Hydration kinetics of CA2 and CA—Investigations performed on a synthetic calcium aluminate cement
Introduction
The main use of calcium aluminate cement (CAC) is in applications which require substances which combine the advantages of casting at ambient temperatures with excellent performance at temperatures above 1700 °C. CACs with 70–80 wt.% Al2O3 can be used to produce castables with a temperature resistance ranging from 1800 °C to 1900 °C. In order to improve technical performance, low-cement castables (LCC) were developed in the mid 1980s. A very low cement content of 5–8 wt.% is a common feature of such LCCs. It also proved that it is possible to achieve a significant strength improvement in these LCCs by optimizing the particle packaging, that is to say, by the mixing of coarser particles with finer filler particles.
The main hydraulic phase of all CACs and LCCs is CA. The specific hydration product which tends to be formed by this CA is strongly dependent on ambient temperatures. According to several studies, the initial hydration product emerging within the temperature range between 0 and 20 °C is predominantly CAH10 [1], [2] together with amorphous AHx. After longer hydration periods the amorphous phases can convert into crystalline phases, such as CAH10, C2AH8/C2AH7.5 and AH3 [3], [4], [5]. In the temperature range between 20 and 30 °C C2AH8 and AH3 are the prevailing hydration products. In the temperature range between 30 and 90 °C, however, it is predominantly hydrogrossular C3AH6 together with AH3 that is precipitated [6]. According to the phase equilibrium [7] CAH10 and C2AH8/C2AH7.5 are metastable phases which will finally be converted into stable C3AH6. In refractory applications the conversion into hydrogrossular (C3AH6) plays no role because the hardened castable will always be heated up before use. In white, Al2O3-rich CACs monocalcium dialuminate (CA2) is the second main phase which emerges. CA2 tends to react very slowly within the first 48 h. For this reason, in many earlier studies it has been the hydration of CA alone that has been intensively investigated. For a long time the hydration of CA2 was an open issue [8] and CA2 was even assumed to be inert. Then different investigators such as [9], [10], [11] reported hydraulic reactivity of CA2, although the reactivity reported was a lower one than the reactivity of CA. Chudak et al. [12] also investigated pure CA2 with 70 wt.% Al2O3, using the DCA technique, and describes a slow CA2 hydration over several days. Negro et al. [13] concluded that the solubility of CA2 is much lower than that of CA. This could explain the very low hydration reaction of CA2. Edmonds and Majumdar [14] reported hydraulic activity of CA2 in Secar 71, beginning after 28–48 h and lasts up to several months. AH3 was observed by Song et al. [15] to precipitate during hydration of CA2 at ambient temperatures.
In this study different ratios of CA/CA2 were investigated using quantitative XRD and heat flow calorimetry. Combination of data from both experimental methods led to the result that the heat contribution during hydration from dissolution of each phase for itself could be described by calculation from standard enthalpies of formation for CA, CA2 and the hydrate phases.
Section snippets
Material and methods
For the synthesis of CA and CA2, CaCO3 (Sigma-Aldrich) and α-Al2O3 (Alfa Aesar) were homogenized in a vibrating disk mill with agate tool according to their stoichiometric composition. Both mixtures were decarbonated for about 17 h at 1000 °C ± 30 °C. CA and CA2 were sintered two times, for 4 h and 5 h respectively, at 1400 °C ± 30 °C with a milling process taking place between the first and the second sinter processes. CA and CA2 were ground to a specific surface of 3200 cm2/g ± 300 cm2/g (CA) and 4500 cm2/g ±
Heat flow calorimetry for the hydration of CA–CA2 mixes
Fig. 1 shows the heat flow in combination with the heat of hydration for the mixtures (10/0) and (10/10) at a w/s of 0.45 together with their heat of hydration. The standard deviation for the heat of hydration after 22 h (H22h) was determined from three independently measured data sets. The plotted curves are in each case the averaged curves.
The heat flow of the mixture (10/0) consisting of CA alone, without CA2, (represented by the black solid line) reaches an induction period after the initial
Hydration of CA2
The influence of CA2 on the heat flow during hydration of synthetic CAC, which is shown in Fig. 1, is best observed after the main reaction, due to CA dissolution. We were able to prove, in the case of the mixture investigated by us (10/10), which contained 10 wt.% CA and 10 wt.% CA2 at the beginning of hydration, CA2 was dissolved later – approximately after 4 h – and very slowly compared to CA. The synthetic CAC mixture (10/0) without CA2 shows no further reaction after 14 h of hydration. After
Conclusions
In this work, we have been able to show by calculation of heat flow from the course of dissolution of CA and CA2 (Fig. 3, Fig. 4) that hydration of CA and CA2 proceeds with a complete different kinetics (Fig. 8). The same course of dissolution for both phases can again be observed in the mixes with various amounts of CA and CA2. There is no change of dissolution behavior of CA in the presence of CA2.
The heat flow occurring during main reaction of CAC hydration is brought about primarily by CA.
Acknowledgments
The authors wish to acknowledge Almatis for the financial support.
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