Investigating the nanocrystalline structure and properties of C-A-S-H gels.

Abstract

The dire need for sustainability and environmental protection, as well as the recent environmental degradation around the planet, makes increasing efforts towards alternative binder systems in concrete a necessary step in tackling one of the biggest sources of CO2 emissions worldwide. The nanocrystalline structure of calcium (-sodium) aluminosilicate hydrate (C-A-S-H) gels and their occurrence in various alkali-activated cements has been contested in past decades. Various claims have been made about the similarities between calcium silicate hydrate in ordinary Portland cement (OPC) systems and such gels, but the differences in the nano-scale structures as well as their chemical properties cannot be overlooked. In this investigation, a combination of X-ray diffraction (XRD), pair distribution function analysis (PDF), attenuated total reflectance (ATR) - Fourier transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA) was deployed to offer a more holistic understanding of the structural properties of gels. It is shown that C-A-S-H follows well-ordered patterns in short-ranges, and sodium (in the form of an alkaline solution) is catalytic for the definition of the nanocrystalline structure over time. The elucidation of such similarities and differences can influence the macroscale material properties of the gels, such as durability and weather resistance of alkali-activated cements.