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Time-resolved in situ investigation of Portland cement hydration influenced by chemical admixtures
(2016)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
Here, we report on a first mechanochemical synthesis of ettringite, an important cement hydrate phase. The mineral compound ettringite ([Ca3Al(OH)6]2·(SO4)3·26H2O) occurs rarely in nature, but is common for cement-based materials. Ettringite has wide technical application in the ceramic and paper industry. However, its typical wet-chemical synthesis is cumbersome and produces waste water and CO2 emissions. Here, we investigate the first mechanochemical synthesis of ettringite for developing an easy and sustainable alternative for industrial application. The mechanosynthesis was monitored in situ by coupled synchrotron X-ray diffraction (XRD) and infrared thermography (IRT). The consumption of the reactants and the formation of the reaction product were monitored with time-resolved XRD. IRT showed the temperature increase based on the exothermic reaction. The reaction conversion was significantly improved changing the strategy of the mechanosynthesis from a one- to a two-step process. The latter included neat pregrinding of solid reactants followed by a delayed addition of the stoichiometric amount of water. Thus, an increase of reaction conversion from 34 to 94% of ettringite could be achieved.
The crystallization of ciprofloxacin - an antibacterial fluoroquinolone compound - with salicylic acid resulted in the isolation of five distinct solid forms of the drug, namely, an anhydrous salt, two polymorphic forms of the salt monohydrate, methanol and acetonitrile solvates, and the salt-cocrystal hydrate. The salicylate salts were investigated by different analytical techniques ranging from powder and single crystal X-ray diffractometry, differential scanning calorimetry, thermogravimetric analysis, variable temperature powder X-ray diffraction, dynamic vapor sorption analysis, dissolution, and solubility investigations. Real-time in situ Raman spectroscopy was used to investigate the mechanochemical formation pathways of the different solid polymorphs of ciprofloxacin salicylate. The mechanism of the phase transformation between the crystalline forms was evaluated under mechanochemical conditions. It was found that the formation pathway and kinetics of the grinding process depend on the form of the starting material and reaction conditions. The analysis of the solid-state thermal evolution of the hydrated salts revealed the two-step mechanism of dehydration process, which proceeds through a formation of the distinct intermediate crystalline products.