April, 1957
HYDROTHERMAL REACTIONS BETWEEN CA(OH)
AND
AMORPHOUS SILICA
473
HYDROTHERMAL REACTIONS BETWEEN CALCIUM HYDROXIDE AND AMORPHOUS SILICA; THE REACTIONS BETWEEN 180 AND B%O0 BY GUNNAR0. ASSARSSON Chemical Laboratory of the Geological Survey, Stockholm 60, Sweden Received October 83, 1966
The reaction between calcium hydroxide and amorphous silica was studied within the tem erature range 180-220°. Three stages of the reaction are distinguighed. The first one is the ra id combination of the c a k u m hydroxide with the reactive silica, forming the poorly crystallized low temperature phase which has no definitive proportion lime : silica, as has been described earlier. The second reaction stage is a recrystallization of fhe hase B forming distinct compounds; lower content of lime, 0-1 mole CaO per mole silica, yields a compound, hitherto uninown as a mineral and designated Z in this pap"'& the content of 0.67-2.0 mole CaO per mole silica yields the tobermorite-like compound; with a lime content of 1-2 mo e a 0 per mole silica, uncombined lime represents an unstable phase. The third reaction stage leads to the final formation of phases in equilibrium; the are the compounds resembling gyrolite, xonotlite and hillebrandite. The existence ranges of the unstable as well as the statle com ounds overlap each other according to the phase rule. With a lime content >2 mole per mole silica, uncombined lime is tffe compound existing besides the hillebrandite compound; no phase richer in lime than the dicalcium silicate was detected within the temperature range examined.
8,
Results and Discussion The Identification of the Phases.-As some new results concerning the X-ray diffraction of the compounds have been available in the literature it may be expedient t o give a short summary of the possibility of identifying the phases in mixtures which are formed during the hydrothermal reactions. For the present there are six calcium silicate hydrates which occur in the autoclave products at 220' : namely, the low temperature phase B, the compounds resembling gyrolite, tobermorite, xonotlite, hillebrandite and finally a new phase which will be designated phase Z in this paper, in the absence of a better name. I n connection with the Xray identification of the phases it must be remarked that objections could be raised against some older measurements. The imperfection of the X-ray cameras was mentioned above, but also the purity of some mineral specimens, used as comparison material, may be questioned. Some of these minerals seem to have been very difficult to obtain in a pure state. Minute crystallites with low degree of repeating patterns could generally be a consequence of the rather short periods of reaction time which have been one of the principal conditions in the present investigation. I n most cases, however, the measurements of the X-ray photographs are easily performed upon the substances produced after a reaction period of 24 hr. at 220") showing a sufficient growth of the crystallites, even if it is impossible to investigate the products microscopically. The X-ray powder data most suitable for the identification of the phases are listed in Table I. Experimental New measurements of the spacings have given rise The autoclave and the materials used for the mixtures cal- to a short discussion of some of the phases below. cium oxide-amorphous silica were the same as those earlier Phase 2.-A re-examination of the reaction described; the amorphous silica was the pure silica gel slightly products poor in lime showed that there must occur desiccated on the steam-bath (80"). By the use of some new X-ray cameras of good resolving an unknown new-formed phase. The examination power it was possible to distinguish the diffraction lines more reported below shows an existence range which is accurately than in the earlier investigation. The smallest Bragg angle which could be measured with reasonable ac- very limited with regard t o both the molar ratio curacy using the improved instruments corresponds to a d- 1ime:silica and the reaction temperature and time. value of about 35 A. The phase Z can only exist during a relatively short period of time and as a consequence of this there is (1) G. 0.Assarsson, et at., THIS JOURNAL, 60,397(1956); 60,1559 not sufficient time to ensure its being well crystal(1956). lized. The only characteristic spacing which has (2) C. F. Claringbull and M. H. Hey, Mia. Mag., 29, 960 (1952); J. D.C. MoConnell, dbid., SO, 293 (1954); H.Megaw, Nature, 177, 390 been discerne4 hitherto in the X-ray photographs (1956). is a line 15.3 A. which has a very strong intensity I n some earlier papers' the present author has given the results of the hydrothermal reactions between amorphous silica and calcium hydroxide within the range of 120-220" and 0-24 hr. Two types of products were to be distinguished. A low temperature phase which is poorly crystallized and which obviously consists of more than one compound is formed below 180" and during short periods of time; its X-ray pattern, however, is very indistinct; it has been considered to correspond t o the mineral tobermorite.2 The high temperature phases are well crystallized and rather easy to distinguish. The subsequent work with the problems showed that special reactions take place under certain conditions. It was also found that the reactions between calcium hydroxide and quartz or some silicates could be elucidated by the reactions calcium hydroxide-amorphous silica, these reactions being of importance in the solution of mineralogical and theoretical as well as of practical problems. It seems therefore t o be necessary to study the reactions a t special temperatures more in detail. For this purpose the temperature range immediately above the transition boundary low temperature-high temperature phase a t 180°, 24 hr. is considered t o be the best basis for a further investigation, as the results of the earlier particularized investigations can be employed. The present investigation therefore involves the formation of the phases in saturated steam at the temperature 220°, and the molar ratio calcium oxide :amorphous silica in the reaction mixtures was changed gradually.
474
GUNNAR0. ASSARSSON
Vol. 61
TABLE I CHARACTERISTIC SPACINGS (A,) USED TO IDENTIFY THE PHASES Symbols for intensity see Table I1 Phase
..
Ref.
Phase 8 ss 15.3 .... .... ...... .... ..... 511.1 . . . . w w 5 . 4 9 m 4 . 7 6 . . . . . Gyrolite 55.22 . . . . . . . . ss 11.4 . . . . .... Tobermorite , , m5.47 ..... Xonotlite .. .... . . . . as 7 . 0 3 . . . . . . . . . . s 3.638 Hillebrandite .. .... .... .... . . . . . . 554.76 . . . . .
when the crystals of the phase Z are most well developed. The line 15.3 8. usually occurs at the same time as the two lines 22 and 11.1 of the compound resembling gyrolite; on the other hand, there may be other Iines of weak intensity, or the lines are coincident with those of the gyrolite-like compound and/or the low temperature phase B, and therefore not distinguishable. The measurements of two of the photographs are compared in Fig. 1 with those of the gyrolite-like compound; they show the occurrence of the characteristic line 15.3 8. of the phase Z. As far as is known to the author, no compound or mineral has been described containing lime, silica and water having a spacing corresponding to 15.3 8. Ca0:Si02-Q67:l 20O:lZ hr Z I/
0.67:l ZOO,' 24 hr
0,671 220,' 24 hr-
.... .... ....
..
TableV . . Table I1 .. 2 . . . . 3.23 TableIV s s 3 . 3 3 ... 3
TABLE I1 POWDER DATA(A.) FOR THE SYNTHETIC COMPOUND 2 C a 0 . 3 S i 0 ~ 2 H ~220°, 0, 144 hr. and for Natural Gyrolite (Mackay and Taylor*)
Synthetic compd. Gyrolite Int. d Int. d as 22 8s 22 a 11.1 s 11.0 ww 9.5 m, d 8.4-7.4 m-w 8.36 m-w 7.87 m-w 7.67 w , d 7.4-5.4 w, d 6.13 ww, d 5 . 4 9 4.75 m-w 4.76 w w 4.67 w 4.45 ww 4.34 s 420 s 4.20 ww 4.06 w 3.902 ww 3.800 m 3.724 w , d 3.72-3.45 w 3.551 ms 3.65 w 3.475 ass 3.160 m, d 3 21-3.02 as 3.097 ss 3.12 ss = very strong; 8 = strong; = very weak; d = diffuse.
Synthetic compd. Gyrolite Int. d Int. d' ww 3.005 s 2.834 m , d 2.80-2.61 m 2.795 m-s 2.80 m, d 2.679 m, d 2.643 w 2.555 m 2.424 w 2.42 W W , d 2 . 3 8 4 WW, d 2.31-2.03 w w , d 2.283 w 2.31 ww 2.189 w 2.17 ww 2,128 w 2.063 w 2.06 ww 2.016 w 1.919 w 1.90 s-m 1.830 ww, d 1.82-1.73 m 1.800 s 1.82 w,d 1.765 w, d 1.740 w 1.595 w 1.575 m-w 1 57 m 1.343 m-w 1.34 m = moderate; w = weak; ww
some shadows a t those positions where the mineral, according t o Mackay and Taylor, has bands. The lines, however, were very easy to measure with good accuracy. The shadows obviously originate from the first poorly crystallized material with patterns Gyrolita to a certain degree disordered or insufficiently repeated. It is therefore plausible that the bands mentioned by Mackay and Taylor have been dissolved in more distinct lines because of a better crystallization of the synthetic compound. The reFig. 1.-The occurrence of the characteristic X-ray reflec- flection lines of both the types are listed in Table tion 15.3 A. of the phase 8, and the reflections of the autoclaved preparations (Table 11) compared with the powder 11; Fig. 1 also gives a representation of these measurements. Mention should be made of one special data of natural gyrolite (Mackay and Taylor).a point. The spacing of 3.12 8. (Mackay and TayAt a control exposure of a mineral specimen des- lor) is a line of very strong intensity and it is also ignated gyrolite from Skye, the Hebrides, Scotland, broader (3.17-3.08 B.) than the other lines of the a broad line of weak intensity was discernible be- photographs of the preparations of the gyrolite-like tween 16-14.5 8. Probably this band indicates substances of the present investigation. This line the presence of a small amount of phase Z, which 3.12 8. has nevertheless very sharp edges and it is has been formed at the same time as gyrolite. therefore considered to be a double line which was Gyro1ite.-In the list of the measurements by not divided into its two components by the camMackay and Taylor4 there are some bands of me- eras. The X-ray photographs of the gyrolite specdium and weak intensity. I n order to be able to imens from Skye mentioned above show very dismeasure the lines accurately for a comparison with tinctly that the line about 3.12 8. is composed of the mineral a mixture of calcium oxide-silica of the two lines. Even if the photographs of the Skye molar ratio O.G7:1 was autoclaved at 220" for 24 gyrolite diverge from that mentioned by Mackay hr. and for 144 hr. The X-ray photographs of and Taylor with regard to the relative intensities of these two preparations showed the same spacings, some lines the two photographs agree almost comgenerally very distinct lines. The first one had pletely. It eeems therefore to be most plausible that the reflection at about 3.12 A. should be com(3) L. Heller, Min. Mae., 80, 150 (1853). posed of two lines. A further discussion of the (4) A. L. Mackay and H. F. W. Taylor, ibid., SO, 80 (1953). 220,' I44 hr
.
.
HYDROTHERMAL REACTIONS B E T W E E N C.4(0H)a AND AMORPHOUS SILICA
April, 1957
Skye mineral is not within the scope of this investigation. Xonot1ite.-In the present investigation the X-ray photograph of a specimen of xonotlite from Xonotla, Mexico, has been used a t the comparison of the synthetic compounds. As far as known t o the author X-ray powder data of an analytically and microscopically controlled specimen of the mineral have not been published5; some notes will therefore be made in connection with this investigation. The mineral is a white, tough, fibrous material. The analysis (Table 111) shows a monocalcium silicate which contains a small amount of Mn, Fe and Mg replacing Ca in the pattern; the content of water is somewhat lower than the ideal formula 3:3:1 shows. The X-ray powder data of a preparation synthesized by autoclaving a mixture lime-silica = 1: 1 a t 220" for 144 hr. are given in Table IV together with the powder data of the mineral. The cell dimensions of the mineral are calculated to be a = 16.90, b = 7.32, c = 7.03 A. The analysis of the synthetic preparation showed the composition CaO :Si02 :HzO = 1: 1: 0.34. The synthetic compound has one line (11.1 k., w) showing the presence of a small residual amount of the tobermorite compound which had not been transformed into the xonotlite compound in spite of the extended time of autoclaving; in other respects the agreement between the two series of measurements is excellent (see also Fig. 2).
475
TABLE IV X-RAYPOWDER DATAFOR THE SYNTHETIC PREPARATION CaO:Si02 = 1:1, 220°, 144 hr. and Xonotlite (Table 111) Synth. prep. Int. d
Xonotlite Int. d
ww
(w) (11.1) ww 8.44 s 7.03 ww 6.29 S 4.23
ww s ww s
ww, d
ww
ww, d 3.638 s ww,d ww,d 3.230 s 3.076 sss 3.018 ww, d ww,d 2.817 s 2.696 s-m 2.628 w
m
2.497 m
8
s
ss ww s S
Synth. prep. d
Int.
ww
8.50 6.98 6.27 4.24 3.98 3.88 3.634 3.484 3.395 3.230 3.076 2.98G 2.915 2.820 2.696 2.623 2.583 2.500
w w w, d s s
m
w ww m ww ww
ww
w
2.427 2.340 2.246 2.189
Xonotlite d
Int.
ww w m
2.415 2.330 2.246 ww, d 2.188 ww, d 2.122 2.037 s 2.032 1.949 s 1.943 1.833 m 1.832 1.770 w 1.765 1.7,53 1.708 m 1.707 1.642 w 1.643 1.588 ww 1.593 1.571 ww 1.575 ww 1.543 1.518 m 1.517 w 1.433 W 1.395
the formation of crystals of better growth. The autoclave products at 200" show its characteristic line with strong intensity after 12 and after 24 hr., but a t higher temperatures (220 and 240') autoclave treatment produces the phase during short
TABLEIII CHEMICAL ANALYSIS OF XONOTLITE, TETELE DI XONOTLA,
Toberrnorlte
I
a
I
MEXICO (Mus. of Nat. Hist. of Sweden, Min. Dep. No. g. 20272) Anal. A. Aaremae
IIII
I
1 I I II I
Molar ratio
Si02
A1208 FeO MnO CaO MgO
KzO, NaRO
HzO c02
48.55 0.30 0.51 1.00 45.17 0.28 0.10 3.57 0.32 99.80
0.809
...
.007 .014 .SO6 .007
...
1:1 220,' 144 hr
Calcd. for (Ca, Mg, Fe, Mn)O :Si02 :HzO = 1.02:1:0.24 (CaCOa = 0.73%)
.198 ,007
The Existence Range of the Phase Z.-To the phase Z must be ascribed a certain importance from the mineralogical point of view and the conditions of its formation have therefore been studied more thoroughly in the present investigation. A survey of its existence range is given in Table V. It is clear from the experiments cited in the table that the phase Z is formed only in mixture low in lime (Ca0:Si02 =
