As several studies have already shown, the addition of fine calcium carbonate (calcite, Cc) is one possible way of limiting the phenomenon of hydrate phase conversion known to occur in calcium aluminate cement (CAC). As a result, the metastable hydrate phases CAH10 and C2AHX, which are converting to C3AH6, are precipitated only as secondary phases next to the stable hydrate phase monocarbonate, especially at room temperature. Research is therefore needed to determine the extent to which current knowledge on CAC hydration in the combinationn with application-relevant additives can be transferred to the hydration path of a CAC-calcite binder. Therefore, the three scientific publications discussed and presented here examine the addition of calcium sulfate, NaOH (accelerator) and fruit acids (retarders) to a CAC-calcite binder. The first two publications focus on the addition of 2. 4 wt. % calcium sulfate in the form of gypsum, hemihydrate or anhydrite. The first publication examined the late hydration (1-365 d) depending on temperature (10, 23 and 40 °C) using quantitative X-ray diffraction analysis (QXRD) and thermodynamic modelling (GEMS). The second publication investigated early hydration (0-24 h) using heat flow calorimetry, in-situ QXRD, pore solution analysis and solubility tests. The results showed that calcium sulfate accelerates the hydration of the CAC-calcite binder depending on the solubility rate of the calcium sulfate (C). In addition, a model for early hydration was developed, which describes that early hydration occurs in two steps. At first, the initially dissolving CA reacts with the calcium sulfate and forms ettringite. In the second step, after the sulfate has been completely consumed, the actual main reaction of CA takes place. At 10, 23 and 40 °C, monocarbonate and AH3 precipitate as stable main hydrate phases in the presence of calcium sulfate. The conversion of initially (during the main reaction of CA) formed CAH10 or C2AHX is accelerated and completed at all investigated temperatures after 14 d at the latest, resulting in a composition close to thermodynamic equilibrium. The third publication analyses the addition of a common accelerator (0. 5-2 wt. % NaOH) and retarder (0. 1-0. 75 wt. % citric acid) and the combination of both. The fundamental reaction mechanism and phase composition (0 h - 28 d) were investigated using heat flow calorimetry, pore solution analysis and QXRD. In addition, application-relevant properties such as workability (Gillmore needle test using an I-meter) and strength development were studied. The acceleration of the reaction by NaOH is achieved by rapidly reaching the maximum supersaturation required for the precipitation of hydrate phases. The formation of monocarbonate alongside AH3 dominates, the strength is increased but the workability is reduced. The retardation due to citric acid addition is caused by preventing the growth of nuclei. Workability is improved, but strength is reduced. For the beginning of the reaction (major drop in CA content), it was possible to demonstrate that the concentration of citric acid in the pore solution needs to drop below a critical level of 115-125 mmol/l. When both additives are combined NaOH improves strength and higher concentrations (0. 5-0. 75 wt. %) of citric acid improve workability again, but the dominant hydrate phase composition shifts from monocarbonate to CAH10. Finally, it was shown that an increased hydration degree within the first 24 h is connected to an increased strength after 28 d.
Pauline Rost (Thu,) studied this question.