SOME REMARKS OK T H E COLLOIDAL THEORY OF CEMENTS BT TCTOMU MAEDA
There are two rival theories, the crystal and the colloid theories, for the setting and hardening of cements. M y studies on the setting and hardening of magnesium oxychloride cement lead me to the conclusion that these two theories are complementary and not antagonistic. The so-called colloid theory must bemodified, however, because it uses a colloid explanation for a phenomenon which plays a minor part in the settingand hardeningof cements and omits the explanation of more important phenomena from the collaid standpoint. My studies, so far, have covered the following five topics, and I will discuss the phenomena in view of what I have found. The full details can be found in the original papers. I. The composition of magnesium oxychloride cement.l Viscosity changes during the reaction between magnesium oxide and 2. an aqueous solution of magnesium chloride; and a new consideration with respect to the setting of cements.? 3 . A study of magnesium oxychloride cement by x-ray^.^ 4. The aqueous vapor pressure of magnesium oxychloride cement, and the state of water in it.4 j. The hardening of magnesium oxychloride cement, and the functions of free water, water of crystallization, and adsorbed water.4 With the exception of W. Michaeliss, the supporters of the colloid theory consider that the colloid phenomenon consists in the adsorption of fluid on the surfaces of the solids in the cement mixture. Thus CavazzP assumes the direct formation of the so-called gelatinous calcium sulphate; but such adsorption phenomena are not limited merely to cement mixtures. They may also occur as the first step in the reaction when any salt is brought in contact with a liquid or a solution. According to my understanding Cavazzi’s gelatinous calcium sulphate can be considered as CaS04(o.5+x)H20,which decomposes. as the second step, into CaS04 . zH20 and (x- I . j)H20. I n this form the colloid theory does not differ much from the crystal theory, for the latter also attributes the setting of cements to the formation of small, interlocking crystals. I wish to ask, however, the function of the water set free (x- 1.j) HZO per mol CaSOl. zH20. The most important weapon for the supporters of the so-called colloid theory is the function of the added chemicals which modify the rate of setting. IVhich of the two reactions-the formation of the gelatinous sulphate or the formation of the crystalline sulphate-is Maeda and Yamane: Sci. Papers Inst. Phys. Chem. Research, 4 , 85 (1926). Maeda: Sci. Papers Inst. Phys. Chem. Research, 4, 102 (1926). hlaeda: Sci. Papers Inst. Phys. Chem. Research, 5 , 95 (1926). In press: A11 these five papers are written in Esperanto. Kolloid-Z., 5 , g (1909). Iiolloid-Z., 12, 196 (1913).
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influenced by these chemicals? Various chemicals, of course, play an important part in the coagulation of a colloidal system. To which of the two reactions does the coagulation correspond? The colloid theory does not explain the cause of hardness from a colloid stand-point, but adopts the latter part of the crystal theory. Are there any crystals in a hardened gelatine? I am willing to accept the hypothesis of the first adsorption of a fluid on the surfaces of the solids in a cement mixture; but I do not consider it important for the phenomenon of setting, because it is evident that an immediate solution of the solids takes place to some extent, since the fluid has some solvent action on the solids. Some C a S 0 4 . o.gH20 will therefore dissolve in water and some magnesium oxide will dissolve in the magnesium chloride solution. There will certainly be formed, as postulated by the crystal theory, a solution supersaturated with respect to a new solid phase, and this solid phase will be formed sooner or later. Jolibois and Chassevent' have already proved by the conductivity method, that this occurs for CaSOl. o.gH?O and I have showed it for magnesium oxychloride cement by measurements of the viscosity changes according to the method of Wo. Ostwald and P. Wolski2, though there was more liquid present in both cases than in real cements. Would it, therefore, be just to assume that supersaturation does not occur in the case of real cement mixtures, as some supporters of the so-called colloidal view say? Quite unjust. Owing to the large extent of active surface in the case of real cements, we may suppose that the supersaturation and consequently the recrystallization of a new solid phase occur very quickly, in which case the above-mentioned gelatinous calcium sulphate is not in the form of C a S 0 4 .o.gHzO xHpO; but is in the form of C a S 0 4 . zH20 yHpO or C a S 0 4 . 2Hz0 y H 2 0 zCaS04. o.sH20. So far the classical crystalline theory does not offer any serious difficulties; but, if carried further, it fails to explain phenomena and a new colloidal standpoint comes into play. It must be remembered that the so-called colloidal theory cannot here explain anything more because it denies the supersaturation and the recrystallization. When a new solid phase appears from the supersaturated solution, surface hydration, somewhat according to my own term, occurs on the surfaces of very tiny crystal particles, which may have colloidal dimensions. By surface hydration is meant a phenomenon, whose true nature is obscure aa yet; but which occurs quite frequently during crystallization. Adsorbed water molecules on the surface possibly explain it, but in the case of magnesium oxychloride cement, Mg' ' or C1' also can take part in the adsorption. But as the ions themselves are hydrated, water molecules here again play parts. I n the above example, I have mentioned the surface hydration on C a S 0 4 . 2H20 with the form of C a S 0 4 . z H 2 0 yHnO. This recrystallization accompanied by the surface hydration means the beginning of the setting of a cement, because the free water becomes less by changing into the forms of water of crystallization and adsorbed water, of which the latter plays an important -
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Compt. rend., 177, 113 (1923). 27, 78 (1920).
* Kolloid-Z.,
THE COLLOIDAL THEORY OF CEMENTS
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rBle in the viscosity relation. Here the crystals formed are not effective for the setting by themselves, but through water. According to my view, the adsorbed water acts like gelatine; in the setting of gelatine, it is clear, that no crystals play a rBle. My X-ray diffraction patterns obtained by the powder method, however, show clearly the existence of crystalline 3MgO . MgClz IzH~O in a hardened magnesium oxychloride cement; 3MgO PVlgClz . I z H ~ O is a new solid phase formed in the recrystallization; the composition being determined from my phase rule research. But crystals with colloidal dimension may give a diffraction pattern, and therefore the above fact does not prove the correctness of the classical crystalline theory. The surface hydration is very important for the strength of cements, as the adsorbed water film prevents brittleness. The adsorbed water reminds us of a thin water film between two glass plates. Furthermore, rny opinion is against the crystal growth or the perfection of imperfect crystals as the causeof the hardening. Von Weimarn’s crystallization theory of colloids*is very full of hints for the setting and hardening: it treats the crystallization from a solution, the solubility and the amount of the crystals formed, the viscosity of the medium, and so on. According to his theory, the general scanty solubility of the newly formed solid phase, the large amount of the above phase and the high viscosity of the medium all postulate, that the formed crystals in cement mixtures are very tiny, although they may form large aggregates, and they can remain in the original tiny form for a very long time. But X-ray diffraction patterns taken at intervals will give a decisive answer to the above questions as to the perfection of crystals, for the patterns by and by will become clear, if the perfection of crystals occurs. From my own X-ray research, it became clear, that when the cement mixture contains a comparatively large amount of the liquid part, it gives a clearer diffraction pattern than that of a mixture with less liquid part. The so-called colloidal theory can not obtain any support from von Weimarn’s theory, because it denies the crystallization from a solution. Also from my own researches, it is clear, that added chemicals and the change of the surface property of a solid pari influence thc velocity of the setting. According to my considerations, the setting means the appearance of a new solid phase, and therefore added chemicals and the change of the surface property of a solid part mean the change of the velocity of the formation of a ncw solid phase; and the velocity of the formation of a new solid phase is influenced by two factors: the rate of solution of the solid part into the liquid part and the vclocity of the recrystallization of the new solid phase. In the case of magnesium oxychloride cement, the casual standing of magnesium oxide in the open air means the decrease of the velocity of the setting and also of the hardness; this means magnesium oxide became less reactive to the solution. Whether added chemicals affect the reactivity of the surface of the solid part or the velocity of the recrystallization of the new solid phase, I do not know clearly. Furthermore I suppose that the degree of the surface
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“Zur Lehre von den Zustanden der Materie” (1914).
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hydration may also be influenced. Conductivity measurements will give some valuable informations with respect to the effects of added substances and the change of the surface. According to my researches, the hardening, which means the hardness increase without external change after the setting, is a continuation of the setting, and the loss of water to the external system is not important; the change of free water into the forms of water of crystallization and adsorbed water, however, is very important for the hardening. Finally I will discuss the function of free water, w t e r of crystallization and adsorbed water. The first added liquid part in a cement mixture means the free water, which is necessary for practical working. The free water then becomes water of crystallization and adsorbed water by the recrystallization process; this change is necessary to increase the hardness, for the existence of the free water means a soft mass. To prevent brittlencss, the adsorbed water is necessary, and interlocking crystals without adsorbed water make a brittle mass, as magnesium oxide and water illustrate. From the above considerations, the setting and hardening of cements are not essentially different from the setting of gelatine or starch paste, with cements, however, a gelatinous mass i s obtained through the crystallization of tiny crystal particles from a solution with the surface hydration on their surfaces. This means the necessity of both crystalline and colloidal ideas expressed in a new light. Summary I have studied magnesium oxychloride cement with special reference to its setting and hardening, in order to clear up the mechanism of the above phenomena. 2. From the above studies, I came to the conclusion, that neither the classical crystalline theory, nor the so-called colloidal theory does not account for the true mechanism. 3 . I have proposed a theory, which involves both crystalline and colloidal ideas: the crystalline idea is necessary for the recrystallization from a solution; the colloidal idea, for the surface hydration on the new solid phase. I.
The Institute of Phystcal arid Chemtcal Research, Hongoku, Tokyo, Japan, Oct 9, 1926
