Synthetic calcium carbonate improves the effectiveness of treatments with nanolime to contrast decay in highly porous limestone

Advanced mapping of inorganic treatments on porous carbonate stones by combined synchrotron radiation high lateral μXRPD and μXRF

Understanding the effects of consolidating inorganic mineral treatments on carbonate stones of cultural heritage, and on the nature and distribution of newly formed products within the matrix, poses a significant challenge in Heritage Science and Conservation Science. Existing analytical methods often fail to deliver spatial and compositional insights into the newly formed crystalline phases with the appropriate high lateral resolution. In this study, we explore the capabilities and limitations of synchrotron radiation (SR) micro-X-ray powder diffraction (μXRPD) mapping combined with micro-X-ray fluorescence (μXRF) to give insight into compounds formed following the application of ammonium oxalate (AmOx) and diammonium phosphate-based (DAP) solutions on porous carbonate stone. Ultimately, the integration of μXRPD mapping and μXRF analysis proved itself a powerful asset in providing precise qualitative and quantitative data on the newly formed phases, in the case of both calcium oxalates (CaOxs) and calcium phosphates (CaPs), and their complex stratigraphic distribution, thus opening a new route for applications to a more comprehensive study of inorganic treatments applied to carbonate substrates.

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam): A Novel Precursor of Calcium Oxalate Coating for Carbonate Stone Substrates

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam), synthesized from O-methyl-N-(pyridin-2-ylmethyl)oxamate, was spectroscopically and structurally characterized and assayed as a novel precursor for the protection and consolidation of carbonate stone substrates. An in-depth characterization of treated and untreated biomicritic limestone and white Carrara marble samples was carried out by means of SEM microscopy, X-ray powder diffraction, helium pycnometry, determination of water transport properties, and pull-off tests. The improved solubility (1.00 M, 16.5% w/w) of the title compound with respect to ammonium oxalate (0.4 M, 5% w/w) results in the formation of a thicker protective coating of calcium oxalate (CaOx) dihydrate (weddellite) on marble and biomicrite samples after the treatment with 5% and 12% w/w water solutions, producing a reduction in the stone porosity and increased cohesion. Theoretical calculations were carried out at the DFT level to investigate both the electronic structure of the N-(pyridin-2-ylmethyl)oxamate anion and the hydrolysis reaction leading from AmPicOxam to CaOx.

Electrochemical Capture and Storage of CO2 as Calcium Carbonate

A carbon dioxide capture, conversion, and utilization technology has been developed that can be powered by renewable energy with the potential to mitigate CO₂ emissions. This relies on an electrochemical process whereby the dissolution of carbon dioxide into carbonate ions is accelerated by a locally induced pH change at the cathode. The carbonate ions can then complex with metal cations, such as Ca²⁺ , Sr²⁺ , or Mn²⁺ , present in solution to form their respective metal carbonates, which precipitate out of solution. To ensure the cathode is not fouled by deposition of the insulating metal carbonate, the process is operated under hydrogen evolution conditions, thereby alleviating any significant attachment of the solid to the electrode. This process is demonstrated in CO₂ -saturated solutions while the possibility of direct air capture is also shown, where the precipitation of CaCO₃ from atmospherically dissolved CO₂ during electrolysis is observed. The latter process can be significantly enhanced by using 5 vol.% of monoethanolamine (MEA) in the electrochemical cell. Finally, the process is investigated using seawater, which is also successful after the initial precipitation of metal sulfates from solution. In particular, the use of renewable energy to capture CO₂ and create CaCO₃ while also generating hydrogen may be of particular interest to the cement industry, which has a significant CO₂ footprint.

Synchrotron radiation Ca K-edge 2D-XANES spectroscopy for studying the stratigraphic distribution of calcium-based consolidants applied in limestones

In Heritage Science, the evaluation of stone consolidation treatments by investigating the nature of in situ newly formed products and their penetration depth within the consolidated matrix is a grand challenge. A number of analytical methods have been proposed, but, currently, most of them are not able to supply a full overview of the spatial, structural and compositional information of the newly formed crystalline and amorphous phases with a submicrometric lateral resolution. Here, we examined, the capabilities of synchrotron radiation (SR)-based two-dimensional X-ray absorption near-edge structure (2D-XANES) spectroscopy at Ca K-edge for determining the structural and compositional properties of the compounds formed after the application of a calcium acetoacetate-based consolidant on a porous carbonatic stone (limestone) and for investigating their stratigraphic distribution at the submicrometric scale length. We evaluated advantages and drawbacks of three Ca K-edge 2D-XANES-based approaches: (i) transmission mode full-field-XANES (FF-XANES) imaging; (ii) micro-X-ray fluorescence (μ-XRF) mapping above the Ca K-edge combined with the acquisition of XRF mode μ-XANES spectra at a limited number of spots; (iii) full-spectral µ-XANES (FS µ-XANES) mapping in XRF mode and its variant called selectively induced X-ray emission spectroscopy (SIXES) mapping. Overall, Ca K-edge 2D-XANES spectroscopy provided accurate qualitative and semi-quantitative information on the newly formed calcium carbonates (i.e., amorphous calcium carbonate, vaterite and calcite) and their stratigraphic distribution at the submicrometric scale, thus opening a new scenario to study the carbonatation process of calcium-based consolidants in limestones.