Multiscale structure of calcium- and magnesium-silicate-hydrate gels

10:00 AM - 11:00 AM
Wei-Shan Chiang, Ph.D., Massachusetts Institute of Technology, Cambridge, MA
Materials Science & Technology Division
Building 4500S, Conf Room A-177

Email: Dr. Michael Manley

Calcium-Silicate-Hydrate (C-S-H) gel is the main binding phase of all commercial Portland cements (CaO-based cements), which are the most commonly used construction material in the world. The general composition and structure of C-S-H have been described in the famous Jennings’ Colloidal Model-II (CM-II), which models C-S-H gel formed by packing of the basic building block, called a “globule,” into a fractal structure. However, CM-II lacks detailed information on the microstructure of the globule itself. We derived an elaborate form factor for the C-S-H globules and applied it to analyze the small-angle scattering (SAS) data. As a result, we can extract important structural parameters of both C-S-H globules and C-S-H fractal structures. In addition, by combining SAS analysis with wide-angle scattering and scanning electron microscopy, we are able to investigate the multiscale structure across wide length-scale from a few angstroms to several micrometers. In this talk, I will discuss the impacts of 1) water content, 2) additives of comb-shaped polycarboxylic ethers (PCEs), 3) methylhydroxyethyl cellulose additive, and 4) Ca/Si ratio on the multiscale structure of C-S-H gels. Despite their common use, a disadvantage of CaO-based cements is that their production involves carbon-rich limestone, which generates more than 5% of carbon dioxide emissions worldwide. Possible alternatives are the MgO-based cements (green cements) because their production process produces low or even negative CO2 emissions if part of the MgO reacts with CO2. Nevertheless, efforts need to be made to improve the poor strength and durability of MgO-based cements before implementing widescale use. Due to this concern, we investigated the structure of magnesium-silicate-hydrate (M-S-H), which is the main binding phase of MgO-based cements. Also, a C-S-H/M-S-H blend was studied in order to understand the compatibility between C-S-H and M-S-H gels at the different length-scales. This work can provide essential insights from a structural point of view to improve mechanical properties of the widely used CaO-based cements and the potential green cements based on MgO.

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