Composition: Alumino silicate glass

Description: Hollow, grey free-flowing spheres

Physical properties:

Under an optical microscope, the appearance is of shiny, colourless spheres relatively close in size. Some darker spheres with iron impurities occur, the proportion varying with source.

Electron microscopy reveals most of the particles as near-perfect spheres. A few are disfigured by blisters possibly fly ash particles fused on to the surface of gas bubbles formed within the shell. Each individual size fraction exhibits a remarkably consistent average particle density, demonstrating that average wall thickness as a percentage of sphere diameter is essentially independent of particle size.

Comparisons are made with synthetic hollow glass spheres. Cenospheres are darker in colour and have a higher density. Their thicker walls result in higher crushing strengths so that cenospheres can be used in applications where synthetic spheres would break. Cenospheres also have a higher softening temperature – about 1,200oC compared with 600oC for the borosilicate glass, most commonly used in synthetic spheres.

The physical properties of both DT Cenosphere grades are presented in the table below:

Physical properties of DT Cenospheres
DT 300 TD 150
Particle size range microns 5 – 300 5 – 150
Average size (approx) microns 120 110
True particle density (S.G) gm / cc 0.60 – 0.65
Bulk density kg / m3 370 – 450
Particle form Hollow Spheres
Colour Grey
Hardness mohs 5 – 6
K value Wm-1 deg C-1 0.11
pH in water <6 – 8/td>
Melting point °C 1250
Sintering point °C 900
Internal pressure atmospheres 0.2
Chemical Analysis of DT-150 and DT-300
SiO2 55% – 65%
Al2O3 25% – 33%
Al2O3 25% – 33%
Fe2O3 1% – 6%
CaO 0.2% – 0.6%
MgO 0.5% – 2%
NaO2 0.5% – 2.%
K2O 1% – 2%

Commercial grades: 

Properties of finished product grades reflect the raw material properties and the degree of processing. Most attention is paid in processing to accurate control of maximum particle size. Because of the low proportion of small diameter spheres in the raw material and the difficulty of extraction, yields of fine grades are low and such fine grades are consequently expensive.

While normal grades are centred around average particle densities of 0.7 kilograms per litre, special grades with lower densities such as 0.5 kg/l and 0.3 kg/l may be produced.