Optimization of Alkali Activated Portland Cement—Calcined Clay Blends Based on Phase Assemblage in the Na2O–CaO–Al2O3–SiO2–H2O System
In the development of low-CO2 emission binders, the inclusion of small amounts of alkali activators in hybrid Portland cement systems may be used to increase the Portland cement replacement level above that in common Portland cement – SCM systems. The co-precipitation of C-S-H and N-A-S-H-type hydration products in these systems has been investigated by studying the influence of Ca2+ and Na+ ions on the formation and compatibility of C-A-S-H and N-A-S-H phases, prepared from alumino-silicate materials with fixed Si/Al ratios and sodium concentrations but variable calcium contents. The structural characterization, by means of solid-state MAS NMR, thermal analysis and powder X-ray diffraction, shows that C-A-S-H and N-A-S-H phases can coexist in the low-Ca part of the CaO-Al2O3-SiO2 system, as seen by the presence of zeolitic phases by XRD and C-A-S-H phases by NMR. The formation of N-A-S-H phases seems to be less dependent on the Si/Al ratio, at least for the studied molar ratios of Si/Al = 1.0 and 2.0. For high fractions of calcium, C-A-S-H phases and strätlingite are the main hydration products. The results from the studies of the synthetic samples in the Na2O – CaO – Al2O3 – SiO2 – H2O system have been used to propose optimum compositions for hybrid cement blends based on Portland cement, metakaolin (as SCM) and small amounts of an alkali salt.
KeywordsPortland Cement Hydration Product Alkali Activator Calcium Aluminate Cement Main Hydration Product
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The Danish Council for Strategic Research is acknowledged for financial support to the LowE-CEM project.
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