In the previous paper concerning with Na
2O-containing phase in portland cement clinker, it was indicated that the X-ray diffraction pattern of the ternary compound of Brounmiller and Bogue to which the composition NC
8A
3 is assigned, is not correct, and the prismatic dark phase in clinker of the systems C-A-F-S-M-N-(SO
3) is the metastable phase of NC
8A
3, containing small amount of SiO
2.
Successively, some studies of K
2O-containing phase in portland cement clinker have been made. From the microscopic observation and powder X-ray diffraction, the following results have been obtained.
a. KC
23S
12 is not a ternary compound, but a stabilized α′-C
2S by the formation of substitutional solid solution of K
2O in C
2S.
b. The prismatic dark phase in preparation of the systems C-A-(F)-S-K is the metastable phase of KC
8A
3, containing small amount of SiO
2 as solid solution or an integral part of the unit cell structure.
Accordingly, the fact that the addition of increasing amounts of K
2O to a raw mixture results in progressively increased amounts of free CaO in clinker is not only due to the formation of substitutional solid solution of K
2O in C
2S, but also the following reaction:
3C
3A+K
2O=KC
8A
3+CaO
Furthermore, the manner in which K
2O is combined in portland cement clinker has been discussed.
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