The early age reaction kinetics and microstructural development in alkali-activated slag binder are discussed. In-situ isothermal calorimetry was used to characterize the reaction progression in ...sodium hydroxide and sodium silicate-activated slag binders cured at ambient temperature. Microstructure and strength development were monitored to correlate the heat evolution with the property development. In-situ isothermal calorimetric data for sodium hydroxide-activated systems exhibited only one major heat evolution peak with no dormant period. Sodium silicate-activated pastes exhibited multiple peaks and extended dormant periods. Microstructural evolution, monitored using BSE–SEM, showed rapid product formation on the surface of slag grains in sodium hydroxide-activated systems, forming thin reaction shells—the thickness of which was related to the activator concentration—and leading to diffusion controlled hydration at a very early stage. Sodium silicate-activated systems exhibited slow and progressive product formation, predominately nucleated from the solution. These results are supported by electron mapping and electron dispersive X-ray spectroscopy.
The influence of starting material on the hydration kinetics and composition of binding gel in alkali activated binder systems was evaluated. The starting materials used were ground granulated blast ...furnace slag, class C fly ash and class F fly ash. All starting materials were activated using alkaline solution with a SiO2/Na2O ratio of 1.5. The hydration kinetics were monitored using in situ isothermal conduction calorimetry and the chemical compositions of the binder gels were determined by energy dispersive X-ray spectroscopy. In the fly ash systems, the calorimetric curves had only one peak, which occurred in the first 30min of reaction, and lacked an induction period. Two peaks were distinguishable in slag systems, though the induction period was much shorter than that of a typical OPC system. The gel composition ratios, including Ca/Si, Na/Si, Na/Al and Al/Si, were different in each of the systems and are discussed in detail.
•The effects of activators and temperature on activation kinetics are investigated.•Elevated temperature and increased activator alkalinity greatly accelerate hydration.•Increased silica retards ...hydration but improves later-age strength.•The main product is C-S-H with varying levels of hydrotalcite.
The early-age reaction kinetics of alkali-activated ground granulated blast-furnace slag (GGBFS) binders as determined by in-situ isothermal calorimetry are discussed in this paper. Particular attention is paid to the effects of activator type (sodium hydroxide and sodium silicate) and concentration, as well as curing temperature (23°C and 50°C). The mechanical strength development, microstructure, and product phase composition are also discussed to provide context for the phenomena observed in the kinetics results. It is shown for both activators that elevated temperature curing greatly accelerates hydration, resulting in more rapid product formation and strength development. High-molarity sodium hydroxide activators are shown to accelerate early hydration at ambient temperature, but tend to present a barrier to advanced hydration thereby limiting the later-age strength. Elevated temperature curing is shown to remove this barrier to advanced hydration by improving solubility and diffusivity. Hydration of sodium silicate-activated slag is comparatively slow, resulting in the delayed formation of very dense products with higher mechanical strength. Increasing sodium oxide tends to accelerate hydration, resulting in improved early- and later-age strength, while increasing the silica tends to retard the reaction, resulting in slower, more complete hydration as well as improved mechanical strength.
An experimental investigation into the micromechanical properties of alkali-activated slag cement (AASC) binders was carried out using targeted and grid nanoindentation. The results of grid ...indentation techniques were deconvolved using Gaussian mixture modeling with Bayesian model selection to determine the appropriate number of component phases for the model. The information given by the resulting mixture models and from targeted indentation experiments was disseminated in the context of existing information about the composition and development of the microstructure in AASC binders. The microstructure of sodium silicate-activated slag cement contains only two components (ground mass gel and unreacted slag cement) upon microscopic examination, but indentation data suggest that it is much more complex and varied. The microstructure of sodium hydroxide-activated slag cement contains ground mass gel, unreacted slag cement, and an inner product ring surround the unreacted slag. The inner product is denser, harder, and stiffer than the surrounding product phases. The micromechanical properties in sodium hydroxide-activated slag cement are not affected by activator molarity; the macroscale strength is similarly unaffected. Conversely, the micromechanical properties of sodium silicate-activated slag show a slight improvement with increased silica modulus, while the macroscale strength shows a significant improvement. The macroscale improvement is likely due to the increased size of unreacted slag cement grains, which are shown to be very hard and stiff.
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