study using bulk PDF characterization63.

Figure 2b shows the distribution of slag and C-(N)-A-S-H gel voxels overlaid on the 2D cross-section of the tomogram. These voxels align well with the light slag and dark gel matrix regions of the tomogram cross-section, and account for 21.0% and 7.4% of all voxels, respectively. The low extent of reaction after 24 h of curing is in agreement with observations from isothermal calorimetry18,65. Figure 2c, d show the average PDF of these voxels compared with the reference PDFs14,63 out to 8 and 30 Å, respectively. For both phases, the local atomic structure, Ca/Si ratios (from G(r = 2.37 Å)/G(r = 1.63 Å) ratio), and structure at distances up to 30 Å all show excellent agreement with the reference values, demonstrating the validity of using PDF-CT to identify separate phases within a heterogeneous mixture and extract quantitative information on the structure of each phase.

Carbonationofalkaliactivatedslag
XRD-CT and PDF-CT have been used to assess how Mg content within the slag affects carbonation. Figure 3a, b show the 2D cross-sections of the tomograms, measuring carbonated AAS samples with low and high Mg content, respectively. As described in the previous section, light gray partially reacted slag particles are surrounded by a darker matrix. Figure 3c, d shows spatially resolved PDF data for the two samples (voxel dimensions of 4×4×10 µm3), using the peak intensity at r = 1.23 Å, corresponding to the C–O bond length. For the low Mg sample a higher intensity of C–O is observed outlining a large solid region of AAS (see highlighted box in Fig. 3c). A similar effect is observed around the two largest solid particles for the high Mg sample (highlighted box in Fig. 3d). Higher values indicate regions of carbonation, which initiates from the outside of this solid region as CO2 diffuses in through the pore network25. Similar observations are seen for Ca-O correlations, which are mapped in Supplementary Fig. 4a, b, while an absence of Ca-Si correlations is generally observed in these regions, as shown in Supplementary Fig. 4c, d. Figure 3e compares the average PDF for the two samples. The peak at 1.2 Å is attributed to the C–O bond from carbonation. In addition to this peak, other changes are also observed with respect to the non-carbonated sample, for instance, the peak attributed predominantly to Ca–Si at 3.65 Å is diminished (compare with Fig. 1d), indicating decalcification of the C-(N)-A-S-H gel. The peak is more diminished in the low Mg sample than the high Mg sample, corresponding with a greater extent of decalcification, and consistent with a previous in situ bulk PDF study from Morandeau et al.39. Figure 3f compares the average diffraction patterns for the two samples. While the sample remains dominated by amorphous content, there are also individual peaks attributed to crystalline calcium carbonate phases, which appear more prominently for the low Mg sample, in agreement with observations from Morandeau et al.39. The two samples also both display a small peak attributed to an LDH at 0.8 Å−1.

Phase identification of the carbonated AAS samples has been performed from PDF-CT data to identify the primary phase present for each of the voxels. Due to the multitude of phases present, including unreacted slag, C-(N)-A-S-H gel, decalcified gel, and calcium carbonate, this was conducted according to the logic flow diagram in Fig. 4a. The initial ratio G(r = 2.37 Å)/G(r = 1.63 Å) refers to the intensity of PDF at the position of the Ca–O bond divided by the intensity at the position of the Si/AlIV–O bond to approximate the Ca/Si ratio. Ratios <0.25 were assigned as aluminosilica voxels (i.e., decalcified gel) due to the lack of Ca. Ratios between 0.25–0.75 were identified as unreacted slag, which has a reference value of ~0.514, if a threshold G(r = 3.65 Å)/G(r = 1.63 Å) ratio of 0.2 was met. This threshold, as described previously for Fig. 2a, corresponds to intensity at the peak position attributed predominantly to Ca–Si correlations divided by intensity at the Si–O peak position, and is important to consider here to distinguish slag from voxels of similar G(r = 2.37 Å)/G(r = 1.63 Å) ratios that contain significant calcium carbonate and/or decalcified gel and thus lack Ca–Si correlations. Similarly, a ratio of G(r = 2.37 Å)/G(r = 1.63 Å) between 0.75−1.25 was identified as C-(N)-A-S-H gel, again if the threshold G(r = 3.65 Å)/G(r = 1.63 Å) ratio of 0.2 was met, as a reference C-(N)-A-S-H gel has G(r = 2.37 Å)/G(r = 1.63 Å) of ~1.063. Voxels that did not meet these criteria were checked for the presence of calcium carbonate phase by assessing G(r = 1.23 Å), corresponding to the presence of C–O bonds. If this was >0, G(r = 2.37 Å)/G(r = 1.63 Å) was considered again, where values <5 were identified as containing both CaCO3 and aluminosilica phases, and values >5 were identified as CaCO3 only. Values <0 for G(r = 1.23 Å) were assigned as ‘other’ phase. Spot checks were performed for each phase, shown in Supplementary Fig. 5, which validates the distribution of phases obtained from the logic flow diagram.

The relative amounts of each phase are given in Table 1, while Fig. 4b, c show the map of each phase on the tomogram 2D cross-sections for the low and high Mg carbonated AAS samples, respectively. More voxels containing CaCO3 are observed for the low Mg AAS sample, indicating that the low Mg sample has undergone a greater amount of carbonation. Most voxels with calcium carbonate also contain aluminosilica, revealing that these phases generally intermix at the length scale below the instrument resolution. The spatial distribution shows that these voxels (yellow) are located around the perimeter of large AAS solid regions with C-(N)-A-S-H gel and partially-reacted slag found in the interior of the particles (particularly evident in Fig. 4b). This agrees with initial spatial analysis for the boxed region in Fig. 3c, and provides evidence of carbonation proceeding inwards through the pore network of the material. The carbonation front (width of yellow regions surrounding uncarbonated paste) is ~10 to 40 μm, although this range may be an overestimate due to the 10 μm dimension of the 4×10 μm2 beam size.

Figure 5 compares average PDFs for each of the different identified phases of the low and high Mg carbonated AAS samples. In Fig. 5a, b the voxels identified as slag and C-(N)-A-S-H gel, respectively, are compared for the two samples and with the same reference as previously mentioned (see Fig. 2). For both the slag voxels and the C-(N)-A-S-H gel voxels, the two samples show good agreement with each other, while differences with the reference include the presence of C–O bonding at 1.23 Å, a slight decrease of intensity for the Ca–O bond peak at 2.37 Å, and a larger decrease of the peak at 3.65 Å from Ca–T and Ca–Ca correlations, which is attributed to decalcification of the slag and C-(N)-A-S-H gel phases due to carbonation. This reveals that CO2 has infiltrated the slag and gel regions identified in Fig. 4b, c.

In Fig. 5c, the average PDF of voxels identified as aluminosilica gel for the two samples is compared with a reference PDF of pure fumed silica (Sigma-Aldrich)67. Both samples show peaks between 3–4 Å not observed in the reference, likely due to the presence of additional phases in the voxels, given that the Ca–T and Ca–Ca peak position at 3.65 Å is present, and some calcium carbonate phases possess Ca–Ca contributions in this region68. The low Mg AAS sample displays a more prominent C–O bond peak than the high Mg sample, indicating that more calcium carbonate is present in the vicinity of aluminosilica for these voxels. The PDF comparison also shows a less intense tetrahedrally bonded Si/AlIV–O bond peak for the low Mg AAS sample, and an additional prominent peak at 1.95 Å not seen in the spot-checked region for this phase. On further investigation, this latter peak has been identified as an artifact introduced by the tomographic reconstruction algorithm, producing some unrealistic negative I(Q) values for voxels on the periphery of the sample. The PDFs of all other phases for both samples have also been checked for artifacts in the PDFs arising in this way, and none were observed.

The average calcium carbonate PDFs were obtained by calculating the average PDFs of voxels containing both calcium carbonate and aluminosilica and subtracting the PDF of fumed silica, since the majority of voxels identified as containing calcium carbonate in Table 1 also contain aluminosilica. Figure 5d shows these PDFs for the two samples compared to the simulated PDF of calcite and the experimental PDF of ACC from Radha et al.46. The local atomic structure is clearly closer to that of ACC46, for instance in terms of the lack of prominent longer-range peaks (i.e., beyond 3 Å) and from the position of the Ca–O bond peak, located at 2.45 Å for both samples. Discrepancies between the samples and references between 3–4 Å are attributed to Ca–Si correlations, which arise from silicates stabilizing ACC69.

There is notable similarity between the slag PDFs in Fig. 5a and the C-(N)-A-S-H gel PDFs in Fig. 5b. For instance, the peak split seen in the reference C-(N)-A-S-H gel PDF in Fig. 5b at ~4.0–4.3 Å is missing in the corresponding gel voxels, and similar distinct peaks between 5.5 and 6.6 Å are also missing. We attribute this to the intermixing of these phases at a length scale below 10 μm and/or partially decalcified slag being the dominant phase of the solid paste regions. Hence, although with the use of PDF-CT we can distinguish calcium carbonate-rich regions from those of partially-reacted slag and gel regions, the latter two phases are more challenging to distinguish from each other with a voxel size of 4×4×10 μm3, which is in contrast to the non-carbonated sample that has a voxel size of 2×2×10 μm3. A higher spatial resolution or implementing a different algorithm are potential methods for improving the identification of the gel voxels.

Although the experimental calcium carbonate PDFs in Fig. 5d are dominated by the ACC phase, it is clear that the PDFs also contain a small degree of crystallinity since the atom-atom correlations are seen to continue across the full r range. This implies that there are calcium carbonate crystals present with crystallite sizes above the resolution limit of the measurement (above ~8 nm), and there is a slightly higher percentage of these crystals in the low Mg AAS compared with the high Mg AAS. For each voxel identified as ACC, the coherent domain size of this phase has been quantified in terms of nanocrystallite size from the decay of PDF intensity with distance. This was achieved by fitting the following envelope function to PDF peak intensities, with the assumption that particles are spherical in shape70:

f(r,d) = 1 - (3r/2d) + (r^3/2d^3) (1)

where d is the diameter of the nanocrystallite, r is the atomic pair distance, and the function is zero for r > d. For each of the voxels identified as calcium carbonate, PDF correlations out to 60 Å were fit using Eq. (1) to determine crystal size. Results show an average domain size of 1.2 nm for the low Mg sample and 1.1 nm for the high Mg sample. The average PDF and fits for the two samples are shown in Supplementary Fig. 6, while the spatial variation of nanocrystallite size across the voxels is mapped for the two samples in Fig. 6. The quantification of the nanocrystalline size of the calcium carbonate corroborates previous results that showed higher Mg content in AAS is associated with less crystalline calcium carbonate after carbonation39. Moreover, the low Mg carbonated AAS sample displays a larger range of sizes, with nanocrystals up to 3.5–4 nm being observed in Fig. 6a. These nanocrystals are mostly concentrated in a single region away from the carbonated AAS solid regions, and for low Mg AAS coincide with a region identified as purely calcium carbonate using PDF peak ratios (Fig. 4b, green region).

Further analysis of the carbonated AAS samples has been conducted with elemental mapping using nano-XRF. In Figs. 7, 8, elemental maps for Ca and Si are shown for three low Mg and two high Mg carbonated AAS particles, respectively. For particle A and the upper right section shown in particle D, there is a clear difference in the spatial distribution of Ca and Si, with Ca-rich regions generally located adjacent to the Si-rich regions. Regions high in Ca are attributed to calcium carbonate phases, while the regions with more Si are attributed to decalcified C-(N)-A-S-H gel, as discussed in more detail below by analyzing Fig. 9. Maps showing Ca/Si ratios for particles A and D are displayed in Fig. 9a, b, respectively, while maps of the other particles are provided in Supplementary Fig. 7. The low Mg particle in Fig. 9a contains Ca-rich domain sizes that range between 100–500 nm, located on the exterior of the particle. Similar behavior is seen for the smaller particles in Supplementary Fig. 7a–c. The high Mg particle in Fig. 9b has three sub-particles. Two are uniform with a low Ca/Si of 0.7, identified as unreacted slag since the reference high Mg slag measured with nano-XRF has an average Ca/Si ratio of 0.73. The third contains high Ca regions at the bottom and sides of the particle, varying from 200 nm to 1 μm in size. This, combined with the crystal size calculated previously from PDF-CT, suggests that these are regions rich in ACC as opposed to distinct crystallites. The identification of slag particles in Fig. 9b indicates they remain relatively unchanged during CO2 exposure in dry conditions, with some loss of calcium as witnessed from PDF-CT (Fig. 5a).

Line profiles of the Ca/Si ratio have been extracted from the maps shown in Fig. 9a, b, as shown in Fig. 9c, d, and from Supplementary Fig. 7a–c in Supplementary Fig. 7d. These regions were examined as they transition from regions of high Ca/Si to low Ca/Si. In Fig. 9c, d, the low Mg slag clearly shows a much lower minimum for the Ca/Si ratio on the interior of the particle. A slightly lower Ca/Si for low Mg particles is also seen for the smaller particles in Supplementary Fig. 7d. The lack of extensive Ca-rich outer regions for the particles in Supplementary Fig. 7a–c means that their identification as decalcified gel remains tentative, although analysis of other elements confirms they are not impurities in the slag. Nevertheless, overall, these data provide evidence that high Mg AAS better resists decalcification and retains a higher amount of calcium within the gel. For both low and high Mg AAS, Ca-rich regions (i.e., the edge regions in Fig. 9a, b) have a significant amount of Si. Ca/Si ratios in these Ca-rich regions are observed to be 5–6, with Si up to 4× the background noise. As mentioned earlier, PDF-CT voxels of calcium carbonate also show signs of potential silicates incorporation via peak intensities between 3 and 4 Å that are absent in pure ACC. The presence of silicates within the calcium carbonate regions supports previous studies by Kellermeier et al. that find silicates stabilize ACC69,71,72, and our previous study that hypothesizes both Mg and silicates contribute to the formation of ACC (as opposed to crystalline calcium carbonate phases) in carbonated AAS40.

With distinct regions of calcium carbonate and decalcified gel, the particles analyzed in Fig. 9 correspond to the yellow areas identified in Fig. 5 (PDF-CT data), where extensive carbonation has occurred and each PDF-CT voxel consists of both ACC and decalcified gel. The finer resolution accessible using nano-XRF reveals phase segregation on the order of hundreds of nanometers to a few microns. Furthermore, Fig. 9a, b shows that exposure to CO2 leads to calcium migrating to the outer region of these particles, since before carbonation the Ca/Si ratio maps of AAS particles were largely homogeneous. This migration behavior of calcium has been previously observed by Saeki et al. via analysis of elemental maps obtained using energy dispersive X-ray spectroscopy for OPC carbonated at 95% relative humidity and natural CO2 levels (~450–470 ppm) for 60 days73. Calcium migration is thought to be enhanced in wet conditions (i.e., saturated pores) and at ambient CO2 levels, where CO2 diffusion through the pores is relatively slow compared with the rate of outward migration of calcium73. Interestingly, the migration behavior only becomes visible from nano-XRF data by analysis of the Ca/Si ratio maps, since the original element-specific maps do not show indications of phase segregation (Figs. 7, 8). Given that 100% dry CO2 was utilized and the relative humidity during carbonation was not actively controlled, calcium migration may still be occurring under semi-saturated and/or accelerated carbonation conditions, but remains largely undetectable using conventional analysis approaches. As seen in Supplementary Figs. 8, 9, water molecules are detected in the low and high Mg AAS samples both before and after exposure to 100% dry CO2 using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, see H2O bending mode at ~1650 cm−1). Hence, water molecules remain in the samples even after exposure to 100% dry CO2, as further confirmed from the TGA weight loss curve of carbonated high Mg AAS pastes shown in Supplementary Fig. 9, where ~4% sample mass is lost between ambient temperature and 105 °C (compared with ~18% prior to carbonation).

Methods
Alkaliactivatedslag
Two sodium silicate-activated slag pastes were prepared, one using a high Mg content (12.7% MgO) blast furnace slag and the second with a low Mg content (7.3% MgO) blast furnace slag (see Supplementary Table 1 for full slag compositions). The solution-based sodium silicate activator was prepared by mixing deionized water, solid sodium hydroxide (reagent grade, Sigma-Aldrich), and Type-D sodium silicate solution (PQ Corporation) for 24 h to obtain an activator solids composition of Na2SiO3, a Na2O to slag weight ratio of 0.07, and a water to slag weight ratio of 0.44. The resulting solution had a composition of 12.1 wt.% Na2O, 11.7 wt.% SiO2, and 76.2 wt.% H2O. AAS paste samples were prepared by mixing the slag with the activating solution for 2 min by mechanical stirring at 1500 rpm using a Caframo mixer. The pastes were cured for 24 h in sealed plastic cuvettes prior to analysis or CO2 exposure to mimic the short curing duration typical of well cements.

One AAS paste, high Mg AAS, was analyzed prior to carbonation to determine the extent of phase heterogeneity at a resolution of ~10 μm using PDF-CT and the chemical heterogeneity at a resolution of ~40 nm using nano-XRF. Prior to synchrotron measurements, the paste was coarsely crushed using a mortar and pestle until particles passed through a 1 mm sieve and were retained on a 0.6 mm sieve. Then the alkali activation reaction was halted by the addition of 1.5 g powder to 150 mL reagent-grade isopropyl alcohol. After a 24-h soak, the alcohol was decanted, the sample dried at 60 °C for 24 h, and then crushed using a mortar and pestle into a fine powder (~50 μm). Prior to measurement, the sample was stored in a sealed vial.

Two AAS pastes were analyzed after being exposed to CO2, low Mg and high Mg AAS. For these pastes, after the initial 24 h of curing, they were then crushed into a fine powder (~50 μm) using a mortar and pestle. Carbonation was carried out in 100% dry industrial-grade CO2 for 24 h at room temperature with a CO2 flow of <0.5 L/min using a custom carbonation chamber. Prior to measurement, the samples were stored in sealed vials.

XRD-CTandPDF-CT
XRD-CT and PDF-CT were performed at the 1-ID beamline at the Advanced Photon Source, Argonne National Laboratory, using monochromatic 71.676 keV (λ = 0.173 Å) X-ray radiation. Powder samples were loaded into ~350 μm outer diameter polyimide capillaries, using a thin metal rod to pack the samples and prevent movement during measurements, then sealed at both ends using quick-set epoxy. The sample was securely mounted with beeswax onto a rotatable and translatable sample stage. Using an X-ray absorption detector, the capillary was centered so that the axis of the capillary coincided with the axis of rotation. Live X-ray absorption images were also used to choose a region of interest of the sample that consisted of large particles with a size close to the diameter of the capillary. When a region of interest was chosen, a conventional X-ray tomography scan was performed by taking a series of X-ray absorption images while rotating the capillary. The resulting voxel resolution obtained was 1.17 μm3. Tomographic reconstruction by back projection was performed to obtain a 3D representation of the region of interest of the sample.

For XRD-CT and PDF-CT, measurement data were obtained using a 2×10 μm or 4×10 μm beam size. A 2D detector (GE-RT41) positioned 300 mm away from the sample allowed for a maximum momentum transfer, Qmax, of 20 Å−1 to be measured. The sample was oriented such that the 10 μm aligned with the length of the capillary and 2 μm or 4 μm with the cross-section. A series of X-ray scattering patterns was measured by rotating the sample 180° using a fixed step size, with a scan taken at each step. The sample was linearly translated through the beam in fixed-length increments, with a series of rotation measurements repeated for each linear position. For the non-carbonated AAS sample, data were acquired using a focused X-ray beam (refractive focusing optics installed during a beamline upgrade), sample rotation of 0.6° increments, linear translation of 2 μm increments (200 translation steps in total), and a 0.6 s exposure time for each scan. The carbonated AAS samples were measured prior to installation of the refractive focusing optics, with sample rotation increments of 2.0°, linear translation increments of 4 μm (100 translation steps in total), and a 7.0 s exposure time for each scan. This resulted in voxel resolutions of 2×2×10 μm3 and 4×4×10 μm3 for the non-carbonated and carbonated samples, respectively.

Data for each 2D scattering pattern were processed using in-house beamline software by subtracting the dark-field background and azimuthally integrating to obtain the 1D scattering pattern. Tomographic reconstruction was then performed by back projection with Hann filtering to obtain a 2D map of scattered X-ray intensity I(Q) through the cross-section of the sample77. To obtain PDF data, the 200×200 (non-carbonated sample) or 100×100 (carbonated samples) distinct 1D scattering patterns were processed in batch mode using the PDFgetX3 software, setting Qmax to 16 Å−1 and rmin to 1.3 Å. The structure factor S(Q) was obtained by normalization followed by smoothing of Q > 8 Å−1 using a 10-point rolling average to reduce noise in the high Q region and thus improve the signal-to-noise ratio78. A 2D grid of PDFs was obtained and analyzed using a custom script in MATLAB. An ellipse fit to the spatially resolved data included only voxels within the capillary. Finally, a silicon standard was measured on the beamline to calculate the instrument parameters Qdamp and Qbroad using the software PDFgui, which were 0.0251 Å−1 and 0.0027 Å−1, respectively. These parameters were used to generate a simulated PDF of calcite using PDFgui for subsequent comparison with calcium carbonate-containing PDF voxels in the PDF-CT data.

Nano-XRFspectroscopy
Nano-XRF spectroscopy was performed at the 26-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory, using monochromatic 10 keV (λ = 1.240 Å) X-ray radiation. The sample holder consisted of an aluminum bar with a ~1 mm hole, which was covered by plastic film (Ultralene film 0.16 mil). The samples, in powder form, were dispersed in isopropyl alcohol, and then a drop was placed on the plastic film. The alcohol was allowed to dry, leaving particles spread on the film in the sample holder. To perform the measurement, the sample holder was placed on a piezoelectric-controlled stage. The sample stage was in a vacuum chamber that was pumped down to 10−7 torr with a turbomolecular pump. The wavelength-dispersive X-ray spectrometer detector was oriented 70° from the X-ray beam to minimize detecting scattered X-rays. Slag powders were ground further before preparing carbonated AAS samples to reduce particle size distribution by using a ball mill (Retsch, PM100) with 10 mm zirconia milling beads for three 15-min intervals. The high Mg slag was also measured using this technique as a reference.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work reports the nanostructural details of one alkali-activated slag paste before carbonation and two alkali-activated slags pastes after carbonation using a set of state-of-the-art synchrotron techniques: (i) conventional X-ray tomography [1.2 μm voxel size]; (ii) X-ray diffraction-computed tomography [XRD-CT], (iii) pair distribution function-CT [PDF-CT [2x2x10 and 4x4x10 μm voxel size, for untreated and carbonated], and (iv) nanoprobe X-ray fluorescence [nano-XRF, non-specified pixel size].\nThe data analyses provide very interesting information concerning the slag_C-N-A-S-H distribution as well as the resistance to carbonation. The formation of amorphous/disordered calcium carbonate is quite firmly established. I have minor comments/remarks to clarify some issues to help the readability and understanding of this work.\n1. It is not explicitly stated, but I deduced that the nano-XRF spatially-resolved study is 2D. Please state this (including the reconstructed voxel size) or otherwise elaborate.\n2. The carbonation conditions are weird. (I) Carbonation rate strongly depends upon the RH, with a maximum close to 80% for PC. Here, dry conditions have been selected. Why? Please elaborate on this. (II) Moreover, carbonation (both degree of reaction/rate and polymorphism) also strongly depends upon the microstructure of the specimens. Here, loose fine powders are carbonate, which is not very relevant for CO2-uptake (CO2-curing: intentional uptake), nor for carbonation resistance. Please comment on this choice for the experimental conditions of this carbonation.\n3. Moreover, pastes were cured just for 24h and then subjected to CO2 exposure. These conditions are closer to CO2-curing than to CO2 resistance. Why such short period of 'water curing'?\n4. Finally concerning carbonation, I would suggest replacing \"... consumed additional CO2 causes a reduction in pore solution pH, which instigates steel corrosion...\" by \"... consumed additional CO2 causes a reduction in pore solution pH, which may instigate steel corrosion...\" as carbonation does not necessarily lead to steel corrosion.\n5. Data are taken to Qmax=20 A-1 but the data are processed to Qmax=16 A-1. This is clearly stated in the experimental section, but it could also be clarified in the main section of the manuscript.\n6. Why, low-Mg content slag paste was not studied (prior carbonation)? The comparison between both pastes could further increase the relevance of this study.\n7. I suggest replacing 1.22 and 1.08 by 1.2 and 1.1 nm, respectively. In my opinion, the accuracy of these analyses does not ensure reporting results with two decimal places.\n8. Concerning Fig. 1a, the reconstruction of the capillary is not smooth. I wonder about the sample-to-detector distance (not reported in the experimental section) and if Paganin-type phase retrieval was carried out.\n9. Is the orthoslice shown in Fig. 1a (μCT) the same region/section than that shown in Fig. 1b (XRD-CT)? Please elaborate on any type of volume registration carried out.\n10. If the XRD-CT / PDF-CT studies are carried out just in a region of 10 μm (vertical), this must be explicitly stated. Otherwise, please elaborate on the volume studied in the different X-ray imaging modalities. This is not clear at all. If a stack of images were acquired, 3D rendered figure could be added to S.I.\n11. Please, could you comment on the REV(s) for these studies for the different modalities? This is always a concern in any nanoimaging work.", "reviewer_2": "This paper investigates the behavior of alkali-activated materials (AAMs) before and after carbonation, which are prepared by adding alkali activators (Na₂SiO₃, NaOH, H₂O) to blast furnace slags with varying Mg content. The analyses are conducted using XRD-CT, PDF-CT, and nano-XRF, and the carbonation behavior is examined. The proposed chart flow for phase separation in the PDF analysis is original and offers innovative insights; the validity and robustness of the methodology are clearly demonstrated. However, the discussion concerning the effects of Mg and Si on the formation and stability of amorphous calcium carbonate (ACC) remains insufficient and requires further analysis and consideration.\n1. First, the initial state of the AAMs is not clearly defined. For different slag compositions, the reaction degree, the dissolved elements, the composition of the gels, and the phase in which Mg is present must be clarified, as they play a critical role in the subsequent carbonation behavior.\n2. Second, the discussion should be deepened regarding the compactness of the hardened matrix, including their porosity.\n3. In addition, the initial water content must be clarified. Combined with point (2), this is essential for evaluating the extent of water present during carbonation, and thereby, how equally the two samples had the opportunity for ACC to transform into crystalline phases. Since carbonation is inherently a dehydration process involving the loss of hydroxyl groups, a discussion is required on how the initial water and the water generated through carbonation affected the location of calcium carbonate precipitation and the transformation processes of carbonate phases.\n4. A comparison between the two samples should be preceded by a discussion of the degree of carbonation achieved in each. Only after establishing that they underwent a comparable reaction process can a meaningful comparison be made.\n5. Recent studies (e.g., T. Uno, et al. (2024). https://doi.org/10.1021/acs.jpcc.4c01714) have pointed out that under moderate humidity conditions, metastable calcium carbonate phases such as vaterite can precipitate within the aggregated structure of C–S–H, and be stabilized by surrounding walls composed of silica. Furthermore, Mg is known to prolong the existence of ACC due to its strong hydration energy and its ability to suppress the crystallization process when incorporated. However, these aspects are not demonstrated in the PDF data presented in this study. From this perspective, the role of Mg should be discussed in terms of (1) differences in gel formation, (2) variations in the resulting gels (in terms of quantity, quality, distribution, and network), (3) differences in the carbonation process of each gel type, and (4) phase composition and distribution of the resulting calcium carbonate. It is essential to discuss how these differences relate to (a) the nature of the gel, (b) the influence of water, and (c) the roles of Si and Mg in the carbonation process.", "reviewer_3": "Review of “Spatially resolved nanostructural analysis of disordered phases in carbonated alkali-activated slag”, by McCaslin et al.\nDear Editor\nI have read with great interest the paper by McCaslin et al on the carbonation of Mg-bearing slag samples. The authors have used a panel of advanced techniques with ~micron resolution (better for XRF) to describe the textural information of the samples before and after carbonation, including detailed spatially-resolved structural information thanks to the use of PDF-CT and XRF. The use of PDF allows interpreting the local order of the disordered materials in the samples.\nThe study is of great interest because described in detail structural details that were not described in the literature before, including the intermixing of phases (at the resolution level). This is great new information. However, the claims about some of the carbonation textures and processes are not that evident from the data. I include some comments below.\nIn my opinion, the paper could fit Nat Comm if the authors are more precise about the carbonation process, the textures, and provide more evidence for some raised points below.\nLines 106 and others: what do the authors mean by ‘tetrahedral atoms’?\nFigure 1. The three PDFs shown in (d) are a bit difficult to interpret, due to the overlapping at low r, I suggest separating them more. Also, the PDF from ‘+’ seems to show more long-range order (small peaks maybe also visible in the diffraction pattern). This is not discussed in the text.\nLines 108-110: This means that Mg is not visible by PDF using these settings.\nLines 290-291, XRF results: It is not clear from the images that the Ca-rich regions are located on the exterior of the Si-rich regions. Also, in the following sentence, on what is based the attribution?\nLines 308-310: The authors claim that the observed low Ca/Si in the interior of the low-Mg slag particle support the ‘evidence’ that high Mg AAS resists better decalcification. One could wonder about the statistical significance of this observation, based here on one particle.\nLines 345-347. It is difficult to get from the data the evidence that the CaCO3 regions were encasing carbonate-deficient cores. Could the authors provide more evidence on this textural information?\nLines 350-352. Is this difference (1.08 vs 1.22 nm) significative? What is the uncertainty? I am very skeptical about this being significative.\nMethods. Is the difference in Mg content between the two samples ‘high enough’? to expect differences in the carbonation behavior?"}, "author_response": "Thank you for the invitation to revise and resubmit our manuscript. Please see below our point-by-point response to the reviewer comments. Reviewer comments are given in bold italics and our responses in normal text.\nReviewer 1\nThis work reports the nanostructural details of one alkali-activated slag paste before carbonation and two alkali-activated slags pastes after carbonation using a set of state-of-the-art synchrotron techniques: (i) conventional X-ray tomography [1.2 μm voxel size]; (ii) X-ray diffraction-computed tomography [XRD-CT], (iii) pair distribution function-CT [PDF-CT [2x2x10 and 4x4x10 μm voxel size, for untreated and carbonated], and (iv) nanoprobe X-ray fluorescence [nano-XRF, non-specified pixel size].\nThe data analyses provide very interesting information concerning the slag_C-N-A-S-H distribution as well as the resistance to carbonation. The formation of amorphous/disordered calcium carbonate is quite firmly established. I have minor comments/remarks to clarify some issues to help the readability and understanding of this work.\nWe thank the reviewer for their thoughtful comments and interest in the data. Below are our responses to the requested items of clarification:\n1. It is not explicitly stated, but I deduced that the nano-XRF spatially-resolved study is 2D. Please state this (including the reconstructed voxel size) or otherwise elaborate.\nWe have clarified in the main text that nano-XRF is spatially resolved in 2D with 40 nm resolution. (see lines 93-94 of the revised manuscript).\n2. The carbonation conditions are weird. (I) Carbonation rate strongly depends upon the RH, with a maximum close to 80% for PC. Here, dry conditions have been selected. Why? Please elaborate on this. (II) Moreover, carbonation (both degree of reaction/rate and polymorphism) also strongly depends upon the microstructure of the specimens. Here, loose fine powders are carbonate, which is not very relevant for CO2-uptake (CO2-curing: intentional uptake), nor for carbonation resistance. Please comment on this choice for the experimental conditions of this carbonation.\nWe appreciate the reviewer’s comment on carbonation conditions and agree that what we used in our manuscript is not the same as those used for CO -curing or assessing carbonation resistance. In fact, our accelerated carbonation conditions of 100% dry CO 2 are most directly relevant to carbonation resistance of cement in oil and gas wells and 1 geological storage of CO . This was mentioned in the Introduction, but we now also 2 include this in the Abstract and clarify the use of these accelerated carbonation conditions in the Discussion.\nThe following text has been added to the Abstract:\n“aggressive carbonation (100% dry CO ), conditions experienced in applications such as 2 oil and gas wells and geological storage of CO ” 2\nThe following text has been added to the Discussion on line 374 for clarity:\n“under accelerated 100% carbonation conditions.”\nThe following text has been amended in the Discussion on lines 411-414 (changes have been highlighted):\n“Moreover, not only are the findings of this investigation relevant for increasing the resistance of cements such as AAS to CO -induced degradation caused by aggressive 2 conditions (i.e., 100% CO ), they provide clues as to which chemistries are best avoided 2 if the aim is to maximize CO uptake when using 100% dry CO .” 2 2\nThe carbonation conditions we use are consistent with our previous studies in this regard (see e.g. A. Morandeau (2015) https://doi.org/10.1021/acs.chemmater.5b02382, S. Wang et al. (2020) https://doi.org/10.1016/j.cemconres.2020.105979, and E. McCaslin et al. (2021) https://doi.org/10.1016/j.cemconres.2021.106454).\nFor the reviewer’s comment on microstructure, the carbonation of loose fine powder was undertaken to study change of chemistry, i.e. so that the chemistry of the AAS could be studied independently from effects of transport phenomena. We have added the following text to the revised manuscript on lines 87-88 to clarify this point:\n“Powders have been used to limit the effects of transport-dependent processes during CO exposure and therefore isolate how slag chemistry affects CO -induced chemical 2 2 changes.”\n3. Moreover, pastes were cured just for 24h and then subjected to CO2 exposure. These conditions are closer to CO2-curing than to CO2 resistance. Why such short period of 'water curing'?\nGiven that the accelerated CO exposure conditions used in this study are relevant for 2 well cements (e.g., oil and gas operations, CO sequestration), and that placement of 2 well cement is quickly followed by other drilling operations that often involve CO , we 2 2 deemed 24h to be an appropriate curing duration. We have used 24h carbonation in our previous studies to identify phase changes successfully, sometimes alongside longer curing times (e.g. A. Morandeau (24h), E. McCaslin (24h, 2day, 7 day), & S. Wang (24h, 3 day, 7 day), linked above).\nWe have added the following text on lines 432-433 of the revised manuscript:\n“prior to analysis or CO exposure to mimic the short curing duration typical of well 2 cements.”\n4. Finally concerning carbonation, I would suggest replacing \"... consumed additional CO2 causes a reduction in pore solution pH, which instigates steel corrosion...\" by \"... consumed additional CO2 causes a reduction in pore solution pH, which may instigate steel corrosion...\" as carbonation does not necessarily lead to steel corrosion.\nWe have made the suggested change regarding instigation of steel corrosion on line 41 of the revised manuscript.\n5. Data are taken to Qmax=20 A-1 but the data are processed to Qmax=16 A-1. This is clearly stated in the experimental section, but it could also be clarified in the main section of the manuscript.\nWe have clarified data collection to Q =20 Å-1 but processing to Q =16 Å-1 in the max max body of the manuscript where the processing is discussed (line 129).\n6. Why, low-Mg content slag paste was not studied (prior carbonation)? The comparison between both pastes could further increase the relevance of this study.\nThe low Mg uncarbonated AAS sample was not studied with PDF-CT/XRD-CT due to limited access to beamtime; this is now added at the beginning of the Results section for context. We agree that there could be some benefit to comparing the low and high Mg uncarbonated samples with this technique, but we consider the high Mg sample sufficient for the purposes of establishing spatial phase separation for an uncarbonated sample with PDF-CT technique as described in the manuscript.\n7. I suggest replacing 1.22 and 1.08 by 1.2 and 1.1 nm, respectively. In my opinion, the accuracy of these analyses does not ensure reporting results with two decimal places.\n8. Concerning Fig. 1a, the reconstruction of the capillary is not smooth. I wonder about the sample-to-detector distance (not reported in the experimental section) and if Paganin-type phase retrieval was carried out.\nThe reconstruction of μCT in Figure 1a is not smooth because of the usual ring artifacts in the reconstruction. Since at high X-ray energies (72 keV in this case) the projection images of light materials are lacking contrast, any small detection anomaly in a pixel of the detector can cause a ring artifact in the reconstruction, generally everywhere. Because these are not a problem for visual studies by human eyes, we have not removed them, so the reconstruction data is as intact as possible. The tomography detector distance from the sample was 80 mm. There was no Paganin type phase retrieval used. (Again, the very small detectable signal would make such methods unreliable.)\n9. Is the orthoslice shown in Fig. 1a (μCT) the same region/section than that shown in Fig. 1b (XRD-CT)? Please elaborate on any type of volume registration carried out.\nThere is a very robust way worked out to register multimodal experimental data at the 1- ID beamline, where such experiments are routinely conducted. The samples never leave the sample stage, hence they are precisely registered with the X-ray beam. For tomography measurements the X-ray slit positions determine the measured volume location, and the same slits are used to position the focused beam onto the same sample for conducting the PDF-CT. By tracking the slits and sample stage positions the exact location of the reconstructed PDF map orthoslice can be determined and the corresponding XRD-CT orthoslice can be exactly matched. (The orientation of the reconstructions are determined by the orientation of the common rotation axis.)\n10. If the XRD-CT / PDF-CT studies are carried out just in a region of 10 μm (vertical), this must be explicitly stated. Otherwise, please elaborate on the volume studied in the different X-ray imaging modalities. This is not clear at all. If a stack of images were acquired, 3D rendered figure could be added to S.I.\nThe reviewer is correct that the PDF-CT/XRD-CT slice is 10 µm thick. This has been clarified in the main body of text for all samples.\nOn lines 109-110, the following text has been added to the revised manuscript:\n4 “(XRD-CT and PDF-CT voxel dimensions of 2 × 2 × 10 µm3 with 10 µm aligned along the length of the capillary)”\nOn line 183 the following has been added:\n“(voxel dimensions of 4 × 4 × 10 µm3)”\nWe have also added text to the figure captions of Figure 1 and 3 stating the voxel resolution and that the 10 µm dimension is aligned along the length of the capillaries.\n11. Please, could you comment on the REV(s) for these studies for the different modalities? This is always a concern in any nanoimaging work.\nWe consider the results to be within the respective representative elementary volumes (REVs) for the techniques. For XRD-CT and PDF-CT, individual voxels for the non- carbonated sample are demonstrated to be comparable to the orthoslice (see Figure 1c and 1d respectively), and the PDF-CT orthoslice is comparable to bulk studies with the same slag sample (see A. Morandeau et al. (2015) linked above). For nano-XRF, we studied multiple particles (including slag and AAS prior to carbonation, not reported due to brevity) and ensured particles shown in Figures 7-9 were representative of the desired carbonated AAS phase. Note we have added more particles of the carbonated AAS pastes to the nano-XRF analysis as seen in the revised manuscript to strengthen the findings of our investigation. The resolution of nano XRF is comparable to that of EDX which has been used by others for 2D spatial phase mapping of AAS and analysis of Ca/Si ratio (see e.g. I. Richardson et al. (2018) https://doi.org/10.1016/j.conbuildmat.2018.02.034).\nThis paper investigates the behavior of alkali-activated materials (AAMs) before and after carbonation, which are prepared by adding alkali activators (Na₂SiO₃, NaOH, H₂O) to blast furnace slags with varying Mg content. The analyses are conducted using XRD-CT, PDF-CT, and nano-XRF, and the carbonation behavior is examined. The proposed chart flow for phase separation in the PDF analysis is original and offers innovative insights; the validity and robustness of the methodology are clearly demonstrated. However, the discussion concerning the effects of Mg and Si on the formation and stability of amorphous calcium carbonate (ACC) remains insufficient and requires further analysis and consideration.\nWe thank the reviewer for their thoughtful comments and praise for the methodology. Below are our responses to the comments:\n1. First, the initial state of the AAMs is not clearly defined. For different slag compositions, the reaction degree, the dissolved elements, the composition of the gels, and the phase in which Mg is present must be clarified, as they play a critical role in the subsequent carbonation behavior.\nWe have addressed this by adding the following to the Results and Supplementary Information:\nLines 97-99:\n“Isothermal conduction calorimetry (ICC) data given in Figure S1 show similar cumulative heat after 24 hours of curing, indicating similar extent of reaction (i.e., extent of slag dissolution and amount of C-(N)-A-S-H gel precipitation).”\nLines 102-104:\n“Additional insights on AAS paste properties prior to CO exposure are given in the 2 Supplementary Information, based on existing literature59-62 and our recent work. 39–41”\nSupplementary Information text:\n“This investigation complements our previous analysis of the same alkali-activated slag (AAS) pastes and CO exposure conditions (i.e., 100% dry CO after 24 hours of curing) 2 2 using standard characterization techniques.39–41 For instance, we have reported that the paste pore structures at 1 day for the low and high Mg AAS systems are similar, consisting of gel pores of ~7 nm in size,41 and that these AAS pastes also form LDH phases, with greater extent of formation of the Mg-based hydrotalcite-like LDH for the 6 high Mg slag paste.40 In the literature, analysis of low and high Mg AAS confirms that Mg does not get incorporated into C-(N)-A-S-H gel.59 Thermodynamic modeling has shown similar C-(N)-A-S-H gel compositions for mature low and high Mg AAS pastes, with Ca/Si ratios of ~1.00,60 while pore solution compositions reveal higher concentrations of Si and S, and lower concentrations of Al, in the high Mg AAS paste compared with the low Mg counterpart.61 29Si nuclear magnetic resonance (NMR) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) data have shown that gel composition (e.g., Ca/Si ratio) remains essentially unchanged from 1 to 100 days.62”\nSupplementary Information figure:\nFigure S1. Isothermal conduction calorimetry (ICC) data for low and high AAS, highlighting the similar cumulative heat values at 24 hours after mixing. Note that the initial 45 minutes of ICC data have been excluded from the cumulative heat curves due to known instrument disturbances during loading of samples. Samples were measured in a TAM Air (TA Instruments) set at 25 °C, each consisting of 5 g of paste with 5 g of deionized water as the reference sample. Data have been normalized per gram of paste.\n2. Second, the discussion should be deepened regarding the compactness of the hardened matrix, including their porosity.\nAs we mention in the response to reviewer 1, samples were crushed so that the chemistry was studied independent of porosity. With the carbonation time set at 24 hrs for 100% CO , the transport properties of the pastes for crushed powders would have 2 had a minimal impact (i.e., carbonation would have proceeded throughout the pore structure).\nSee Reviewer 1 #2 for specific changes we made to the revised manuscript to clarify this point.\nRegarding porosity, pore structure, and related properties, we have previously investigated these aspects for the low and high Mg AAS pastes before and after exposure to 100% CO , as reported in ref. 41 of the revised manuscript. In the original manuscript 2 we made reference to this paper in the Introduction, specifically: “Furthermore, we have observed that high Mg AAS undergoes less pore structural change upon accelerated carbonation.41”\nIn light of the reviewer’s comment we added the following text to the Supplementary Information:\n“we have reported paste pore structures at 1 day for the low and high Mg AAS systems are similar, consisting of gel pores of ~7 nm in size”\n3. In addition, the initial water content must be clarified. Combined with point (2), this is essential for evaluating the extent of water present during carbonation, and thereby, how equally the two samples had the opportunity for ACC to transform into crystalline phases. Since carbonation is inherently a dehydration process involving the loss of hydroxyl groups, a discussion is required on how the initial water and the water generated through carbonation affected the location of calcium carbonate precipitation and the transformation processes of carbonate phases.\nWe have updated the manuscript to include the following text on lines 99-102 (beginning of Results), which outlines the initial water content of the samples:\n“The water content of the low and high Mg AAS pastes is visible in the thermogravimetric analysis (TGA) data reported in Figure S2, where the low Mg AAS paste possesses a slightly higher amount of surface and weakly bound water visible via more weight loss between ambient temperature and 105°C.”\nWe have also added FTIR data before and after carbonation, along with a representative TGA curve after carbonation, that shows the presence of water molecules even after exposure of the samples to 100% dry CO for 24 hrs. As such, the following text has been 2 added to lines 360-366 of the revised manuscript:\n8 “As seen in Figures S8 and S9, water molecules are detected in the low and high Mg AAS samples both before and after exposure to 100% dry CO using attenuated total 2 reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, see H O bending mode at 2 ~1650 cm-1). Hence, water molecules remain in the samples even after exposure to 100% dry CO , as further confirmed from the TGA weight loss curve of carbonated high 2 Mg AAS paste shown in Figure S9 where ~4% sample mass is lost between ambient temperature and 105°C (compared with ~18% prior to carbonation).”\nHence, overall, we show that the water content of the two AAS pastes prior to carbonation is similar, and after carbonation both samples retain water as visible from FTIR data (4 wt.% for high Mg AAS paste compared to 18 wt.% prior to carbonation, quantified using TGA).\nRegarding the reviewer’s comment “a discussion is required on how the initial water and the water generated through carbonation affected the location of calcium carbonate precipitation and the transformation processes of carbonate phases”, we did not directly quantify the amount and types of water before and after CO exposure, which would 2 require 1H NMR, quasi elastic neutron scattering, or a similar technique. Indeed, use of such techniques would be an investigation in its own right. However, we now reiterate the role of Mg solvation energy in the ability for ACC to transform into crystalline calcium carbonate in lines 63-65 (part of the Introduction) of the revised manuscript:\n“It has been shown that Mg stabilization of ACC is due to the higher solvation energy of Mg2+ compared with Ca2+, making it harder for dehydration to occur for transformation into anhydrous crystalline CaCO .” 3 9\nFigure S2. Thermogravimetric analysis (TGA) curves for low and high Mg AAS at 1 day of curing. Samples were measured in a Pyris 1 thermogrametric analyzer (Perkin Elmer) using platinum crucibles and N as the purge gas. The ramp rate was set at 10 °C/min 2 from ambient temperature to 900 °C with a hold time of 2 hours at 105 °C.\na) b)\nFigure S8. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR- FTIR) of (a) uncarbonated and (b) carbonated low and high Mg AAS paste. Samples were measured using a Perkin Elmer Frontier FT-IR Spectrometer and a Frontier ATR attachment. The instrument was purged with N gas to eliminate atmospheric H O and 2 2 CO contributions. 32 scans were taken for each sample, with a scan speed of 1 cm/s, a 2 resolution of 4 cm-1 and a wavenumber range of 500 to 4000 cm-1.\n4. A comparison between the two samples should be preceded by a discussion of the degree of carbonation achieved in each. Only after establishing that they underwent a comparable reaction process can a meaningful comparison be made.\nThe carbonation time was the same for the two samples. We have previously reported that the time for the C-O peak in PDF to reach equilibrium is ~40 min (A. Morandeau et al. (2015) https://doi.org/10.1021/acs.chemmater.5b02382). Thus, we consider carbonation conditions to be sufficiently similar. The FTIR data now given in the Supplementary Information also support this evidence via the prominent carbonate band at ~1450 cm-1 in both samples. (Note that we were unable to locate the TGA data for the carbonated low Mg AAS paste, otherwise we would have reported %wt. loss associated with the carbonate phases, as visually seen in the TGA data for the carbonated high Mg AAS paste sample.)\n5. Recent studies (e.g., T. Uno, et al. (2024). https://doi.org/10.1021/acs.jpcc.4c01714) have pointed out that under moderate humidity conditions, metastable calcium carbonate phases such as vaterite can precipitate within the aggregated structure of C–S–H, and be stabilized by surrounding walls composed of silica. Furthermore, Mg is 11 known to prolong the existence of ACC due to its strong hydration energy and its ability to suppress the crystallization process when incorporated. However, these aspects are not demonstrated in the PDF data presented in this study. From this perspective, the role of Mg should be discussed in terms of (1) differences in gel formation, (2) variations in the resulting gels (in terms of quantity, quality, distribution, and network), (3) differences in the carbonation process of each gel type, and (4) phase composition and distribution of the resulting calcium carbonate. It is essential to discuss how these differences relate to (a) the nature of the gel, (b) the influence of water, and (c) the roles of Si and Mg in the carbonation process.\nWe thank the reviewer for bringing this recent paper to our attention and mentioning items (1) through (4), and (a) through (c). Here we respond to each item individually:\n(1) There are minimal differences in gel formation (including extent of formation, quantity, gel pore size) according to slag type (i.e., Mg content of the slag), with relevant literature and information now given in the revised manuscript (see responses #1 and #2)\n(2) See our response to (1).\n(3) We have already outlined the carbonation process for each gel type extensively in our prior publications (A. Morandeau (2015) https://doi.org/10.1021/acs.chemmater.5b02382, S. Wang et al. (2020) https://doi.org/10.1016/j.cemconres.2020.105979, and E. McCaslin et al. (2021) https://doi.org/10.1016/j.cemconres.2021.106454), including the role of Mg, and outline succinctly this process again in the Introduction of this manuscript, with the text reproduced below.\n“Our previous research39-41 found that a high Mg AAS facilitates formation of amorphous calcium carbonate (ACC) during accelerated carbonation, where we hypothesized that the capability of Mg to stabilize ACC45,46 leads to improved carbonation resistance in AAS. It has been shown that Mg stabilization of ACC is due to the higher solvation energy of Mg2+ compared with Ca2+,47 making it harder for dehydration to occur for transformation into anhydrous crystalline CaCO .48 With 3 respect to AAS, since ACC is less thermodynamically stable than crystalline CaCO ,49 3 the higher solubility product increases Ca2+ concentration in the pore solution which lowers the driving force for C-(N)-A-S-H gel decomposition.39 Because of this 12 difference in ACC stability according to availability of Mg, we have observed that high Mg AAS undergoes less pore structural change upon accelerated carbonation.41”\n(4) The analysis of the PDF-CT data, as already carried out in the manuscript, shows the phase composition and distribution of the resulting calcium carbonate at the ~10 microns scale, while nano-XRF data provide insight on composition (i.e., Ca/Si) and distribution at a scale below 1 micron. These data were discussed extensively in the original manuscript, particularly in the context that Mg and Si stabilize ACC, with more disorder in the carbonate-containing regions for carbonated high Mg AAS (compared with the low Mg counterpart).\n(a) We now discuss initial gel compositions of the C-(N)-A-S-H gel at the start of the Supplementary Information, as mentioned in Reviewer 2 response #1.\n(b) We now discuss the water content before and after carbonation, as mentioned in Reviewer 2 response #3. See response #3 for our explanation as to why we refrain from including a detailed discussion of the role of water in the carbonation process in this manuscript.\nMoreover, we have added the following text to the end of the Discussion so it is clear to readers that the findings are dependent on the humidity level:\n“However, it is important to note that humidity is known to play a critical role in the carbonation of cement-based materials,75,76 and as such, these implications should be reassessed across a range of humidity levels to determine their extent of relevance.”\n(c) We already discuss the role of Si and Mg in the carbonation process throughout the manuscript, including toward the end of the first paragraph in the Discussion section:\n“This quantification of nanocrystalline size supports the hypothesis that carbonation resistance in AAS is linked with the formation of a more disordered (i.e., amorphous) calcium carbonate.39 Since ACC is metastable but less thermodynamically favorable than crystalline calcium carbonate,49 the equilibrium of dissolution favors a higher concentration of Ca2+ in solution where ACC is present, which reduces the driving force for decalcification of the C-(N)-A-S-H gel. PDF-CT data showed that both magnesium and silicate are linked with the formation of ACC, although no direct evidence of magnesium incorporation was found in the PDF data due to resolution limitations.”\nI have read with great interest the paper by McCaslin et al on the carbonation of Mg-bearing slag samples. The authors have used a panel of advanced techniques with ~micron resolution (better for XRF) to describe the textural information of the samples before and after carbonation, including detailed spatially-resolved structural information thanks to the use of PDF-CT and XRF. The use of PDF allows interpreting the local order of the disordered materials in the samples.\nThe study is of great interest because described in detail structural details that were not described in the literature before, including the intermixing of phases (at the resolution level). This is great new information. However, the claims about some of the carbonation textures and processes are not that evident from the data. I include some comments below.\nIn my opinion, the paper could fit Nat Comm if the authors are more precise about the carbonation process, the textures, and provide more evidence for some raised points below.\nWe thank the reviewer for their thoughtful comments and interest in the article. Here are our responses to the comments:\nRegarding more details on the carbonation process, we have augmented the manuscript to include more details in light of comments from Reviewer 2 in addition to other changes made below in response to the numbered comments from Reviewer 3.\nRegarding carbonation texture, it is unclear exactly what the reviewer is referring to here. Nano- XRF findings? If so, we have added additional nano-XRF data to the manuscript, which reinforces that the carbonated high Mg AAS particles suffer less decalcification compared with the carbonated low Mg AAS particles.\n1. Lines 106 and others: what do the authors mean by ‘tetrahedral atoms’?\nIn lines 106 and others, we have clarified that ‘tetrahedral’ atoms are ‘tetrahedrally bonded’ atoms. The text at line 106 (now line 122) now reads as:\n“The peaks at 1.63 Å and 2.37 Å correspond to tetrahedrally bonded Si and AlIV atoms (labeled T) bonded with oxygen, and Ca-O bonds, respectively.63 The peak at 3.15 Å represents the correlation (i.e., distance) between two T atoms (T-T correlation) in addition to a correlation between calcium and tetrahedral atoms (Ca-T), while the peak at 3.65 Å predominantly contains contributions from a Ca atom and a tetrahedrally bonded atom (Ca-T) along with those from two calcium atoms (Ca-Ca).64,65”\n14\n2. Figure 1. The three PDFs shown in (d) are a bit difficult to interpret, due to the overlapping at low r, I suggest separating them more. Also, the PDF from ‘+’ seems to show more long-range order (small peaks maybe also visible in the diffraction pattern). This is not discussed in the text.\nWe have updated Figure 1d to separate the lines and mentioned the longer-range order of ‘+’ in the text as suggested:\n“[…]although the ‘+’ voxel appears to have some longer-range order.”\n3. Lines 108-110: This means that Mg is not visible by PDF using these settings.\nRegarding lines 108-110, the reviewer is correct that the experiment setup is such that the small Mg-O peak typically observed in bulk PDFs of AAS (see for example, A. Morandeau (2015) https://doi.org/10.1021/acs.chemmater.5b02382) is not visible with the PDF-CT data. In fact, when a correlation is present at ~2.0 Å this is attributed to both Mg-O (if Mg is present) and octahedrally bonded Al atoms with oxygen (making differentiation between Mg-O and AlVI-O difficult).\n4. Lines 290-291, XRF results: It is not clear from the images that the Ca-rich regions are located on the exterior of the Si-rich regions. Also, in the following sentence, on what is based the attribution?\nIn lines 290-291, the reviewer is correct that it is not immediately obvious in the context of Figure 7 and Figure 8 that the Ca-rich region is on the exterior of the Si-rich region, so we have clarified that the Ca-rich regions are observed to be adjacent to the Si-rich regions, which is observable in particle A and the upper right section of particle D (which used to be particle C in the original manuscript). We have also reworded the following sentence so it’s clear that analysis of Figure 9 leads to attribution of calcium carbonate and decalcified gel:\n“Regions high in Ca are attributed to calcium carbonate phases, while the regions with more Si are attributed to decalcified C-(N)-A-S-H gel, as discussed in more detail below by analyzing Figure 9.”\n5. Lines 308-310: The authors claim that the observed low Ca/Si in the interior of the low- Mg slag particle support the ‘evidence’ that high Mg AAS resists better decalcification. One could wonder about the statistical significance of this observation, based here on one particle.\n6. Lines 345-347. It is difficult to get from the data the evidence that the CaCO3 regions were encasing carbonate-deficient cores. Could the authors provide more evidence on this textural information?\nIn lines 345-347, it has now been made clearer what we are referring to in the Discussion regarding CaCO regions encapsulating carbonate deficient cores in PDF-CT. 3 We have updated the text to the following (now line 377-382):\n“After exposure of young AAS pastes synthesized with high and low Mg slag to 100% dry CO , the PDF-CT data contained amorphous calcium carbonate (ACC) regions intermixed 2 with decalcified gel (i.e., aluminosilica gel) (see yellow regions in Figures 4b and 4c and corresponding colored regions in Figure 6 for enhanced clarity). These regions were found to encase carbonate-deficient cores consisting of slightly decalcified partially- reacted slag (blue and magenta, particularly evident in the lower half of Figure 4b).”\n7. Lines 350-352. Is this difference (1.08 vs 1.22 nm) significative? What is the uncertainty? I am very skeptical about this being significative.\nIn lines 350-352, given the particle size is averaged from the model in Eq. 1 being applied to the PDFs in several hundred voxels, we consider the difference observed to be significant, albeit small. In fact, the difference is visually apparent as seen in Figure S6. Hence, although subtle, this size difference is complementary to other acquired data that support the hypothesis that Mg improves the carbonation resistance of AAS via stabilization of highly disordered ACC.\n8. Methods. Is the difference in Mg content between the two samples ‘high enough’? to expect differences in the carbonation behavior?\nYes, we do consider the difference in Mg content (7.3% vs 12.7%) to be sufficiently high. We refer the reviewer to comparisons of MgO content conducted by others, such as Ben Haha et al. https://doi.org/10.1016/j.cemconres.2011.05.002 (7.7-13.2% MgO) as well as results from our previous studies of the same slags (S. Wang et al. and E. McCaslin et al linked above).\n16"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I consider the revision adequate. The authors have carried out a competent work and I now recommend publication as it is.", "reviewer_2": "The manuscript has been improved by considering and reflecting the reviewer's comments.\nNow manuscript fits the journal publication.", "reviewer_3": "My comments have been addressed. The manuscript can be accepted."}, "author_response": ""}]

Decision letter