The Hydration of Calcium Aluminate Cements

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Ordinary Portland cements (OPC), generally used in the construction sector, are mainly based on lime-silica mineral phases. On the contrary, lime-alumina compounds are the core reactive phases in calcium aluminate cements.

Calcium aluminates (CACs) may be known by many other names such as aluminous cement or high alumina cement (HAC). CAC was developed following the demand to produce sulfate-resistant cements.

Cement chemistry nomenclature used in a few of the notations mentioned in this article, is listed in Table 1.

Table 1. Cement Chemistry Nomenclature

Hydration of Calcium Aluminates

The key hydraulic mineral of HAC is CA. Hydration of CA, or any calcium aluminate compound, takes place in a three-stage process. The first stage in this process is the dissolution step. The anhydrous grains contained in CA react instantly upon adding water, dissolving simultaneously, to form calcium ions and aluminate ions.

Thus, pH and conductivity of the resulting solution increase until a point of super-saturation is attained. The dissolution process is typically said to be steady, but there is often more lime than alumina present in the solution.

CaAl₂O₄₍₅₎ + 4H₂O -> Ca²⁺_(aq) + 2Al(OH)^-_(aq)

Barret and Bertandie claimed that this was due to the fact that small quantities of alumina gel precipitate out of the solution at the time of hydration, conforming to the following equation:

Al(OH)₄^-_(aq) -> Al(OH)_3(s) + OH ^-_(aq)

The First Stage

The hydration reaction’s first stage is exothermic, and calorimetric methods can be used to easily detect associated heat evolution. The reaction attains dormant or nucleation stage upon reaching a state of super-saturation of Ca²⁺ and Al(OH)₄⁻_(aq) species.

The Second Stage

In the second stage of the hydration reaction (nucleation), the solution remains super-saturated with ions. Dissolution and hydrate formation occur very slowly, which maintains a very high concentration of Al(OH)₄⁻_(aq) and Ca²⁺ species. A clear reflection of the fact is that the pH and conductivity of the solution stay constant.

The Final Stage

In the reaction’s last nucleation phase, considerable precipitation and growth of the hydrate species occur. This instantaneously results in the decrease in the volume of ions present in the solution; therefore, if there is any reacted CA present, it quickly dissolves to attain super-saturation once again. However, this will precipitate almost instantly to create hydrated species.

Dissolution and hydrate precipitation will now proceed concurrently, though the reaction rate will eventually become considerably large with the decrease in the anhydrous CA. At this point, the conductivity declines radically, equal to the decline in the ionic species present in the solution.

Mass precipitation of such species in the hydration reaction takes place along with a considerably large exotherm (which can be identified to define the end-point of the hardening reaction).

Formation of Hydrated Species

The nature of the hydrated species formed is very much dependent on temperature (ambient air). Certain hydrates form at particular temperatures.

Conversion

The hydration of the CA phase is temperature-dependent. CAH₁₀ forms at lower temperatures (typically below 20 °C). C₂AH₈ forms in the intermediate temperature range from 21 °C to 30 °C. C₃AH₆, which forms at higher temperatures, is highly thermodynamically stable and least soluble of all the calcium aluminate hydrates. Apart from the hydrates developed at intermediate temperatures, crystalline gibbsite (AH₃) is also formed.

Transformation to Hydrogarnet

CAH₁₀ and C₂AH₈ are metastable at higher temperatures or over longer time periods, causing both phases to undergo conversion into the hydrogarnet phase (C₃AH₆). This reaction is known as conversion.

Conversion is initiated by the nucleation of C₃AH₆ and takes place in the solution. In addition to obvious variations in the chemistry during conversion, there are related alterations in the physical properties, which are mentioned in Table 2.

Table 2. Dehydration of calcium aluminate hydrates

When CAH₁₀ is converted to C₃AH₆, there is a 52.5% reduction, and when C₂AH₈ is converted to C₃AH₆, there is a 33.7% reduction. Apart from this, there is a boost in porosity.

Changes that Occur During Conversion

Another outcome of the conversion is the discharge of water from the hydrates. A massive impact of this increase in porosity and shrinkage, is a decrease in mechanical strength. At present, conversion is a hot issue in the field of calcium aluminate cements, as it has resulted in the failure of several buildings and bridges.