This study investigates the effects of three salt-lake by-product–derived and two magnesite-derived MgOs on the compressive strength and depth-dependent carbonation behavior of reactive magnesium cement (RMC). Mineralization products, morphology, and pore structure were characterized to reveal reaction mechanisms. The results show that salt-lake by-product–derived MgO possessed a high specific surface area and low to medium reactivity, forming abundant plate-like Mg(OH)2 and amorphous hydration products that filled pores and disrupted connectivity. Although substantial surface carbonation was observed with nesquehonite as the dominant phase, CO2 diffusion toward the interior was limited. Carbon analysis confirmed this strong carbonation gradient, with CO2 uptake decreasing from 5.62–8.82 wt% near the surface to only 0.70–1.37 wt% at a depth of 16–20 mm. In contrast, magnesite-derived MgO exhibited higher reactivity and lower specific surface area. XCT analysis further revealed a pronounced surface-to-interior porosity gradient in salt-lake-derived RMC, whereas magnesite-derived RMC exhibited a uniformly dense microstructure throughout the specimen. These results demonstrate that MgO source governs carbonation pathways, pore evolution and strength development.
Sodium iron silicate (NFS) samples were synthesised at initial Fe/Si molar ratios of 0.3 and 0.8 and equilibrated at 50 °C for up to 180 days to elucidate reaction pathways in high-Fe alkaline environments. The resulting solid phases comprised siliceous ferrihydrite, sodium‑iron-silicate-hydrate (N-F-S-H) and nontronite. N-F-S-H was isolated via selective dissolution using citrate-bicarbonate-dithionite (CBD). Its features include a broad X-ray diffraction (XRD) hump centred at ~38° 2θ, a SiOFe stretching band at 965 cm−1 in Fourier transform infrared spectroscopy (FTIR), and a mass-loss feature at ~130 °C corresponding to water release. Mass balance calculations identified the early-age (7 and 28 days) N-F-S-H composition as (Na2O)0.3 (Fe2O3)0.4–0.5(SiO2)·0.7 H2O. Identifying these secondary, poorly crystalline phases preceding nontronite formation advances the understanding of FeSi reaction pathways. These insights are highly relevant for Fe-rich blended cements, nuclear waste disposal, and reinforced concrete.
Strätlingite (Ca2Al2SiO2(OH)10·2.25H2O), a mineral rarely found in nature, which is believed to be responsible for their long-term stability due to its lamellar microstructure that hinders crack propagation along the interfaces between the constituents of concrete. However, the identification of the mineral during early hydration times is often complicated by low crystallinity and absence of a reliable disorder-free structural model. Published crystal structures of strätlingite often contain positionally-disordered atoms in both brucite-like layer and half-occupied double-tetrahedra (DT) layers. In this work we reinvestigate the average structure of strätlingite combining bulk high resolution powder X-ray diffraction (XRPD) and 3D electron diffraction (3D-ED), demonstrating that the R3¯ space group describes the brucite-like layer of strätlingite more appropriately than R3¯m, without positional disorder of the oxygen atoms. Partial occupation in the DT-layer is addressed via solid state nuclear magnetic resonance (ssNMR) and pair distribution function (PDF) analysis, showing that the condensation of Si(Al)O4 tetrahedra is best described by a Q3 configurational arrangement in the form of deformed double six-membered rings, exhibiting local displacements within the ab-plane.
This study investigates the oxidation of iron sulfides in concrete aggregates under conditions simulating accelerated mortar bar test (AMBT) protocols. An aggregate bearing both pyrrhotite and pyrite was exposed to NaOH (80 °C), NaCl (50 °C), and bleach (23 °C) solutions for 30 days. The chemical-mineralogical evolution was tracked using in situ techniques: micro X-ray fluorescence (μXRF), micro X-ray diffraction (μXRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA); and by bulk analyses such as X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), and total sulfur analysis.The results indicate an apparent sulfide reactivity order of pyrrhotite > framboidal pyrite > cubic pyrite. μXRF showed local sulfur depletion of 98% at the sample surface after bleach exposure, against 46% and 38% after NaOH and NaCl, respectively. Over the same period, the bulk sulfur content of the powdered aggregate decreased by approximately 27% in bleach, while remaining unchanged in NaOH and NaCl. The two measurements therefore describe different quantities: the oxidation attained at an exposed sulfide surface and the sulfur effectively removed from the aggregate. The alkaline solutions were neutralized from pH 12.5 to 7, consistent with acid generation by sulfide oxidation, which may in turn have limited the alkali-silica reaction (ASR) during the test period.These results indicate that the oxidizing capacity of the immersion solution, rather than its pH alone, governs sulfide oxidation, and identify solution stability as parameters worth considering in the optimization of accelerated test protocols.
