Shirasu is a naturally occurring volcaniclastic deposit found extensively across southern Kyushu, Japan, particularly in Kagoshima Prefecture. Derived primarily from pyroclastic flow deposits associated with the Aira Caldera eruption approximately 29,000 years ago, this volcanic resource consists mainly of glass-rich particles. Its low density, porous structure, chemical stability, and heat resistance have generated interest in applications ranging from construction materials to functional fillers.
Although large reserves are locally available, Shirasu has historically created challenges for agriculture, civil engineering, and land development because of its high water permeability and limited nutritional value for plant growth. Research into higher-value uses is helping reposition the material as a potentially useful industrial resource. Investigated applications include cement admixtures, lightweight aggregates, porous adsorbents, thermal insulation materials, and fillers for composite materials.
Shirasu Balloons are hollow particles produced by heating Shirasu to temperatures above 1000°C. Expansion associated with gas inclusions creates an internal void structure and changes the morphology of the original particles. The resulting characteristics, including low density, thermal insulation, and fire resistance, make Shirasu Balloons relevant to the development of lightweight composites and functional inorganic fillers.
Understanding this transformation requires more than a measurement of chemical composition. Particle size and particle shape can affect how a powder flows, packs, and performs within a finished material. Morphological variation may also provide information about the consistency of the thermal expansion process. Quantitative particle analysis is therefore valuable for process optimization, product quality control, and the development of new applications.
This application note examines raw Shirasu and Shirasu Balloon samples sourced from Kagoshima Prefecture using the Morphologi 4 automated imaging system. Dry dispersion with the integrated Sample Dispersion Unit enables individual particles to be presented for imaging. More than 20,000 particles were analyzed for each sample during the particle size comparison, providing a high-statistics view of the two populations.
Circle Equivalent Diameter is used to compare particle size distributions, while High Sensitivity circularity and aspect ratio are used to investigate particle shape. Circularity indicates how closely a particle approaches a perfect circle. Aspect ratio helps distinguish isotropic particles from more elongated forms. Recorded particle images and two-dimensional scatter plots provide complementary ways to examine relationships between these shape parameters.
The analysis demonstrates that particle size alone does not fully describe the physical effects of Shirasu expansion. The raw and expanded materials show similar particle size distributions, indicating that thermal expansion has a limited effect on overall particle size. Their shape distributions, however, show clear differences. Shirasu Balloons have higher circularity values than raw Shirasu, quantitatively confirming spheroidization during expansion. They also display a broader shape distribution, which suggests variation in expansion behavior among individual particles.
These measurements can support several practical objectives:
The document is relevant to scientists, materials developers, process engineers, quality control specialists, and researchers working with volcanic resources, mineral powders, lightweight aggregates, inorganic fillers, or composite materials. It also provides an example of how automated image analysis can distinguish morphological changes that may not be apparent from particle size measurements alone.
Register to access the full application note, including the measurement conditions, particle size data, morphology comparisons, particle images, and two-dimensional shape-correlation results for raw Shirasu and Shirasu Balloons.
Shirasu is a volcaniclastic deposit widely distributed throughout southern Kyushu, Japan, and is primarily derived from the Ito pyroclastic flow deposits associated with the Aira Caldera eruption approximately 29,000 years ago. It consists predominantly of volcanic glass and is known for its low density, porous structure, chemical stability, and excellent heat resistance. [1][2]
Enormous reserves of Shirasu occur throughout southern Kyushu, particularly in Kagoshima Prefecture. Historically, it has been often regarded as a constraint for agriculture, civil engineering, and land development due to high water permeability and low nutritional value for plant growth. In recent years, however, increasing attention has been directed toward the effective utilization of locally available natural resources to support a circular economy and reduce environmental impact. As a result, research and development efforts aimed at high-value applications of Shirasu have expanded significantly. [3]
A wide range of industrial applications for Shirasu have been explored, including cement admixtures, lightweight aggregates, porous adsorbents, thermal insulation materials, and functional fillers. Consequently, Shirasu has attracted considerable interest as a means of effectively utilizing an underexploited mineral resource. [3][4] Among these applications, Shirasu Balloons, produced by heating Shirasu at temperatures above 1000°C leading expansion of gas inclusion, are particularly noteworthy (Figure 1). These hollow particles exhibit low density, thermal insulation, and fire resistance owing to their internal void structure, making them attractive for lightweight composites and functional inorganic filler applications. [3][5][6]
![[AN260916-figure1.png] AN260916-figure1.png](https://dam.malvernpanalytical.com/b6d1a2f8-18a4-4627-92f3-b4c700534bbf/AN260916-figure1_Original%20file.png)
The performance characteristics of both Shirasu and Shirasu Balloons depend not only on their chemical composition but also on their particle size distribution and morphology. Quantitative assessment of shape changes and particle uniformity resulting from the expansion process is therefore important for process optimization, quality control, and the development of new applications. However, studies directly comparing the particle size and shape distributions of raw Shirasu and Shirasu Balloons using a single high-statistics analytical technique remain limited.
In this application note, the Morphologi platform was used to characterize the particle size distribution and morphological properties of Shirasu and Shirasu Balloons sourced from Kagoshima Prefecture. The objective was to provide fundamental insights into the high-value utilization of this volcanic resource.
The particle size distributions of raw Shirasu and Shirasu Balloon samples were evaluated using the Morphologi 4 automated imaging system under dry dispersion conditions using the integrated Sample Dispersion Unit (SDU). Measurements were performed using a 5× objective lens using diascopic illumination, and more than 20,000 particles were analyzed for each sample.
Volume-based Circle Equivalent Diameter (CED) distributions are shown in Table 1 and Figure 2. Both materials exhibited similar particle size distributions.
| Sample | # of particles | Dv10 (μm) | Dv50 (μm) | Dv90 (μm) |
|---|---|---|---|---|
| Shirasu | 21466 | 41.02 | 71.41 | 115.81 |
| Shirasu Balloon | 25979 | 33.63 | 80.02 | 150.22 |
![[AN260916-figure2.png] AN260916-figure2.png](https://dam.malvernpanalytical.com/a9e01a0f-dea0-44b8-b8fa-b4c700534a3b/AN260916-figure2_Original%20file.png)
Morphological characteristics of raw Shirasu and Shirasu Balloons were subsequently compared. Shape analysis was conducted on particles larger than 30 μm, comprising 11,802 particles for the Shirasu sample and 6,015 particles for the Shirasu Balloon sample.
Results for High Sensitivity (HS) circularity, where a value of one equals a perfect circle, are presented in Figure 3. Shirasu Balloons exhibited higher circularity values than raw Shirasu, confirming spheroidization during the expansion process. Aspect ratio distributions (Figure 4), where a value closer to zero equates to an elongated particle, showed a similar trend, with a greater proportion of highly isotropic particles observed in the Shirasu Balloon sample.
![[AN260916-figure3.png] AN260916-figure3.png](https://dam.malvernpanalytical.com/0c0dd427-b697-42ff-ac88-b4c70053490c/AN260916-figure3_Original%20file.png)
![[AN260916-figure4.png] AN260916-figure4.png](https://dam.malvernpanalytical.com/3d88e58f-214a-4e49-b05f-b4c70053475a/AN260916-figure4_Original%20file.png)
Relationships between circularity and aspect ratio were further evaluated using the recorded particle images and two-dimensional scatter plots (Figure 5a,b). Visual inspection revealed clear morphological differences between the two materials. The scatter plots showed a relatively strong correlation between HS circularity and aspect ratio for raw Shirasu particles. In contrast, Shirasu Balloons exhibited a shift toward higher circularity values together with a broader shape distribution, likely reflecting variations in particle expansion during thermal foaming.
![[AN260916-figure5.png] AN260916-figure5.png](https://dam.malvernpanalytical.com/ff7ae55d-5403-4fc2-bed6-b4c700534614/AN260916-figure5_Original%20file.png)
Because particle morphology influences flowability, packing behavior, mechanical properties, and thermal performance, quantitative evaluation of these shape changes is valuable for both quality control and application development of both Shirasu and Shirasu Balloon materials.
Particle size and morphological characteristics of Shirasu and Shirasu Balloons were compared using automated image analysis. Results showed similar particle size distributions for both materials, indicating that thermal expansion had only a limited effect on particle size. In contrast, Shirasu Balloons exhibited increased circularity, quantitatively demonstrating particle spheroidization during expansion. Shape distributions were also broader than those of raw Shirasu, suggesting variability in expansion behavior among individual particles.
These findings demonstrate that particle imaging analysis provides an effective means of quantifying morphological changes associated with Shirasu expansion and offers valuable information for quality control and development of advanced materials based on Shirasu Balloons.