SETTING CONTROL OF CALCIUM ALUMINATE CEMENT USING CALCIUM SALTS OF SUBSTITUTED ACETIC ACIDS

摘要

Organic admixtures such as organic acids or their calcium salts typically act as retarders on the setting of calcium aluminate cements (CAC). In the present work the calcium salts of different systematically selected derivatives of acetic acid, i.e. calcium formate, acetate, propionate, iso-butyrate, pivalate, acrylate, methacrylate, mono-, di- and trichloroacetate, glycolate, glyoxylate, lactate and pyruvate, were tested as retarding agents by isoperibolic heat flow calorimetry. It can be demonstrated that the affected setting delay depending on the admixture concentration strongly correlates with the pKa values of the corresponding acids. Within the applied admixture concentration range 0.001M ≤ c_(admix) ≤ 0.05 M the strength of the retarding effect can be measured by the exponent m as a retardation power factor or m·A as the retardation power product of the functional dependence of the times of onsets t_(onset) or heat flow maxima t_(max): ln(t) = A·e~(mc). Both values allow for a comparison of the retarding effects of the different admixtures. The calcium salts of the a-hydroxyl acids were found to be the strongest retarders. However, for this group the functional dependence of the setting times are better described by polynomial functions. The retardation mechanism of glycolic acid identified as a dissolution-precipitation action of calcium complexes implying also the stronger retarding effect of calcium glycolate. Within the tested concentration range the effect on the hydration products was found to be negligible. For all tested compounds the ranges of thermal stability of the hydrated as well as the anhydrous phases are given. Crystallographic data are provided for phase identification and quantification. Moreover, it can be shown that the strong retarding effect of the calcium salts of the a-hydroxyl acids are due to the involvement of the hydroxyl groups in the bonding of calcium.