In-situ microscopic investigations of the interaction of admixtures and solid surfaces in cementitious systems

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The production of mineral based high performance materials is not possible without the use of organic admixtures. The increasing diversity of application of these concretes and mortars and the rising number of supplementary cementitious materials necessitates a higher specialization and efficiency of the organic admixtures. Superplasticizers, like polycarboxylate ethers, are necessary for concretes with a low water/binder ratio like ultra-high performance concrete (UHPC). Another organic admixtures used in cement based systems is redispersible polymer powder (RP). For instance, they are used in tile adhesive mortars to improve the mechanical properties of the hardened mortar. Especially for improving the adherence on non-porous surfaces, adding RP to cementitious materials is effective. Though, these RP are also used in cement based adhesives for joining structural elements of fibre-reinforced UHPC especially improving the adherence to the steel-fibres, which is part of a current research project conducted in the department of the applicants. For both examples the interaction of organic admixture and mineral or metallic surfaces is crucial. The mechanisms of action of both types of organic admixtures were already investigated in general, but no in-situ-method for detailed analyses of the interaction of mineral or metallic surfaces and organic admixtures exists. The aim of this project is a development of such an in-situ-method using fluorescence microscopy, which is well known from other disciplines like biology. Admixtures are usually not visible in conventional microscopic methods. Therefore, the admixture molecules will be stained with a fluorescing dye making them visible in fluorescence microscopy. In cementitious materials, the parameters pH value, ion concentration, concentration of admixture and character of solid surface are crucial and thus will be varied. For validation of this method, the distribution of admixtures in dependency of those environmental influencing factor will be compared to the results of common methods and macroscopic properties of related mortars. This novel in-situ-method allows investigations on the interaction of new admixture-binder-combinations enabling the prediction of macroscopic properties. This fluorescence microscopic method will be developed and validated with this two above mentioned application examples: 1. In dependency on the environment the adsorption of superplasticizers on different mineral surfaces will be analysed in situ. The results will be correlated to rheological properties of related mortar. 2. The improving properties of RP on adhesion strength of mineral adhesives will be tested by pullout-tests of single steel fibres. By fluorescence microscopy the distribution of RP at the interface to the steel fibre will be analysed and compared to the macroscopic pullout tests.