Portland cements (OPC), which are commonly used in the construction industry, are based mainly upon lime-silica mineral phases, whereas in calcium aluminate cements the main reactive phases are lime-alumina compounds. Calcium aluminates (CAC) may go under the other names of aluminous cement or high alumina cement (HAC). Calcium aluminate cements evolved from a drive to develop sulphate resistant cements.
Some of the notation used later in this article uses cement chemistry nomenclature, which is documented in table 1.
Table 1. Cement Chemistry Nomenclature
Hydration of Calcium Aluminates
The principle hydraulic mineral in HAC is CA. Hydration of CA, or in fact any calcium aluminate compound, can be thought of as a three-stage process, of which the primary stage is the dissolution step. For CA, the anhydrous grains react immediately upon their addition to water, dissolving congruently to yield calcium ions and aluminate ions. As a consequence of this, the resulting solution will increase in both conductivity and pH until a point of super saturation is attained. The dissolution process is typically thought of as being congruent, yet there is often more lime than alumina present in solution.
Barret and Bertandie reported this observation to be due to the fact that small quantities of alumina gel precipitate out of solution during hydration via the erquation
The Initial Stage
This initial stage in the hydration reaction is exothermic and associated heat evolution can be detected easily by calorimetric methods. Once a state of super saturation of Ca2+ and Al(OH)4-(aq) species is reached, the reaction reaches a dormant or nucleation stage.
The Second Stage
In the second stage of the hydration reaction (nucleation) the solution will remain super saturated with ions. Dissolution and hydrate formation occurs at a very slow rate, which maintains a very high concentration of Ca2+ and Al(OH)4-(aq) species. A clear reflection of the fact is that pH and conductivity of the solution will be constant.
The Final Stage
At the end of the nucleation phase of the reaction, massive precipitation and growth of the hydrate species will occur. This immediately results in a reduction in the amount of ions present in the solution, so if any reacted CA is present it will quickly be dissolved to attain super saturation once again. However this will precipitate almost instantaneous to yield hydrated species. Dissolution and hydrate precipitation will now proceed simultaneously though eventually the rate of the reaction will become infinitely large as the anhydrous CA diminishes. At this point the conductivity will drop sharply corresponding to the drop of in ionic species present in solution. Mass precipitation of species of this nature in the hydration reaction is accompanied by a considerably large exotherm (which can be detected to determine the end point of the hardening reaction).
Formation of Hydrated Species
The nature of the hydrated species formed is very temperature (ambient air) dependent. Certain hydrates will be formed at specific temperatures. Figure 1 shows the hydration pathway of the CA phase.
Figure 1. Hydration pathway of calcium mono-aluminate.
The hydration of the CA phase is temperature dependent. At lower temperatures CAH10 will form (typically below 20°C). In the intermediate temperature ranges between 21 and 30°C, C2AH8 will form. Under conditions of elevated temperature C3AH6 will form, which is the most thermodynamically stable and the least soluble of the calcium aluminate hydrates. In addition to the hydrates formed at intermediate temperatures crystalline gibbsite (AH3) will form.
Transformation to Hydrogarnet
Due to the metastable nature of CAH10 and C2AH8 over long periods of time or at elevated temperatures, both phases will undergo a transformation into the hydrogarnet phase (C3AH6.) This reaction is known as conversion. Conversion is initiated by the nucleation of C3AH6 and takes place in solution. As well as obvious changes in the chemistry during conversion, there are accompanying changes in the physical properties which are summarised in table 2.
Table 2. Dehydration of calcium aluminate hydrates.
Dehydrating temperature (°C)
In the transformation from CAH10 to C3AH6 a 52.5% reduction takes place, and during the change from C2AH8 to C3AH6 there is a 33.7% reduction. In addition to this, the porosity increases.
Changes Taking Place During Conversion
Another effect of conversion is the release of water from the hydrates. A major impact of this increase in porosity and shrinkage is a decrease in mechanical strength. Conversion is a very topical subject in the field of calcium aluminate cements and the phenomena has been responsible for the failure of many buildings and bridges.