False set of Portland cement is one of so complicated phenomena, that it has not been yet apprehended and controlled perfectlly.
In the present paper the authors explained the mechanism of false set of cement at the view-point of hypothesis based upon the hardening of hemihydrate contained in cement. Although its mechanisim now could not be fully clarified by this hypothesis, by introducing the idea of hardening time of hemihydrate the authors solved distinctly several features of false set, which had not been made clear.
That is, false set of cement only takes place, when the hemihydrate content is in the following certain amount in cement and its hardening occurs in certain period (1-5 min. after mixing by the test procedure of American Federal Specification).
From the authors' experimental series, it will be concluded as follows;
(1) In the specimens of powder prepared by grinding the silicious sand together with a small amount of hemihydrate, false set took place when the SO
3 content is above 0.5% in weight and above 1.2% initial set of cement also was very quick.
But in the same specimens, false set can be eliminated by prolonged mixing or by shortening the hardening time of hemihydrate with additional dihydrate, or by aerating at high humidity, -in the last case a portion of hemihydrate changes into dihydrate, and consequently the hardening of hemihydrate is so shortened, that false set can be eliminated by dead-mixing. On the contrary, false set is induced more severe by reducing the mixing time or by adding such reagents as alkali carbonates which can retard the hardening of hemihydrate.
(2) In the case of cement, false set is not so simple as in above-mentioned case of silicious powder, but nearly the same tendency is recognized. It seems that the hardening time of hemihydrate in cement is considered to be affected by hemihydrate itself, and also by dihydrate, calcium sulfoaluminate, several alkali salts, calcium hydroxide and by many others. As direct measurement of hardening time of hemihydrate in cement is very difficult, the determination of initial set during the hardening time of 1:1 mixture of hemihydrate and cement according to the test method of plaster of Paris is able to indicate the degree of hardening of hemihydrate.
(3) False set of fresh cement direct come from cement grinding mill takes place hardly, because the hardening of hemihydrate in this case is almost very rapid and therefore the dead-mixing is carried out. Hence the grade of false set of this cement occurs undirectly against to SO
3 content. By aeration, however, hardening of hemihydrate is prolonged, and accordingly false set appears, and its degree is generally dependent on the amount of SO
3 in cement. According to Federal Specification, if the hemihydrate efficiency is defined as the quotient of the amount of hemihydrate, which can harden within 1-5 min. after mixing, divided by hmihydrate, it can be regarded that the intensity of false set is proportional to the product of total amount of hemihydrate and hemihydrate effcieney.
(4) In the case of additional dihydrate on the false setting cement or of aeration at high humidity, the reasons of disappearing of false set in cement is seemed to be in quick hardening of hemihydrate in cement. Especially in the latter case the decreasing of hemihydrate is proceeded by its transformation into dihydrate or sulfoaluminate, therefore the hardening of hemihydrate would be fastened by these after-formed compounds.
(5) In cement aerated at low humidity, false set continues unchangeably for a considerable period. It is believed that the hemihydrate at this state exists under the stable condition.
(6) In cement prepared by mixing powder of clinker aerated at high humidity and hemihydrate, false set usually takes place strongly. However in cement composed of above clinker powder and dihydrate instead of hemihydrate, it does not take place. And in
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