Synchrotron X-ray Spectro-microscopy and Micro-diffraction Study on the Hydration of Tricalcium Silicate including High-Volume Fly Ash

Understanding on calcium silicate hydrates (C-S-H) formed in the hydration process of Portland cement (PC) phases and the atomic bonding structure of the C-S-H are very crucial to optimize the PC-based materials. Optimization of PC will contribute to enhance sustainability and energy efficiency of concrete structures by reducing use of PC, however, certain details of C-S-H are partially known.;The major objectives of this thesis were to elucidate a hydration process of pure triclinic tricalcium silicate (C3S). In situ spectro-microscopy was carried out using C3S solution in a wet cell to determine the morphological and structural environment changes surrounding the Ca and Si atoms in the early age hydration of C3S. Synthesized calcium silicate hydrate (Syn-CSH) with different Ca/Si ratios was used to quantitatively evaluate the effect of silicate polymerization on the local binding structure of C-S-H.;As a secondary objective, this thesis was aimed at clarifying the effect of high-volume fly ash (HVFA) on pure C3S and PC hydration. Synchrotron soft x-ray scanning transmission x-ray microscopy (STXM), x-ray diffraction imaging, and monochromatic scanning x-ray micro-diffraction (mu-SXRD) were utilized.;Near edge x-ray absorption fine structure (NEXAFS) analysis showed that Syn-CSH had no variation in peak positions and energy separation for Ca L III, II edge for a wide range of the Ca/Si ratios. The Ca LIII, II edge NEXAFS of 17 days hydrated C3S showed different local structure with anhydrous C3S. Compared to Syn-CSH, C-S-H of C 3S was proved to have a very similar local structure around the Ca but with less crystallinity. Si K edge NEXAFS analysis on Syn-CSH showed a tendency for the peak positions of both the Si K edge and the peak induced by multiple scattering to shift to higher energy levels. The results also indicated that the distance between the two peaks increased with a decrease of the Ca/Si ratio in Syn-CSH. Silicate polymerization influenced the multiple scattering contributions from distant shell atoms more than the binding energy of the core atoms.;In situ STXM analysis using pure C3S solutions in a wet cell showed morphologic changes in the dissolution of the Ca and the precipitation of C-S-H on the C3S particles in the process of hydration. Outer product (Op) layers of C3S showed higher degree of silicate polymerization compared to core area of C3S. The degree of silicate polymerization of Op increased during the hydration process. Ex situ STXM analysis on 17 days hydrated C3S particles showed that Op layer and core area had a different degree of silicate polymerization. The results imply the Op layer reduces the hydration process of C3S in the core area. Soft x-ray diffraction imaging showed Op and inner products of 17 days hydrated C3S had fibrillar and an inter-globular structure with voids, respectively.;In a C3S / HVFA matrix, the Si concentration of C3S particle was very uniform compared to pure C3S matrix. No clear Op layers around C3S particles, as observed in pure C3S, were found. C-S-H in C3S / HVFA matrix showed very similar absorption feature with fly ash. C-S-H in the system, however, showed further lower binding energy of core Si atom and higher degree of silicate polymerization compared to Op in pure C3S matrix. Lower binding energy of core Si atom can be due to Al substitution for Si in hydration products of C3S, and the higher degree of silicate polymerization is assumed to be due to additional silicate provided from fly ash. 29Si magic-angle spinning nuclear magnetic resonance spectroscopy confirmed that C-S-H in C3S/ HVFA matrix have longer average chain length compared to one in pure C3S confirming the silicate polymerization.;mu-SXRD analysis showed that the C-S-H formed in PC / HVFA system containing 50% or more of fly ash had a similar structure as C-S-H(I). Moreover, coexistence of more ordered C-S-H (C-S-H(I)) and stratlingite (2CaO˙SiO2˙Al2O3˙8H2O) was observed in the system containing 80% of fly ash, confirming that the amount of alumina and silicate phases provided by the fly ash is a major factor for the formation of C-S-H(I) and stratlingite in PC / HVFA system.