Abstract:
The energetics of cast calcium sulfate dihydrate-aluminium thermites was explored and its
use as a potential metal-cutting tool was investigated. Thermite is a pyrotechnic composition
that undergoes a highly exothermic reaction that burns relatively slowly. It is often used in
cutting, welding and incendiary devices. Consolidation of thermite by casting was chosen to
enable control of the burning front. The base case thermite comprised 60 wt-% calcium sulfate
dihydrate oxidiser and 40 wt-% aluminium fuel. Addition of additives were considered for their
effect on the cast thermite’s setting time, density, surface temperature, reaction products and
burn rate. EKVI and FactSage thermodynamic simulations were used to determine optimum
compositions for the various systems. The thermite powder compositions were sieved before
mixing with water and casting in a mould. The casts were allowed to set for 3 days to form
calcium sulfate dihydrate-aluminium compositions. The copper sulfate pentahydrate additive
was found to significantly decrease the setting time of the casts. The heat of hydration of the
base case was 59 ± 8 J g−1
. The compressive strength reached 2.9 ± 0.2 MPa, the open air
burn rate was 12.0 ± 1.6 mm s
−1 and a maximum surface temperature of 1370 ± 64 °C was
recorded using a pyrometer. Bomb calorimetry indicated an energy output of
7.96 ± 1.07 MJ kg−1
, slightly lower than predicted by the EKVI simulation. The density of the
castings was varied by either adding hollow sodium borosilicate glass spheres or by adding
excess water. The glass spheres resulted in a burn rate that decreased nonlinearly with
decreasing cast density. The excess water made no changes to the burning, except for
increasing the burn rate of the copper sulfate pentahydrate-based thermite. Calcium sulfate in
the casts was also dehydrated by thermal treatments at 155 °C and 200 °C. This resulted in
significant increases in the burn rate due to the porosity created by the evaporation of the
hydration waters. Castings that were thermally treated in an oven at 155 °C were successful
in puncturing part of an aluminium block in confined burn tests. A hole with a diameter of
~13.6 mm and depth of ~7 mm was produced. It is recommended that the composition with
copper sulfate pentahydrate be used as a binder in further tests.
