Advanced Characterization of Cementitious Materials

Abstract

Early-age hydration reactions contribute to early structure and strength development, which is closely related to transportation and concrete placing on site in real practice. The in-situ hydration reactions of nano Portland cement (PC) and nano PC with nano-silica up to 24 h were realized using liquid cell transmission electron microscopy (LC-TEM). The main hydrated product, calcium silicate hydrate (C-S-H) was found to shed light on a complex early stage process, where C-S-H develops according to both layered structure morphology and aggregation of building blocks. These experiments are the first application of the LC-TEM technique in real-time imaging reaction process of cementitious materials. Also, the effects of nano-silica on microstructure evolution, phase transformation and heat release, especially in early hydration reactions up to 24 h needs further elucidation. The results indicated that the addition of nano-silica provided more nucleation sites for hydrated products leading to more well-distributed precipitates in the reaction. It was also observed that nano-silica had a strong tendency to attach to ettringite crystals and accelerated the conversion of ettringite to monosulfate. In mortar or concrete, the pore systems and distribution of various cementitious phases were characterized using plasma focused ion beam (PFIB) and X-ray micro-computed tomography (CT) to obtain dataset of larger length scale (100s of nm to mm). A workflow working with cement mortar on PFIB was established, in which a hard Si mask was applied to minimize the curtaining effect. The 3D reconstruction models reveal fluid transport pathways through connected pores and cracks. The thickness mesh of anhydrous phases shows different dissolution rates and preferential dissolution locations. This is the first time that PFIB has been used for cementitious materials and bridges a critical mesoscale length scale that dictates some materials properties such as strength and fluid transport properties.