A recent study published in Scientific Reports outlined a breakthrough in the chemical conversion of kaolinite, a two-dimensional (2D) material, into a three-dimensional (3D) amorphous structure. Found in clays like kaolin and metakaolin (MK), kaolinite has a naturally layered structure. This conversion was achieved thanks to kaolinite’s high surface-to-volume ratio and chemical reactivity.
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Background
Clays are layered structures made of sheet-like monolayers, with one dimension typically in the nanometer range. Due to their abundance and affordability, they are widely used in applications such as pollution control agents, biocompatible composites, and construction materials.
Kaolinitic clays are aluminosilicate precursors. When exposed to alkali activation, they form a 3D gel, creating a robust material with high mechanical strength. The sheet-like structure of kaolinite is often exfoliated—through intercalation, chemical, or physical methods—to produce nanoscale sheets.
Calcination, acid leaching, and ultrasonic dispersion can also be applied to raw kaolin to create modified kaolinite nanolayers, which are used in applications like drug delivery. This study explored the relationship between the structure and bonding energetics of kaolinite, MK, and alkali-activated derived materials (AAMs).
Methods
The AAM binder used in this study consisted of MK, composite liquid alkali silicate, and small amounts of silica fume and slag. The mixture contained 33.5 wt.% MK, 52 wt. % liquid alkali silicates, 7 wt.% silica fume, and 7.5 wt. % slag.
The mortar mix was prepared using the AAM binder combined with river sand aggregates in a 1:3 volume ratio. This mortar was cast into prismatic molds (15 × 15 × 30 mm) and cured at room temperature for 28 days.
MK and AAM were exfoliated from the prepared samples using a modified liquid biphasic exfoliation system that utilized water and dichloromethane (DCM) as the exfoliation media.
The MK precursor and AAM samples were characterized using field emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDX). Additionally, the samples underwent analysis through X-ray diffraction (XRD) and wide and small-angle X-ray scattering (WAXS and SAXS, respectively).
Elemental analysis was conducted on the specimens to quantify carbon, nitrogen, hydrogen, sulfur, and oxygen (CNHS-O) using X-ray photoelectron spectroscopy (XPS) and an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES). Their morphology was further examined using atomic force microscopy (AFM).
Finally, density functional theory (DFT) calculations and classical molecular dynamics simulations were performed on modeled kaolinite and MK to investigate the mechanisms underlying their structural transformations.
Results and Discussion
After mixing MK with an alkaline solution, the formation of AAM followed four key steps:
- Dissolution: MK dissolved in the alkaline solution, releasing silicon (Si) and aluminum (Al) species and converting Al(V) and Al(VI) into Al(IV).
- Reaction: The released Si and Al species reacted with silicates in the solution, forming aluminosilicate oligomers.
- Reorganization: The oligomers reorganized through gelation and polymerization to form a gel-like, amorphous phase.
- Polycondensation: This phase underwent polycondensation, producing a structurally stable 3D matrix, influenced by temperature and composition.
The inclusion of minor amounts of silica fume and slag significantly enhanced the material’s compressive strength. ICP-OES analysis revealed a Si:Al mass ratio of 0.38 for the MK precursor, which increased to 5.8 for the AAM—a direct effect of the slag addition. SAXS data indicated primary particle sizes of approximately 20 nm for MK and 5 nm for AAM. Additionally, WAXS results showed lower intensity Bragg reflections for AAM compared to MK, reflecting its more amorphous nature.
FESEM analysis revealed distinct structural differences:
- MK: Featured loosely stacked, disordered sheets with a wide size distribution.
- AAM: Displayed a more compact layered structure embedded within an amorphous phase, resembling a sintered material.
The brittle structure of MK tended to break into a random sheet size distribution, whereas AAM preserved fewer but larger sheets with thinner profiles. AAM approached a monolayer structure while maintaining a large lateral size, even after exfoliation. The amorphous phase of AAM appeared to break down further during this process.
Concluding Remarks
The researchers successfully demonstrated the conversion of kaolinite, a crystalline 2D material, into a 3D amorphous material with short-range 2D order through a chemical reaction in an alkaline medium. This transformation, driven by the high surface chemical reactivity of the 2D material, relied on a combined order-to-disorder mechanism.
However, the chemical reaction was self-limiting, resulting in a complex structure that was intermediate between a 3D glass and a 2D crystal. In this intermediate state, nanoscale 2D crystalline grains were embedded within an amorphous 3D structure exhibiting short-range order.
The researchers observed that the material produced through this process could be used as green cement. Unlike traditional Portland cement, it does not emit CO2 during its structural transformation, offering an environmentally friendly alternative.
Journal Reference
Carrio, JAG., et al. (2025). From 2D kaolinite to 3D amorphous cement. Scientific Reports. DOI: 10.1038/s41598-024-81882-1, https://www.nature.com/articles/s41598-024-81882-1
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