Characterization of the Network Structure of Alkali-Activated Aluminosilicate Binders by Single- and Double-Resonance 29si {27al} Mas Nmr Experiments

Abstract

The past decade has shown a renewed research interest in alkali-activated aluminosilicate binders, often referred to as geopolymers, owing to their environmentally beneficial properties of low energy consumption of production and low CO2 emission. The fundamental binder structure of alkali-activated aluminosilicates is a poorly ordered aluminosilicate network of SiO4 and AlO4 tetrahedra linked by Si–O–Si and Si–O–Al bonds. 29Si and 27Al MAS NMR spectroscopy are key analytical tools in the characterization of these systems, since the 27Al and 29Si isotropic chemical shifts provide information about the second coordination spheres of these elements. However, alkali-activated materials may include several different silicate species, and a severe overlap of resonances may occur, preventing a straightforward interpretation of the 29Si NMR spectra. Attempts to combine the beneficial properties of Portland cement and alkali-activated binders have led to increased focus on hybrid systems including both cement types. These new binders are expected to provide both high mechanical strength and durability, relying on the stable coexistence of reaction products from hydration of Portland cement clinker (mainly C-S-H phases) and alkali-activated aluminosilicates (“geopolymeric” gels). In this work, we investigate the structures of conventional and hybrid alkali-activated materials prepared from aluminosilicate precursors, with and without addition of white Portland cement clinker, using 27Al and 29Si MAS NMR. The more advanced NMR techniques of 29Si {27Al} REAPDOR and 27Al MQMAS are applied to obtain improved structural information, facilitating a reliable interpretation of the 29Si and 27Al resonances in the single-pulse NMR spectra. The 29Si{27Al} REAPDOR experiment is shown to be a valuable method to distinguish between different Si−O−Al connectivities, whereas the high-resolution 27Al MQMAS NMR technique makes it possible to investigate aluminate species with similar local structures and coordination environments. The study of sulfate-activated hybrid binders in the Na2SO4–white Portland clinker–metakaolin system provides evidence for the coexistence of reaction products from the hydration of Portland clinker (mainly C-S-H phases) and the alkali activation of metakaolin (“geopolymeric” gels). The structural evolution of the gels is strongly influenced by the Na2SO4 content in the binders and associated with an increased formation of aluminosilicate gel. The results demonstrate that the production of hybrid binders with a stable structure is achievable by the use of alternative activators such as sulfates in conjunction with widely available precursors, which may be of great value in the development of new cement binders.