Experiments have been made of the influence of the chemical composition of caustic-burned magnesites upon the shrinkage occurring in dead-burning and also that of the fineness of the dead-burned magnesites and moulding pressure upon the quality of the refractories.
The magnesites produced at six important mines near Daisekkio were calcined and small discs were made of the products with or without addition of silica, ferric oxide, alumina, lime. chromic oxide, or manganese dioxide. Then they were fired to several temperatures ranging from cone 8 to 27 and the fire-shrinkage was measured.
A mixture of Seizanhai magnesite 100, ferric oxide 1.5, and manganese dioxide 0.5 was used for the experiments of fire-shrinkage and moulding pressure. It was dead-burned to cone 26 and then ground in 3 grades to prepare fine, medium and coarse powders. Small cylindrical test pieces were formed of each powder with a pressure of 300kg per sq. cm and were fired to cone 20. Similar test pieces were moulded with pressures varying from 50 to 1500kg per sq. cm and were fired to cone 20. The test pieces were examined for the softening behavior under a load of 2kg per sq. cm, the resistance to slaking action of steam at 134°C, and the resistance to spalling.
The results can be summarized as follows: -
(1) Fe₂O₃ accelerates the fire-shrinkage most strongly. Al₂O₃ is active at cone 20 and up. Mn₂O₃ is also effective. A small addition of CaO is probably effective at cone 27 and up. Cr₂O₃ acts to reduce the shrinkage very strongly. The shrinkage is also reduced with an increase of SiO₂.
(2) The finer grinding lowers the softening temperature and elevates the temperatures at which the test pieces submit to the pressure 0.5% or more in height. An increase of the moulding pressure elevates the softening temperature considerably.
(3) The resistance to the slaking action is not markedly influenced by the fineness. An increase of moulding pressure favors the cracking tendency.
(4) An increase of the pressure intensifies the resistance to spalling.

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