The DTA curves of the samples are compared in Fig. 5. It can be seen
that the glass transition temperature, Tg, decreases from 753 °C for sample
×0 to 726 °C for sample ×12. The crystallization temperature is almost
unchanged. However, the crystallization peak becomes weak
upon increasing ZnO. The DTA curves obtained with different ramping
rates and the resultant ln(Tc2
/β) against 1/Tc plots are not shown
here. Generally, the larger the ramping rate was applied, the stronger
the crystallization peak was. The E values calculated according to the
formula (1) are shown with the content of ZnO in Fig. 6. The data indicate
that the initial inclusion of 3.41 mol% of ZnO, E is almost unchanged.
When the content of ZnO is increased from 3.41 to 9.59 mol% increases significantly from 225 to 356 kJ/mol. However, E decreases
to 301 kJ/mol when ZnO is further increased to 12.39 mol%.
The thermally treated samples show surface crystallization as most
calcium-aluminosilicate glasses [20–21]. The XRD patterns of the heattreated
samples are presented in Fig. 7. The main crystalline phases
are ferroan wollastonite (Ca2.87Fe0.13(SiO3)3, PDF card No. 83-2198)
and silicon oxide (SiO2, PDF card No. 77-1060). In addition, there is a decreasing
amount of pure wollastonite (CaSiO3, PDF card No.75-1396)
with the increase of ZnO.
Presented in Fig. 8 are the images of the pressed cylinders recorded
during the sintering process. It can be seen that the temperatures at
which the sintered bodies reach 1/3 and 1/2 of the original heights
(taken at 500 °C) decrease from ×0 to ×12. There is a significant difference
of these characteristic temperatures between ×6 and ×9.