Direct insight into the admixture activity during cement hydration using levitated droplets

Chemical admixtures are widely used to adjust properties of the cementitious mixtures. Different superplasticizers (SP) are typically added to improve the flowability of the fresh cement paste. Their working period is the initial stage of the cement hydration from its beginning until stiffening. Currently, polycarboxylate-based SP (PCE-SP) are the most effective ones. They can act beyond their aimed function causing changes in the hydration course, e. g. retardation effects. The exact mechanisms and influencing factors are still under investigation. However, the occurrence of the retardation effect indicates the involvement of PCE in the hydration reactions. Some observations indicate, that the retardation depends on the type, molecular structure and concentration of PCE, and the presence of other admixtures (stabiliser, ST). These factors influence further the working time of admixture and its amount related efficacy in the flow improvement. We investigate the cement hydration process in situ in levitated droplets. High resolution synchrotron X-ray diffraction allows gathering temporal course of the hydration reactions. The measurement setup is based on an ultrasonic levitator allowing in situ investigation of the temporal changes of the phase composition in the hydrating cementitious system. Further, the hydration can be followed ab initio avoiding the time loss for the sample preparation and filling in the sample holder. The collected data allows to conclude about the mechanisms of the admixture action and their involvement into the ongoing hydration process.