Hydration of a ternary binder model system and the influence of citric acid on phase evolution

摘要

Ternary-binder-like mixes were prepared from synthesized calcium aluminates CA and C_3A, which represent calcium aluminate cement and silicate-free OPC, together with calcium sulfates. The CA-C_3A-Ca-sulfate mixes were investigated with respect to the question of the influence of citric acid on hydration and ettringite formation. For better comparison of the technical with the model system an inert filler was added to be able to use similar w/s ratios and citric acid concentrations as for technical applications. The interesting calcium sulfates, bassanite und anhydrite II, were synthesized from gypsum by temperature-dependent dehydration processes. The cement phases CA and C_3A were synthesized from CaCO_3 and α-Al_2O_3 at 1400°C and ground to an appropriate specific surface area (Blaine 3800 resp. 4700 cm~2/g). The obtained data were refined by Rietveld analysis. During hydration of the mixes with pure H_2O ettringite and minor amounts of monosulfate 14-hydrate are forming. A low amount of 0.1 wt% citric acid in the mixing water leads to more precipitated ettringite during the first 15 min. After the initial hydration of the ternary binder model mix more C_3A is found to be dissolved. But then the dissolution of C_3A is not proceeding until 2 h of hydration and then continues again until 3 h. The dissolution of CA is only little affected when citric acid is dissolved in the mixing water. Bassanite cannot be detected in the pastes after mixing nor with H_2O neither in the citric acid containing solution. Kinetics of ettringite formation is strongly influenced by the presence of citric acid in contrast to its formation in pure H_2O. After 3 h the maximum ettringite content is reached with citric acid. In comparison the maximum ettringite for pure H_2O solution was already reached after 1.5 h. This shows that citric acid is acting as an accelerator during the initial hydration and as a retarder between 15 min and 3 h.