Cubic aluminate hydrate

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. 

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

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... 

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). ... 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

It is also suggested that the cubic hydrate (C3 AH ) does not have a binding capacity. For example, the strengths of the aluminate hydrates are thought to be in the order CAHjq > CjAH > C3AH6. 

The conversion of CAH q to C3AH5 results in a volume decrease to about 50% whereas that of C2AH8 to the cubic phase results in a decrease of about 65% of the original volume of the reactants. It is apparent that methods to identify and determine the amounts of the aluminate hydrates in CAC concretes are useful for a meaningful diagnosis of potential problems. 

In this final peak, the remaining tricalcium aluminate reacts with both gypsum and water to form ettringite and subsequently the tricalcium aluminate monosulfate. C3A hydrates formed are of varying composition but eventually form the stable cubic phase C3AH6 in solid solution with the tricalcium aluminate monosulfate. 

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]. 

In Portland cements tricalcium aluminate usually exists in its cubic form however, in the presence of increased amounts of alkalis in the raw mix an orthorhombic or even a monoclinic modification may be formed instead. The stmcture of tricalcium aluminate is built from rings of six AIO4 tetrahedra and Ca ions. All three modifications of C3A hydrate in a similar way however, their reactivity may differ, depending on the quality and quantity of the foreign ions incorporated in their crystalline lattices. 

Tricalcium aluminate reacts with water to form C2AH8 and C4AH13 (hexagonal phases). These products are thermodynamically unstable so that without stabilizers or admixtures they convert to the C3AH5 phase (cubic phase). In a paste, hydration is slightly retarded in the presence of CH. In dilute suspensions the first hydrate formed is C4AH19. 

The alkali in clinker is combined as a solid solution with the C3A phase. The crystalline stmcture changes from cubic to orthorhombic or monoclininc structure, depending on the content of Na in the C3A phase. Shin and Han studied the effect of different forms of tricalcium aluminate on the hydration of tricalcium silicate by applying DTA, TG, and conduction calorimetry. It was concluded that the hydration of tricalcium silicate is accelerated when orthorhombic, monoclinic, or melt C3A was present in the mixture. The cubic form of tricalcium aluminate was least effective for accelerating the hydration of the silicate phase. 

It has also been observed that a sugar, in certain concentrations, may also act as an accelerator for the hydration of the tricalcium aluminate systems. Thermograms indicate that at 0.025-0.05% sucrose dosage, peaks due to cubic phases are intensified or those due to the hexagonal phases are decreased. This would show that sucrose is an accelerator. At concentrations of 2-5%, however, the number of hexagonal phases formed is diminished indicating that the hydration of C3A is retarded. ]... 

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