Feb., 1958
THEREACTION BETWEEN CALCIUM HYDROXIDE AND AMORPHOUS SILICA
223
HYDROTHERMAL REACTIONS BETWEEN CALCIUM HYDROXIDE AND AMORPHOUS SILICA; THE REACTIONS BETWEEN 120 AND 160' BY GUNNAR 0. ASSARSSON Chemical Laboratory of the Geological Survey,Xtockholm 50, Sweden Received Sepl ember 16, 1967
The hydrothermal reactions between calcium hydroxide and silica over the temperature range 120-160" were investigatetl andothe possibility of occurring equilibria discussed in connection with the results earlier published for the reactions a t 180At about 120-140' two-stage reactions take place. During the first stage the phase B is formed, thifi phase not being 220 a chemical compound in real sense but a mixture of crystallites of nearly sub-X-ray dimensions as described earlier. The second stage yields hases, the characters of which are dependent on the molar proportion calcium hydroxide: @ea of the reaction mixtures; t l e proportion 0 to about 0.67: 1 yields the Z-phase of unknown composition mentioned, In the earlier paper, in mixtures of the molar proportions 0.67 to about 1.25: 1 the tobermorite phase is formed; in the mixtures 1.25 to 2: 1 and with a further excess of lime the a-dicalcium sllicate monohydrate crystallizes. At higher temperatures (>160') begins the range of the thJee-stage reactions earlier described, The possible temperature range of equilibrium of the zphase seems to be 130-150 ; the X-ray diffraction of this phase is given. The tobermorite phase has its upper teinperature boundary for a real equilibrium a t about 150" and above this temperature it recrystallizes into the xonotlite compound. The a-dicalcium silicate seems to have an uppw temperature boundary close to 160' and a t higher temperatures ,the hillebrandite phase is the stable compound. In the light of the results the formation of the corresponding minerals In nature is discussed and the reason for the discrepancies between properties of the synthetic compounds and the minerals was Buggested. The exothermal reaction type of the transformations is emphasized.
.
I n an earlier paper1 were given the results of the investigation concerning the equilibria of calcium silicate hydrates, formed during autoclave treatment of mixtures of amorphous silica and calcium hydroxide at temperatures between 180 and 220". Three reaction stages were shown to take place during the crystallization of the silicate hydrates, and the formation was established of a new phase designated Z. The mineralogical and practical interest concomitant with these results obviously promotes new investigations, designed t o discover some new conditions for the formation of the Z-phase and t o follow the reaction stages a t somewhat lower temperature.
autoclave treatment of lime-silica mixtures that real equilibria are sometimes difficult to reach especially a t lower temperatures. It seems, however, to be possible to get a good survey of the reactions within the temperature range 120-1GO" without reaching definitive equilibria by a combination of the results according t,o the phase rule. The occurrence of the phases is generally estimated by means of the characteristic X-ray diffraction lines.2 The crystals cannot be distinguished microscopically because of the relatively short Crystallization time resulting in only poorly crystallized preparations. I n the discussion of the results given below it is most expedient to group the description of the reactions according to the Materials and Experimental Details.-The materials, compounds which must be considered as the phases silica and lime, were identical with those used in the earlier always characteristic for the ranges. experiments and described in the i,eports. The special The Range of the Phases Gyrolite and 2.-This autoclave, with a manometer control of the pressure and valves for removing the air, was the same as that earlier de- range includes the mixtures of the molar ratios scribed and its use was necessary to reach experimental con- 0-1 : 1. ditions which could be precisely defined. The characteristic phases, the gyrolite compound One object of the present investigation was to find the experimental conditions under which the phase Z is formed and the Z-phase, are formed in substantial amounts in its least adulterated state. The molar ratio CaO:SiOz within 7 days a t 160" (Fig, 1,2,Table I.) The mixin the reaction mixtures was therefore changed in steps of about 0.1 mole CaO per mole Si02 throughout the range 0.25: 1 tures very rich in lime (0.85-1.O:l) do not yield reto 1:1 . The behavior of only some of these mixtures can be action products containing the Z-phase in amounts mentioned in this report. The period of autoclave treat- discernible from the X-ray photographs. Instead ment was, in most cases, 7 days, although where lower tem- of this is formed the compound richer in lime, the perature (120") was used, the period was sometimes extobermorite phase, which belongs to the adjacent tended. In spite of all the precautions concerning the experimental range overlapping the gyrolite range. In mixtures conditions it was sometimes concluded that details of the poor in lime some uncombined silica must also OCexperiments must have varied, as some of the autoclave prod- cur. ucts could be slightly better crystallized than others even Either the autoclave treatment of the mixtures if the phases were the same. The temperature of the furnace for heating the autoclave and of the autoclave itself at 140" was not intensive enough for a crystallizawere controlled automatically, and the heating up period was always identical. There seems therefore to lie n certain tion of the gyrolite compound or it is also possible disposition for a crystallization of the reaction products that this phase is not formed at all. On the other caused by nuclear crystallization centers. These observa- hand, the X-ray photographs show the Z-phase-as tions are especially pertinent to the recrystallization of the the highly predominant phase in the mixtfies wlth phase B, but the variations are without substantial impor- the lime ratios 0-0.67:l; the only other phase tance in the establishment of the phase formation. Some experiments, however, were repeated as a control on the which can be discerned in the photographs is the
formation of the phases and, according to t,he phme rule, the same phases were always recovered.
Results It is well-known from all the experiments with (1) G. 0. Assrtrsson, THISJOURNAL, 61, 473 (1957).
( 2 ) While the author's earlier report' was in printing, a summary of t h e researches on the calcium silicates was published: L. Heller and H. F. W. Taylor, "Crystallographic data for the calcium silicates," Building Research Station, Her Majesty's Stationary Office, London, 1956. Reference can be made t o this book concerning the results of earlier investigations.
GUNNAR0. ASSARSSON
224.
Vol. 62
TABLEI PHASES, ESTABLISHED AT VARYING LIMERATIOS,REACTION TEMPERATURES A N D PERIODS OF TIME Symbols for the phases: B = low temperature phase B; Z = hase Z, unknown composition; A2 = ~ 2 C a 0 8 i O ~ . H t O ; G , T, X, H = compounds resembling the minerals gyrolite fG), tobermorite (T), xonotlite (X), hillebrandite (H). Mixture mole CaO :Si02
Autoclave TepP., Time, C. days
0.25:l .40:1
.5Q:1 .67: 1
B Z (B) Z,G B
7 7 7 21 7 7 21
120 140 160 120 140 160 120 140 120 140 160
z
Mixture mole CaO: SiOa
Autoclave Temp., Time, OC. days
0.85:l
1.OO:l
B Zl
1.35:l €3
B
7 7
ZI
I .50:1
B
Z, G, B
* Uncombiiied lime 3%.
B-phase. I n the photographs of the autoclave products richer in liFe (0.67-1.OO:l)the characteristic Z-line 15.3 A. gradually disappears with increasing lime content; instead of it th,e diffraction line of the tobermorite phase (11.4 A.) becomes visible. The lowest autoclave temperature used in this investigation 120" yields only the phase B, in mixtures poor in lime even with extended periods of reaction time (Table I). CoO:SiOz=0,40:1
2.00:l
Phases shown
B
7
120 120 160 120 140 160 120 160 120 140 160 120 160
Z. G
7 7
Uncombined lime 1%.
Phases shown
21
T (G, B) T, G
7
T
21 7 7
T (X?)
T B B A2
21
7 21
7 7
A2 H A2" Hb
21 7.
The Range of the Tobermorite and Xonotlite Phases.-The range of these phases encloses the mixture of the lime ratios 0.67-2.00 :1, thus overlapping the range of the Z and gyrolite compounds and of the dicalcium silicate hydrates. The mixtures of the lime ratios 0.67-1.OO:lautoclaved at 160" contain the characteristic tobermorite and xonotlite compounds as well as the gyrolite compound as mentioned above (Fig. 2, Table I).
120:. 21 days
CaO:SiO,= 0.s7:l 140: 7 days
140q 7doys
160: 7days
: I Illllrn
160: 7 days
I
I
I I
4
3 2 5
2.n
A
powder data of preparations representing the range of the Z and gyrolite compounds.
COO:S~O~-O.SS:I
Ca0:Si 0,=1,00:l
120: 21 days
120: 21 days 140; 7 days
160: 7days
l60Y 7days
1
Fig. Z-X-Ray
I I
Gyrolite
1 6 8 6
Fig. 1.-X-Ray
1
n n
I I
I Ill1 I IIII I
powder data of preparations representing the range of the monocalcium silicates phases, the tobermorite and xonotlite compounds.
THEREACTION BETWEEN CALCIUM HYDROXIDE AND AMORPHOUS SILICA
Feb., 1958 CaO:Si02= 1,50:1 120: 21 days 140: 7days
225
CaO:Si02=2,oo:l 120", 21 days
I II Ill Ill1
160: 7days
L
Hillebrandite
2.0 A. 16 B 6 4 3 25 powder data of preparations representing the dicalcium silicates. I
3
Fig. 3.-X-Ray
1609 7days
/
25
However, when the lime content of the mixture increases, as in the mixtures 1.25:l and 1.50:1, no detectable amount of the monocalcium silicate hydrates is formed, although there should occur substantial amounts of the monocalcium silicate hydrates, if the whole amounts of the lime and silica are combined. The autoclaving of the mixtures with the molar ratio 0.85-1:l at 120-140" yields only the tobermorite phase as recrystallization product of the phase B. The xonotlite compound is not formed even after a more intensive treatment (21 days).-Within the temperature range 120-160" the recrystallization of the B-phase formed in the mixtures with the lime ratio 1.25:l is very insignificant. I n the autoclaved mixtures 1.50:l the tobermorite or xonotlite compounds cannot be shown a t all. The Range of the Dicalcium Silicate Hydrates. -This range comprises the mixtures containing lime-silica in the molar proportions >1:1. The autoclave treatment a t 160" of the mixtures within this range yields products the X-ray photographs of which are principally characterized by the diffraction lines of the hillebrandite compound. The mixtures poor in lime seem to be less disposed for a recrystallization of the B-phase as mentioned above, but in the mixtures 1.50:l and 2.00:l the X-ray photographs show distinctly the presence of the hillebrandite compound (Fig. 3, Table I). ' l T h e autoclave products formed at 120-140" contain a new phase that does not occur among the compounds in the higher temperature products, namely, the a-dicalcium silicate monohydrate, earlier described in the literature.a The experimental conditions used here (120-140"), are those employed by Heller and Taylor, and the X-ray spacings measured by Heller3 are in very good agreement with those found in the preparations of the present investigation (Table 11). Discussion I n the following discussion, some general properties of the system lime-silica-water will be treated more in detail with special reference to the (3) G. L. Kalousek, "Intern. Symposium on Chemistry of Cement," London, 1952, p. 334; L. Hsller and H. F. W. Taylor, J . Chem. Soc., 2535 (1952); L. Hellcr, Acta Cr~at.,5, 724 (1952), and literature cited there.
io A .
possibilities for equilibria of the phases. The phase rule requires two solid phases present in the mixtures of the present system at an equilibrium, when the proportions of the compounds do not form a one-phase composition. The discussion on possible equilibria of the phases must therefore be based on some premises deduced from the general properties of the system to some extent known from earlier investigations. As mentioned above some recrystallization reactions must proceed rather slowly but the observation of the succession of the formation of the phases is generally enough for a correct establishment of a phase equilibrium. The B-Phase.-In the sense the designation "phase B" has been used in the reports of the present author, the phase B cannot be a real homogeneous chemical compound. It is always formed when calcium hydroxide and amorphous silica are mixed in the presence of a sufficient amount of water. It has been shown in the earlier papers that a recrystallization of the phase B gradually takes place, the rate being highly dependent on the experimental conditions. A combination of a rather large amount of lime in the B-phase is also established, up to about a molar proportion of 1ime:silica = 1.5:l. The X-ray spacings characteristic for the phase B are very few, thus emphasizing the uncertainty of its being a trus chcmical compound: d = 3.07ss, 2.81w, 1.83m (A,), possibly also some others of very weak intensity, all of them rather broad with indistinct edges. When other lines become measurable, they have been found to indicate a beginning recrystallization and by diffraction lines other compounds have been identified. The lines ascribed here t o the phase B are explained by assuming that the different real phases of the mixture of nearly sub-X-ray dimensions have some unit repeat periods in common, as has been assumed by Bernal for other silicate hyd r a t e ~ .Attempts ~ to identify the B-phase by pure chemical methods have hitherto been without success. The 2-Phase.-This compound has been found in the preparations of the lime ratios 0-0.67: 1. If the lime content increases somewhat over the 0.67 content (e.g., 0.85) the characteristic spacing of (4) J. D. Bernal. "Intern. Symposium of Chemistry of Cement." London, 1952, p. 216.
GUNNAR0. ASSARSSON
226
Vol. 62
TABLE I1 CY-DICALCIUM SILICATE MONOHYDRATE A: preparations Ca0:SiOs = 1.5:l and 2.00:1, autoclaved 120°, 21 days B: according to Hellers SPACINGS OF THE
A
Int. W
ww 8
m-s m-8 888
w-m w-m m m
d
d.)
5.34 4.58 4.22 3.87 3.55 3.260 3.060 3.030 2.877 2.790
B Int. W
ww 5s
8 S 858
W
2.690
W
ww m m
2.580
8
W
m
2.523
m cwvw
s-m
2,405 2.313
5s
2.230 2.200
W
W
W
W
5.35 4.63 4.22 3.90 3.54 3.27 3.04
A d
Int.
ww ww m
2.87 2.80 2.77 2.71 2.69 2.65 2.60 2.56 2.52 2.47 2.41 2.31 2.27 2.24 2.18
this phase 15.3 A. disappears in the X-ray photographs. Good X-ray photographs were obtained from preparations autoclaved at 140" for 7 days, but there was no indication a t all in the preparation 0.50:l treated a t 120" for 21 days. It seems therefore to be plausible that the lowest formation temperature boundary of the Z-phase should be between 120 and 140' and the boundary of the highest lime ratio between 0.67-0.85:l. At 160" the Zphase is already transformed into another phase, the gyrolite compound. If there is a real existence range for the Z-phase it should be limited to about 130-150". Because of an obviously certain resistance t o transformation it is shown to be present in some autoclave products of the same lime ratios as those mentioned above but after treatment at higher temperatures for shorter periods of time.' Some experiments were performed in order t o prepare samples of pure Z-phase. These experiments were not successful. The best preparation hitherto was obtained by autoclaving the mixture with a lime ratio 0.40:l. Microscopically it was found t o contain crystal crusts composed of minute crystallites of high optical birefringence without any observable extinction direction. Because the crystallites have been formed by a transformation in the lumps of the B-phase, they are surrounded by a diffuse mass of low refraction, and the mean refraction could not be determined with any accuracy; it seems t o be somewhat lower than 1.5. The optical properties as well as the chemical composition must therefore remain unsolved problems for the present. The X-ray photographs of the reaction product above show a series of diffraction lines which seem to be characteristic for the 2-phase; they are listed in Table 111. Six of the spacings are
B
(A.,
Int.
d
W
ww m-w m-w
2.100 2.055
W
m-w W
w m
'
1.958 1.921
W
W W
1.878
W
1.831 1.780 1.732 1.707
m-w www m
W
S S W
m
a (A.)
m W
m m-w m-w
8
m-w m-w WWW
m-w
1.648
W
1.607
m-w m m m W
(A.1
2.16 2.10 2.08 2.06 2.03 2.02 1.982 1.956 1.926 1.890 . 1.872 1.842 1.820 1.788 1.737 1.712 1.687 1.662 1.654 1.645 1.630 1.608
obviously inverse multiples of 15.3 8. showing a repeat period corresponding to a probable diffraction at the 001 or 002 planes. TABLE I11 X-RAYDATAFOR THE SYNTHETIC PHASE Z Preparation: CaO:Si02 = 0.40: 1, autoclaved 7 days, 140" Int
d
(A.)
15.3 7.8 5.01 4.14 3.800 3.066 1.900 1.817
688
W
ww m-w W S
ww m
Period of (000, oalcd.
1 5 . 3 : l = 15.3 15.3:2 = 7 . 7 15.3:3 = 5 . 0
...
15.3:4 = 3.83 15.3:5 = 3.06 15.3:8 = 1.91
...
The Gyrolite Phase.-The experiments described above show clearly that the lowest formation temperature of the gyrolite phase seems t o be between 140 and 160". There is, for example, substantial formation of the gyrolite phase a t 160" in the reaction mixture 0.67 : 1, but no discernible amount occurs in the same mixture autoclaved at 140" for the same period of time, The boundary of the lime ratio must be close to the monocalcium silicate proportion. As shown in the earlier report' the gyrolite phase is gradually formed in the reaction products during an autoclave treatment, a t the same time as the Z-phase disappears because of a certain degree of metastability. The temperature range within which the gyrolite is a stable phase is therefore about 150 > 220" which is in agreement with the preparation temperatures mentioned by Flint, McMurdie and Wells.5
-
.
Feb., 1958
THEREACTION BETWEEN CALCIUM HYDROXIDE AND AMORPHOUS SILICA
The Tobermorite Phase.-The tobermorite compound is formed rather easily a t 120" by autoclaving mixtures of 0.85-1 :1. I n the mixtures of lower lime ratio (0.67 :1) the gyrolite phase sets bounds to the formation of the tobermorite compound, and in mixtures richer in lime (1.25:l) other reactions seem to take place which will be discussed in connection with the formation of the dicalcium silicate hydrates. The range of the lime ratio within which the tobermorite phase is formed, is therefore limited to lime:silica = 0.67 to 1.25:l. The lowest formation temperature of the tobermorite phase is lower than 120" ; according t o the investigations of Taylor6the tobermorite compound is formed and stable a t ordinary temperatures. I n the earlier paper' it was shown that the tobermorite compound, occurring in mixtures a t higher temperatures, 180-220", must be considered to be a transition phase which is transformed into the xonotlite phase. The lowest temperature a t which this transformation takes place seems to be between 140 and 160". The Xonotlite Phase.-As the xonotlite compound contains three moles of calcium oxide and silica and one mole of water there must be a relationship between this compound and the other monocalcium silicate, the tobermorite phase which contains three times the amount of water, and the xonotlite compound is thus regarded as a recrystallized dehydrated tobermorite. According to the experiments (Table I, Fig. 2) its lower formation temperature is between 140 and 160" which is in agreement with the preparation experiments of Heller and Taylor.7 At higher temperature (180220") the occurrence of the xonotlite phase was established in mixtures of the lime ratios 1.0-2.0:1, though the tobermorite phase was not observed; this behavior of the phases must be connected with the rate of increase of temperature in the autoclaves; the range of the xonotlite phase also extends therefore over the dicalcium silicate range up to a lime ratio of 2:l and with temperatures higher than 220". The Dicalcium Silicate Monohydrate.-The dicalcium silicate formed at 120 and 140" was identified as the a-dicalcium silicate m ~ n o h y d r a t e . ~ The two mixtures 1.5:l and 2.00:l autoclaved at 120 and 140" show the same spacings exactly, which means that the reaction mixture poorer in lime probably contains only this a-dicalcium silicate hydrate. There is no content of uncombined lime in the mixtures 1.50: 1 and only 1%in the mixture 2.00:l (Table I). Thus practically the whole amount of lime present in the mixtures has been combined as a dicalcium silicate. As the mixture 1.5:l has a different proportion from that of a dicalcium silicate, some free silica must exist in the reaction mixtures. No diffraction line belonging to other compounds could be discerned, whereas the agreement of the photographs with Heller's measurements of the a-dicalcium silicate monohydrate is good (cf. Table 11). Some spacings of medium and weak intensities, occurring in the list (5) E. P. Flint, H. F. McMurdie and L. S. Wells, Bur. Sfand. J . Res., 21, 617 (1938). (6) H. F. W. Taylor, J . Chem. Soc., 3682 (1950); 163 (1953). (7) L. Heller and H. F. W. Taylor, ibid., 2397 (1951).
I
AI I the
227
ranges: 1 st stage phase B, unstable
21
{
2nd stage: a.2CoO. SO,: H,OtSiO, unstoble or Taberrnorite
e(
3rd
1
stage: Hillebrandite stable t Xonotlite
3rd stage: GyrolitetSiOZ stable
I
100
Fig. 4.-The
I
I
150
2b0 "C,
existence range of the unstable and stable phases between 120 and 220".
given by Heller, however, are missing in all the preparations of this kind of the present investigation, though they plausibly should be discernible. It can also be stated that the poorly crystallized B-phase does not occur in these preparations. The reaction between the calcium hydroxide and the silica must therefore take place in the following way: While the autoclave is being heated up the calcium hydroxide and the silica a t once react forming the very poorly crystallized phase B. This phase is not stable a t the temperatures 120 or 140" and therefore recrystallizes forming the adicalcium silicate hydrate. If the content of calcium hydroxide is not sufficient to combine with the silica to form dicalcium silicate, an excess of silica must be left unreacted. If there is any lime apart from that present in the B-phase, this component reacts successively with the silica which must become available by the formation of the dicalcium silicate. As these reactions take place a t 120" and the autoclave periods were extended to three weeks, it seems to be plausible that the lowest formation temperature is lower than 120". There is a zone within the range of the lime-silica mixtures where the recrystallization of the B-phase is very slow, namely, the area where the reaction mixtures have a molar ratio of about 1.25:l. This zone forms a transition area between two areas of relatively well crystallized reaction products, the tobermorite range and the dicalcium silicate range. Repeated attempts to prepare in 7 days reaction products sufficiently well crystallized to give a t least an indication of the structure, were without success, obviously because for those mixtures which have this proportion there is a transition range between the monocalcium silicate and the dicalcium silicate areas where the recrystallization of the B-phase is balanced between the two types of crystallized phases. Reaction mixtures very rich in lime (3:l) autoclaved a t 120 and 140" contain only the adicalcium silicate hydrate together with uncom-
228
GUNNAR0. ASSARSSON
bined calcium hydroxide. A silicate hydrate richer in lime than a dicalcium silicate does not occur within this temperature range (120-160"), nor does it do so within the range of some higher temperatures (180-220") as shown ear1ier.l Between 140 and 160" the phase a-dicalcium silicate hydrate is transformed into another phase. The products formed a t 160" after autoclaving for 7 days are rather poorly crystallized and in particular the intensities of the diffraction lines of the X-ray photographs are therefore undecisive. The position of the lines, however, shows a beginning formation of the hillebrandite compound ; furthermore, the products formed on autoclaving a t somewhat higher temperature (180") show the distinct spacings of the hillebrandite compound. The lowest formation temperature of the hillebrandite phase and also the upper formation temperature of the a-dicalcium silicate hydrate must therefore be somewhat below 160". The formation of the phases and their possible existence ranges and equilibria between 120 and 220" according t o the results above are shown in Fig. 4. Obviously some more experiments, observations and measurements must be made. The apparently rather complicated system of the figure is, however, easily understandable. Summing up the results of the investigations, some principal lines are forthcoming which are not exclusively connected with the system lime-silica-water but also with other systems. I n the autoclave experiments there must always be a heating up period; in other reactions of hydrothermal character, such as those occurring in nature, similar periods do occur. During this period phases are formed successively in accordance with the conditions of temperature and vapor pressure, their rate of variation and other factors. If the external factors, temperature and pressure, increase during the hydrothermal process, the existence conditions of the low-temperature and low-pressure phases are passed gradually and the possibility of the appearance of the low-temperature phase depends on its disposition for recrystallization. I n the present system there are two types of such reactions: the recrystallization of the Bphase, and of the phases tobermorite -+ xonotlite on one hand, and the transformation 2-phase the gyrolite phase and a-dicalcium silicate hydrate the hillebrandite phase on the other hand. The rate of the first mentioned reaction is obviously slow; the intermediate stages are distinguished rather easily. The other type of reaction is more rapid and the stages are more difficult t o discern. The more resistant intermediate phases, e.g., the tobermorite phase, can therefore remain under conditions which do not correspond to a real chemical
-
--f
Vol. 62
equilibrium and the crystals once formed partly can be decomposed, by a slight hydrolysis or dehydration resulting in the formation of pseudomorphs. When the temperature and pressure decrease during the formation of the phases, the change in properties of the phases once formed must be similar, paying regard t o the very slow transformation of the high-temperature phases, the gyrolite, xonotlite, hillebrandite compounds. The structure of the synthetic compounds in the author's preparations seems t o be very constant, possibly because of very well defined experimental conditions. The spacings also agree well with those published by other investigators. The autoclave preparations have the best conditions for a constant structure of the phases, more perfect than the reactions of hydrothermal deposits in nature could produce. Because the boundaries of the equilibria are exceeded objection can sometimes be raised concerning the purity of the synthetic preparations. The general existence and formation conditions of the compounds and minerals gyrolite, tobermorite, xonotlite and hillebrandite within the temperature 120-220" were discussed above. Two compounds also occurring within this range have not been found in nature, the 2-phase and the a-dicalcium silicate hydrate. Perhaps their existence range is too limited for an occurrence in hydrothermal deposits in nature, perhaps they do occur but have not yet been discovered. The recrystallization processes of the phases have to be discussed from another point of view. The slowness of the transformation of metastable phases into stable phases causes the ranges of their stability t o be considerably exceeded. The phase B must be considered as an unstable phase within the whole temperature range, the Z-phase within the gyrolite range and so on. Every recrystallization involves a generation of heat as a consequence of the exothermal reactions. Thus, starting from the obviously unavoidable B-phase which represents one period of heat evolution there should be one more period in the two-stage and two more periods in the three-stage crystallization. The determination of these heat quantities would be of greatest interest for an estimate of the properties of the system and should explain its rather exceptional character. If the temperature increase is very rapid the two transformation stages could coincide and give rise t o a more marked evolution of heat. This has been observed by the author when autoclaving lime-quartz and similar materials. Acknowledgment.-The author wishes to express his gratitude t o the International Ytong Cy of Stockholm for the placing a t his disposal instruments required for this investigation.
1
I
e
