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Physical & Chemical properties

Particle size distribution (Granulometry)

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The typical particle size distribution for synthetic amorphous calcium silicate found in the presence of high shear forces ranged from approx. 4  to 9 µm, which appears to correspond to aggregated particles. For synthetic amorphous silica, the structure-analogous substance, the mean particle sizes were a factor of > 10 - ~80 higher under technical conditions representative of the situation encountered under normal and intended use, covering a range of about 200 to 600 µm, which indicates to agglomeration of particle aggregates (see Stintz 2003). 

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Particle Size: Definition and Dynamic Interactions:

With respect to particle size, the German standard DIN 53206 (1972) distinguishes between primary particles, aggregates and agglomerates.

Due to limited data about particle properties of calcium silicate itself, the data of SAS as structure-analogous substance are used in the following considerations:

Primary particles in the case of pyrogenic or precipitated SAS do not exist in isolation.

Aggregates are assemblies of primary particles which are grown together face-to-face in the form of chains or clusters. The aggregates are formed by the collision of primary particles during particle growth and/or by the further deposition of silica onto these aggregates.

Agglomerates are assemblies of aggregates, held together by strong physical adhesion forces.

Shear forces cause a breakdown of agglomerates as well as dilution tends to prevent the  formation of larger agglomerates due to the larger interparticle distances and low collision frequencies. Under the technical conditions of inhalation exposure of experimental animals, in general high shear forces dominate that form aerosols of small particle sizes with a high fraction of respirable particles. This is in marked contrast to the largely stressless settling conditions that prevail under normal handling and use of these materials.

The typical particle size distribution for synthetic amorphous precipitated silica, the parent substance to synthetic amorphous calcium silicate (CS), is between 1 and 250 µm.

Under normal handling and use of commercial products, the silicates in analogy to silica tend to form agglomerates of high mass median aerodynamic diameters that are not respirable (MMAD >>10 µm). The respirable fraction (=< 10 µm) comprises of less than 1 wt% for SAS (see Stintz 2003). Under experimental conditions, the MMADs may range significantly below 10 µm, the respirable fractions accounting for more than 80 %.

(see summary tables under "4. Endpoint summary": Physical chemical properties_Particle characteristics).

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Reference:

ECETOC (2006): Synthetic Amorphous Silica (CAS No. 7631-86-9). JACC Report No. 51, European Centre for Ecotoxicology and Toxicology of Chemicals, Brussels, September 2006