Recently, ceramic bodies containing talc or other magnesium silicates have been called much attention in the field of the electrical insulation owing to their low dielectric loss and high insulation resistance at high temperatures. The electrical resistance of Manchuria talc-Kato kaolin system was determined. The results can be expressed by the equation logρ=B+A/T°K, where ρ is the resistivity in MΩ.
The relation of the resistivity at 400°C with the composition of the bodies is represented by
logρ(400°C)=-0.0296k+4
where k is the percentage of Kato kaolin in the fired bodies.
Kato-kaolin contains 0.85% alkalies in the anhydrous state and according to the previous investigation (S. Moriyasu, J. Jap. Cer. Assoc., 1936, 44 (528), 856), the resistivity of the system Al₂O₃SiO₂ is very much influenced by the presence of alkalies. On the absence of alkalies, the resistivity of 3MgO⋅4SiO₂-Al₂O₃⋅2SiO₂ system should be approximately represened by
logρ(400°C)≅-ak+4
where the coefficient a is probably 0.01 or less.
A simple method of the determination of the resistivity less than 100MΩ (at 400°C) was described. (Elec. Tech. Laboratory, Ministry of Communication, Tokyo.)

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The presents author, in continuing the previous studies (This Journal, 1934, 42, 273, 628, 688; 1935, 43, 215, 572; 1936, 44, 22, 617, etc.), report here the results of various comparative tests on various kinds of cements on the market in the spring of this year, in which high siliceous mixed Portland cements were recently commenced to produce in large scale in some factories. The principal results of the present report were abstracted from the original Japanese paper, as following:
(1) Two series of cement samples were collected from the market and tested their chemical compositions, which are tabulated in the following table 1.
Table 1. Results of Analyses of Chemical Compositions
(2) These 2 series and 14 cement samples were nextly compared on some physical properties, fineness, specific gravity and time of setting, which results are shown in the following table 2.
Table 2. Results of Comparison of Physical Properties
(3) The strengths of mortars of these cement samples were tested by several ways, as following:
Table 3. Results of Strength Tests of 1:3-Cement Sanl Non-Plastic Mortar
(a) 1:3-cement sand non-plastic motrars, which are in both standards of the Japanese Engineering Standard for Portland cement (JES 28) and blast furnace slag cement (JES 29), were tested on their strengths and the results were obtained, in table 3,
(b) 1:2-cement fine sand plastic mortars, which were modified by the present author from the method of Prof. M. Ros in Switzerland or of Dr. G. Haegermann in Germany, were tested on their bending and compressive strengths by using the prismatic test piece (4×4×16cm).
(c) 1:1-cement fine sand plastic mortars were also tested on their bending and compressive strengths by the same size prisms, only different in the amount of water, i.e., 1:2-mortar of 65% water-cement ratio and 1:1-mortar of 45% water-ceweut ratio.
(d) 1:1:2-cement-fine sand-standard sand plastic mortars were also moulded with 50% water to cement and their bending and compressive strengths were also tested by using of the same size prismatic test pieces
All these results are tabulated in the following table 4.
Table 4. Results of Strength Tests of 1:2-, 1:1:2- and 1:1- Cement-Fine Sand or Cement-Fine Sand-Standard Sand Plastic Mortar
It is from these results clearly seen, that low-heat special Portland cements (No. 490 and C) were considerably low grade cements of lowest strength at short curing days (3 and 7 days), and on the contrary high silica mixed Portland cements (No. 492-494 and E-G) were remarkably high grade cements having largest strength already at short curing days.
(4) The author is now further studying on various points, i.e., resistance to sea water and aggressive salt solution, heat of hydration, expansion or contraction, etc., which will he reported in next papers.

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