Hydrogamet

One such Ca-aluminate hydrate mineral was observed in fly ash-water pastes (Tishmack 1999). The fly ash analysed in the study was washed with deionized water and Na-EDTA to selectively remove the soluble S-bearing minerals prior to being blended with additional CaO and water. The XRD analysis of the hydrated pastes showed peaks that closely matched the pattern of hydrogamet with one notable exception, a strong peak at 32.1° 2-theta. The phase may be a tetracalcium aluminate hydrate prototype AFm mineral noted to form in systems that have a low S C3A composition (Highway Research Board 1972). It may also be possible that the phase had a composition close to that of hydrogamet with some substitution of Si for Al. 

MacDowell, J. F. 1991. Stratlingite and hydrogamet from calcium aluminosilicate glass cements. In Special Cement Advanced Properties, Mater. Res. Soc. Symp. Proc., 179, 159-179. 

For the ratio MeO/AljOj >2 1, hydrogamets of 3Me0-Al203-6H20 type are usually formed. In pure water, their synthesis occurs at temperature of 200-400°C and pressure of 50-80 MPa. A 100 % yield is observed when the reagents ratio MeO/AljOj is equal to 3 1. In aqueous solutions of NaOH, the stoichiometry can deviate towards MeO excess. 

Geiger et al. (1991) synthesized pyropes with 200-700 ppm H2O at 2-5 GPa, 800-1,200 °C. They proposed that OH was entering the structure in the hydrogamet substitution based on their X-ray and IR data. They noted that the more comphcated IR spectra of natural water-bearing pyropic garnets implicates dilferent substitution mechanism(s) for these garnets compared to the synthetic ones. 

Nobes RH, Akhmatskaya EV, Milman V, White JA, Winkler B, Pickard CJ (2000) An ab initio study of hydrogamets. Am Mineral 85 1706-1715... 

Taylor and Newbury [24] presented the results of analysis in the system of coordinates (A1+Fe)/Ca-Si/Ca and obtained a triangle which is shown in Fig. 4.7. Near the apexes, where there is most of points, the chemical composition corresponds to the following products CH, C-S-H and hydrogamets. Taylor and Newbury are of the opinion that the points in middle position, not near the apexes, correspond to the mixtures of phases. In the pastes matured for a short time the AFm phase was found [25]. The differences between the inner and outer C-S-H was not observed, perhaps except of some minor components their percentage in onter product is a tittle lower [25, 26]. The Ca/Si ratio higher than 2, results presumably from the presence of C-S-H—AFm mixtures in nanometric scale. This is possible because of the layer structure of both phases this perfect mixture of gels can be thns explained [9]. On the other side the monophase mixtures of CH with C-S-H are observed under the microscope only in the pastes with very low w/c [9]. 

The effect of aluminum ions on the transformation of C-S-H into 1.1 nm tobermorite was investigated by Mitsuda and Taylor [184], The solid solution of aluminum ions in tobermorite enlarges also the stability field of this phase in relation to xonothte [185], Diamond [186] found the maximum Al/(Si+Al) ratio in tobermorite to be 0.15 above this value the hydrogamet phase is appearing. Aluminum is tetrahedrally coordinated and accelerates the formation of 1.1 nm tobermorite. Finally, Mitsuda and Taylor [184] revealed that alkalis (l%Na20+1.7%K20) are activating the tobermorite formation from zeolite and calcium hydroxide. 

Also the hydrogamets are resistant to the aggressive action of the sulphates water solutions [181]. The replacement of H2O by Si02 in amount to get the Si02 content in hydrogamet structure close to 0.30 of mole (C3 AS 28H5 causes practically this phase resistant to sulphates. 

At NaOH concentration of 400 mmoles/1 ettringite is not stable already at temperature of 75 °C, and in this condition C AH j as an aluminate phase is formed. At NaOH concentration equal 1 mol/1 the aluminate ions form the amorphous phases, and at temperature of 100 °C the hydrogamet CjAHg is formed. 

The structure of C-S-H phase formed in alkali activated slag cements was studied by Deja [98], He found the elongation of sihcate chains, that is the ordering of the stmcture with its transformation closer to tobermorite, as a function of lydration time. [98]. The aluminate ions occur in the paste of this cement primarily in C-S-H (even about 7 % AI2O3), however, they can form also hydrogamet phase rich in silicon [98]. 

Locher [13] foimd that the gehlenite hydrate is not stable in the calcium hydroxide containing mixture and is transformed into hydrogamet CjASH. CjASH is not the stable phase in the gypsum and calcium hydroxide solutions and in this condition is transformed into ettringite. However, the gehlenite hydrate is stable in the presence of gypsum and NajSO [13]. 

The C AH transforms into hydrogamet with time and the sodium and potassium ions promote this transition [17],... 

In the mixtures of lime and natural pozzolana Sersale [27] found the hydro-gehlenite C2ASHg. It was formed, apart from hydrogamet at later age, from 70 to 150 days, when the amount of combined calcium hydroxide changed from 46 to 60% [28], The formation of hydrogamets was found also by Takemoto and Uchikawa [23]. 

Mixes of C4A3S with calcium hydroxide, but without calcium sulfate, yield the hydrogamet phase C3AHg and an AFm phase of the approximate composition... 

As expected, the rate of hydration accelerates with increasing temperature of curing. Under hydrothermal conditions crystalline compoimds rich in hme are formed at the expense of Ca(OH)2. They include a-C2SH (C2SHQ3 j ), hydroxyl ellestadite [CaOjQ (Si04)3(S04)3(0H)2], and hydrogamets (CjAS Hg 2 ) (Kropp et al, 1986). 

Klimesch, D.S., and Ray, A. (1998a) Hydrogamet formation during autoclaving at 180°C in imstirred metakaohn-hme-quartz slurries. Cement and Concrete Research 28,1109-1117. 

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