Hydrogamet phase

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. 

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]. 

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

According to Ding and Beaudoin (1996), the addition of small amounts of sodium silicate promotes the formation of strathngite (C2SAHg) in calcium aluminate cement pastes. At the same time the hydration of the cement is strongly retarded. The additive delays, but does not prevent completely, the formation of the hydrogamet phase in the cement paste. 

Just like the strength development, the release of the heat of hydration also depends on the composition of the cement. In cements yielding mainly the hydrogamet phase an intensive exothermic maximum is apparent within the first hour of hydration. Such a maximum is markedly delayed and the overall amount of released heat is significantly reduced if straflingite is formed (Hormnertgen and Odler, 1991). 

The texture of the hardened material also depends on the quality of the formed hydrates. At equal initial water/cement ratios and degrees of hydration the total porosity and medium pore size are distinctly higher if the hydrogamet phase, rather than straflingite, is formed as the product of hydration (MacDowell, 1986 Hommertgen and Odler, 1991). The specific surface area (BET) of the hydrated material typically hes below 10 m /g. 

In a mature hydrated portland cement, the products formed are C-S-H gel, Ca(OH)2, ettringite (AFt phase), monosulfate (AFm phase), hydrogamet phases, and possible amorphous phases high in Al" and SO4 ions. A small amount of cryptocrystalline CH may be intimately mixed with C-S-H phase. 

The iron analogue of the hydrogamet phase can also form ... 

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. 

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]. 

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. 

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 monosulfate group, also known as the AFm phase, is represented by the formula C4ASH12 or C3A CS Hj2. AFm stands for Al-Fe-mono, in which one mole of CS is present. In portland cement this phase forms after the AFt phase disappears. This phase may constitute about 10% of the solid phase in a mature cement paste. In SEM, this phase has a hexagonal morphology resembling that of Ca(OH)2 and the crystals are of sub-micrometer thickness. The principle ionic substitutions in the AFm phase are Fe for AP+, and OH", C03 , Cl", etc. for S04. The density of this phase is 2.02 g/ml. The amount of crystalline hydrogamet present in cement paste is less than 3 %. It is of the type Ca3 Al2(OH)j2 in which part... 

Cement C-S-H, AFm, AFt, Hydrogamet, Hydrotalcite-like phase, Vicatite (C3S2H3) Any one paste may not have all these phases... 

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