Hexagonal hydrate

In the C-A-H system, protected against the CO2 influence, there is a large number of so-called hexagonal hydrates, ciystalhzed in the form of hexagonal plates. These are the metastable phases, because cubic CjAHg is the only stable calcium aluminate hydrate [83, 84]. This phase is, however, formed in the reaction of calcium aluminates with water only at temperature higher than 45 °C [85]. At lower temperatures... 

The layer structure of hexagonal hydrates can be derived from their morphology the thickness of the basic layer corresponds to the largest interplanar spacing on the XRD pattern. These layers are composed of [Al(OH)g] octahedra and [Ca(OH)J" tetrahedra hnked with their comers (Fig. 3.35). The composition of the main layer is [Ca2Al(OH)J+ [73]. The stracture of these phases is related to the Ca(OH)2 stracture in which every third Ca + ion is substituted by Al + and Fe. The smaller aluminium and iron ions cause the disordering of this stracture and influence of some shifting of Ca ions and enable their coordination by H2O molecules from the interlayer plane, beside of the six OH" ions. Therefore Taylor [62] included these molecules to the main layer composition, which then reveals the composition... 

The rapid formation of the hexagonal hydrates can be easily explained based on the C-A-H system. The dissolution of CjA results in the molar ratio CaO/ Al203=3 in the liquid phase (congraent dissolution) or higher than 3 (incongment dissolution). This solution becomes quickly supersaturated in relation to the hexagonal hydrates (point M in Fig. 3.45). 

The addition of gypsum is radically modifying the C3A hydration process. A long induction period appears, followed by the ciystallization of ettringite (Fig. 3.48). The preinduction period depends substantially on the rate of sulphate dissolution [ 127]. As it is commonly known, C3 A reacts with water violently and in the absence of gypsum hemihydrate or sodium and potassium sulphates, a certain amount of hexagonal hydrates is formed in the pre-induction period or even the monosul-phoaluminate [128]. 

The rate of hydration decrease is explained by the densification of the stmcture, as a result of occupying the voids in the centres of [AlgO,g] rings in C3A by sodium ions. The transformation of hexagonal hydrates into the cubic C3AHg is also retarded [139]. 

A rapid reaction of C3 A with water is oidy possible after gypsum consumption this occurs commonly after 24 h of hydration. Then the hexagonal hydrates are formed and react with ettringite to form AFm. The direct reaction of CjAwith ettringite or rather locally present aluminate ions with ettringite is also possible. 

Hydration of CA leads to the formation of two hexagonal hydrates CAHj and C2AHg. CAHjo is formed at lower temperatures, not exceeding 20 °C the ratio of C2AHg increases with temperature. At temperature above 30 °C both hexagonal hydrates transform to the only stable cubic CjAHg phase. 

Usually in the process of CA hydration the mixture of the two hexagonal hydrates together with the colloidal aluminum hydroxide are formed. It is all the more probable that calcium aluminate cement contains always some amount of C,2A2 phase, which hydrating gives at once these two hexagonal phases ... 

The C/A molar ratio is usually 1.06 because of low amount of AH3 precipitation [17]. Exceeding the ratio of C/A> 1.2, for example in the presence of very low CJ2A2 content, causes the rapid crystallization of hydrates, due to solution supersaturation in relation to the hexagonal hydrates (Fig. 9.3). 

The hexagonal hydrates are unstable and transform into CjAHg at temperature range 35-45 °C in cubic CjAHg. It is the only stable hydrate in the CaO-Al203-H20 system. The rate of conversion depends mainly on three factors temperature, humidity and pH. In the dry concrete the conversion does not occm. At higher pH in the presence of alkalis the conversion is accelerated [18]. 

Fig. 9.3 The system CaO-AijOjU O at 2i °C i hexagonal hydrates, 2 miero-crystalline eom-pounds, 3 gibbsite...

The conversion of unstable hexagonal hydrates into the cubic CjAH is associated with elution of high water amount according to the reactions ... 

There are controversial opinions about the corrosion of reinforcing steel in the calcium aluminate cement concretes. This is linked with the less basic paste in comparison with the Portland cement one. However, it appeared in practice, that in the good quality concrete there was no difference related to the stability of steel reirrforcement in comparison with Portland cement. The destmction of reinforced concrete was linked with high w/c, in which the conversion of hexagonal hydrates into cubic caused the porosity and rapid carbonation increase [12]. 

Amorphous hydroxides of Fe and A1 form in the reaction of C4AF. The thermod5mamieally stable product is C3(A,F)Hg and this is the eonver-sion produet of the hexagonal hydrates. Seldom does the formation of these hydrates eause flash set in eements. 

Addition of lignosulfonate retards both the C3A hydration and the conversion of hexagonal hydrates to the cubic phase, Ca lignosulfonate... 

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