A pure limestone and Iceland spar were shock calcined at several constant temperatures in the range 750 to 1300 C in a study of the pore structure of the calcines. The calcines were examined with a scanning electron microscope to follow the development of the pore structure and to correlate changes in the pores and in the related surface area with the properties of the lime and the temperature of calcination.
Limestone calcines prepared at temperatures below 1000 C had larger pores and lower surface areas than Iceland spar calcines prepared at the same temperature. Raising the temperature of calcination caused a decrease in surface area and an increase in pore size of both materials. The decrease in surface area on calcination at 1000 C resulted from the growth of large pores at the expense of smaller pores, but the physical characteristics of the lime were largely unchanged. At temperatures above 1000 C the calcium oxide crystals sintered and grew in size as the pore size continued to increase and the surface area decreased.
The larger initial crystallite size of the Iceland spar resulted in an unfavorable pore size distribution in its calcined products prepared below 1000 C. The limestone calcines prepared at the same temperatures had larger pores and smaller surface areas. Literature reports confirm that there is an optimum temperature of calcination for each stone for producing the proper surface area, pore distribution, and lime condition for maximum solid-fluid reactivity.