Hydrothermal Reactions between Calcium Hydroxide and Amorphous Silica.

BYGUNNAR0. 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 180-
220 At about 120-140' two-stage reactions take place. During the first stage the phase B is formed, thifi phase not being 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
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.
Feb., 1958
THEREACTIONBETWEEN CALCIUMHYDROXIDANED AMORPHOUSSILICA
223
HYDROTHERMAL REACTIONS BETWEEN CALCIUM HYDROXIDE AND
AMORPHOUS SILICA; THE REACTIONS BETWEEN 120 AND 160'
z- 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 phase 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.
One object of the present investigation was to find the experimental conditions under which the phase Z is formed in its least adulterated state. The molar ratio CaO:SiOz in the reaction mixtures was therefore changed in steps of about 0.1 mole CaO per mole Si02throughout the range 0.25: 1 to 1:1 . The behavior of only some of these mixtures can be mentioned in this report. The period of autoclave treatment was, in most cases, 7 days, although where lower temperature (120") was used, the period was sometimes extended.
Materials and Experimental Details.-The materials, silica and lime, were identical with those used in the earlier experiments and described in the i,eports. The special autoclave, with a manometer control of the pressure and valves for removing the air, was the same as that earlier described and its use was necessary to reach experimental conditions which could be precisely defined.
Results
It is well-kno G. 0. Assrtrsson, THISJOURNAL, 61,473 (1957).
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,othe phase rule.
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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.
In spite of all the precautions concerning the experimental conditions it was sometimes concluded that details of the experiments must have varied, as some of the autoclave products could be slightly better crystallized than others even if the phases were the same. The temperature of the furnace for heating the autoclave and of the autoclave itself were controlled automatically, and the heating up period was always identical. There seems therefore to lie n certain disposition for a crystallization of the reaction products caused by nuclear crystallization centers. These observations are especially pertinent to the recrystallization of the phase B, but the variations are without substantial importance in the establishment of the phase formation. Some experiments, however, were repeated as a control on the formation of the phases and, according to t,he phme rule, the same phases were always recovered.