Hexagonal aluminate hydrates

The formation of the hexagonal aluminate hydrate and conversion to the cubic form is accelerated by TEA. 

Ramachandran followed the hydration of CjA (with and without gypsum) containing triethanolamine. It was found that TEA accelerated the hydration of C3A to the hexagonal aluminate hydrate and its conversion to the cubic aluminate hydrate. The formation of ettringite was also accelerated in the C3A-g q)sum-H20 system. 

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... 

It has been found that at full saturation with water the there is about I6H2O in the monosulphate phase (d= 959 pm), while at RH lower than 90 % this monosulphate transforms to the hydrate with 12H2O molecules, having c =899 pm [116]. Monosulphate is formed also as a result of reaction between the ettringite and hexagonal aluminates. 

Spmng and Rechenberg [132] state that in the pastes of cements rich in alkalis C3A dissolves quickly because of the reduced Ca(OH)2 solubility. At low gypsum content the quick set can be observed as a result of the hexagonal calcium aluminate hydrates formation In the presence of gypsum excess the similar effect can be the consequence of ettringite formation. 

At low or medium water/cement ratios the porosity and permeability of hydrated non-converted aluminous cement pastes are sufficiently low to confine the corrosive action of any external chemical agents to the surface region of the concrete structure. However, as the porosity increases in the course of conversion, the susceptibility to chemical attack of concrete based on aluminous cement increases. An effective way to prevent this from happening is to use initial water/cement ratios that are too low for complete hydration. Under these conditions the water liberated in the conversion of the hexagonal calcium aluminate hydrate phases, formed initially, reacts with the non-hydrated fraction of the cement, thus preserving a low porosity of the hardened paste. Note that the permeability is the main factor determining the resistance of aluminous cement concrete to chemical agents, and this has to be kept in mind when calcium aluminate cement is used in practice. 

As a result of exposure to humidity and even a slightly elevated temperature, a conversion reaction may start in hardened cement paste. It develops in the calcium aluminate hydrates where hexagonal crystals are transformed into cubic ones, which have smaller volume. This causes an increase of porosity and considerable decrease of strength. It has been proved that even a temperature over 20°C can initiate the reaction for which the remaining amount of mixing water may be sufficient. The conversion reaction may... 

Tricalcium aluminate hydrates to form, initially, hexagonal phases identifiable by endothermal effects in DTA at 150-200°C and 200-280°C. These are converted to a cubic phase of formula C3AHg which exhibits endothermal effects at 300-350°C and 450-500°C. The addition of ligno-sulfonate influences the rate of formation of these phases and their interconversions to the cubic phase. Depending on the amount of lignosulfonate, the hexagonal phase may be stabihzed even up to fourteen days or more with lignosulfonate, but in that hydrated without the admixture, the cubic form may appear at six hours or earlier (Fig. 1). ... 

The addition of CH (Fig. 18) shifts the second and third peak to an earlier stage. The formation of the calciiun aluminate hydrates are accelerated, as is the conversion of the hexagonal hydrate into CjAHg. 

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. 

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 ... 

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