In continuing the previous studies (This Journal, 60 No. 668, p. 60-65 and No. 676, p. 422-425 (1952); 61 No. 682, p. 152-154 (1953)), the present authors reported in the present paper, the results of further studies on the chromite-magnesia refractories by using chromite ore and special sea-water magnesia clinker obtained by dead burning special magnesium hydroxide contained a few percents of ferric oxide, which was specially obtained from dilute bittern solution by settling with slaked dolomite slurry from calcined dolomite with 3-5% limonite powder.
Among many comparative testings, the best result was obtained as one of the example, having, the following phisical properties and chemical compositions: True density: 3.86, Aparent density: 3.83, Volume density: 2.90, Perosity: 24.1%, Water absorption: 8.3%, Compressive strength: 420kg/cm², SiO₂: about 8%, Al₂O₃: about 12%, Fe₂O₃: about 15%, CaO: about 3%, MgO: about 32% and Cr₂O₃: about 32%. But these contents of SiO₂, Al₂O₃ and Fe₂O₃ are a little too large owing to the low grade of chromite ore, and the content of CaO is a little too large owing to the high lime content of sea-water magnesia clinker.

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The resistivities of various types of cement mortar to chemical attack by 2%-MgSO₄ soln were examined, with the results as follows:
1) Sufficient durabilities were shown in the cases of alumina cement, sulphate resistant and low-heat portland cement.
2) Blast-furnace cement together with silica cement, did not always show a higher durability than of normal and high early strength portland cement, Therefore, a special care should be paid for technics in manufacturing them.
3) The durabilities of portland cement, blast-furnace cement, and silica cement would be raised by increasing the amount of the additional gypsum or anhydrite newly up to 2-4% as SO₃.
4) In the specimens containing portland cement, and finely ground slag, the external part was severely cracked by dint of excessive expansion, while the internal part was kept in secure tightness more stable than the case of portland cement free from slag.
5) In order to improve the resistivity of blast-furnace cement, it seems desirable to raise the additional amount of each modifications of gypsum; to increase the slag content up to about 60%; to grind the slag more coasely than the case of normal cement, especially when the former is contained about 30%. Thus it becomes possible to keep the internal part of the specimen almost stable, as the result of the formation of a dense and stable protective layer over the surface.
6) The superior durability of slag gypsum admixture, such as “Thurament”, for various usages seems to cause partly in it being coasely ground and partly in its including gypsum.
7) Though the external part of the specimens containing gypsum slag cement was finely cracked, their internal part remained entirely unattacked, because of the altered layer being not so thick.

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