FTIR Characterization and Structural-Functional Analysis of Kaolin Powder for Thermal Insulation Applications

Abstract

Kaolin is an alumino-silicate mineral that occurs naturally, and has attracted attention as a suitable thermal insulation material because of its inherent structural stability and chemical functionality. Fourier Transform Infrared Spectroscopy (FTIR) characterization was performed to investigate the structural and functional properties of kaolin powder. The structural analysis was conducted using a Nicolet iS50 FTIR instrument and a Far-infrared spectrum (150–350 cm?¹), which indicated that distinct absorption bands attributed to Si–O and Al–O lattice vibrations were observed; the bands\' representation confirms that the structural integrity and ordering of the crystalline framework was maintained. These attributes are correlated to the inherent high temperature stability of kaolin and the ability to maintain stability under thermal induced stresses. The typical functional analysis was compared in the Low and Mid Far Infrared region (1000–7000 cm?¹); strong bands corresponding to –OH stretching, Al–OH bending, and Si–O stretching vibrations were observed. These functional groups are representative of the presence of surface hydroxyls, interlayer hydrogen bonding, and silanol groups, thus proving the functional benefits that will additionally enhance kaolin\'s low thermal conductivity, refractory properties, as well as compatibility with composite systems. By incorporating structural resilience with functional reactivity, kaolin has the potential to be applied in many applications including insulating bricks, refractory linings, kilns, furnaces and energy-efficient construction materials. All in all, the dual FTIR analysis demonstrates kaolin to be a durable, sustainable and a viable thermal insulation material for modern industrial and greener technology purposes.

Introduction

1. Introduction

Due to increasing demand for sustainable, energy-efficient, and cost-effective materials, naturally occurring minerals like kaolin are gaining attention. Kaolin (a hydrated alumino-silicate) is:

• Abundant, low-cost, and structurally stable

• Traditionally used in ceramics, paper, and paints

• Now being explored for thermal insulation and fireproofing in industrial and building applications

Its layered structure of alternating silica and alumina sheets gives it:

• Mechanical strength

• Chemical stability under heat

• Functional hydroxyl and silanol groups that allow compatibility with composites

2. Role of FTIR in Characterization

Fourier Transform Infrared Spectroscopy (FTIR) is used to study kaolin’s:

• Structural order (via lattice vibrations)

• Functional groups (–OH, Si–O, Al–O)

• Thermal behavior

FTIR distinguishes between ordered and disordered kaolinite and identifies changes during thermal treatment (e.g., dehydroxylation).

3. Methodology

• Sample: Untreated kaolin powder, low in moisture absorption

• Instrumentation: Thermo Scientific Nicolet iS50 FTIR spectrometer (SAIF, IIT Madras)

• Spectral Ranges Analyzed:

  • Far-infrared (150–350 cm?¹)

  • Low-to-mid infrared (1000–7000 cm?¹)

4. Results

A. Far-Infrared Spectrum

• Peaks at 197.6, 212.3, and 346.8 cm?¹ indicate:

  • Al–O and Si–O lattice vibrations

  • Presence of ordered crystalline structure

• Confirms thermal stability and structural integrity (consistent with earlier studies)

B. Low-to-Mid Infrared Spectrum

• 3620–3695 cm?¹: –OH stretching from inner-surface hydroxyls → contributes to thermal stability

• 1656 cm?¹: H–O–H bending from adsorbed/interlayer water

• 1030–1110 cm?¹: Si–O stretching → strong indicator of silica framework

• 750–950 cm?¹: Al–OH bending and Si–O–Al linkages

• >4000 cm?¹: Combination bands indicating complex vibrational interactions

These results support that kaolin has a stable silicate structure and high hydroxyl content, ideal for insulation and composite use.

5. Literature Support

• Studies confirm:

  • OH bands disappear on heating → phase change to metakaolin

  • Far-IR bands (~200–340 cm?¹) indicate crystalline order

  • FTIR reliably detects thermal transformations, modifications, and natural variation in clays

6. Implications for Thermal Insulation

? Advantages of Kaolin:

• Refractory strength: Retains structure at high temperatures

• Low thermal conductivity: Hydrogen bonding scatters phonons

• Composite compatibility: Reacts well with resins and cements

• Sustainable: Naturally available and environmentally friendly

???? Applications:

• Refractory linings

• Insulating bricks

• Energy-efficient building materials

• Polymer-clay composites

Conclusion

The FTIR characterization of the kaolin powder in the far-infrared and mid-to-low infrared range signifies an evident correlation between functional flexibility and structural stability of the mineral. In the far-infrared range (197–346 cm?¹), the highly defined and sharp nature of lattice vibrations signifies a very ordered alumino-silicate structure. Order in the structure is required for crystallinity, and hence the value of kaolin in refractoriness, mechanical strength, and resistance to high temperature with low phase transformations. In the low-to-mid infrared region (1000–3700 cm?¹), we can investigate the chemical functional groups of kaolin by characteristic hydroxyl stretching bands, silicate framework vibrations, and water bending modes. Octahedral aluminum is bonded to hydroxyl groups and plays a dual role, they stabilize the layered lattice through interlayer hydrogen bonding and are possible sites for reactive surface modification or composites with kaolin. The Si–O stretching vibrations verify the structural hardness of a framework of silica, allowing for low thermal conductivity and high durability, and the bending band of H–O–H demonstrates kaolin\'s inherent ability to retain moisture and how this interaction affects dehydration and thermal transformation processes. Together with the current literature, these consistently verify the widely reported FTIR signatures of kaolinite and hence endorse kaolin for industrial application. The three qualities of kaolin which are crucial - refractoriness, low thermal conductivity, and chemical compatibility with other materials - can now be merged with the abundance, low price, and environmental friendliness of kaolin to complement kaolin in green building technologies by offering advantage to sustainability and performance in energy-efficient building.

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Copyright © 2025 Atul Raj, Anurag Shrivastava, Rohit Shrivastava. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.