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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

Despite the enormous amount of concrete consumed, fundamental understanding on the structural properties of hydrated oxides in concrete is still an open question. Due to the structural hierarchies and heterogeneous characteristics of concrete, accurate experimental and theoretical studies have not been well-developed for measuring mechanical properties of crystalline and amorphous materials in concrete. Lack of the information makes the development of constitutive relation of them to the macroscopic concrete properties difficult.

The objective of this thesis is to compute mechanical properties of various phases in concrete. Unconventional methods of high pressure x-ray diffraction, absorption, and first-principles calculation are applied to calcium silicate hydrates (tobermorite 14 Å, 11 Å, 9 Å, and jennite), calcium aluminate hydrates (hemi-carboalumiante, monocarboaluminate, strätlingite, and hydrogarnet), tricalcium aluminate, and alkali-silica reaction gel. From the systematic comparison of both experiment and simulation, a comprehensive understanding of structural mechanism is achieved.

For tobermorites, interlayer thickness which is related to the number of interlayer water molecules determines its compressibility. Hemicarboalumiante and strätlingite shows a pressure-induced dehydration while monocarboaluminate with the full occupancy of carbon oxide group behaves stable and incompressible under pressure. In the cases of tobermorite 14 Å and monocarboaluminate, linear density approximation in the first-principles calculation predicts the experimentally measured bulk modulus with high accuracy. Based on the excellent agreement of bulk moduli between experiment and simulation, it can be concluded that computed elastic properties of shear and Young's modulus, Poisson's ratio, and elastic tensor coefficients are highly reliable. In addition, combining x-ray diffraction and absorption methods allows accurate measurement of density variation under pressure which is used to compute the bulk modulus of alkali-silica reaction gel.

The fundamental structural properties obtained in both experiments and simulations given in this thesis will pave a path toward not just to increase our knowledge of the structural mechanism of concrete but also to optimize concrete design for better structural performance.

    Cover page: Experimental and Theoretical Studies on Mechanical Properties of Complex Oxides in Concrete
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C₃S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C-S-H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C₃S hydration was significantly extended, the degree of hydration of C₃S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C₃S in the C₃S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C₃S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C-S-H in the C₃S-HVFA paste directly indicate that Al substitutes for Si in C-S-H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C-S-H in C₃S-HVFA system and presented results consistent with previous literature.
      Cover page: Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      The understanding and control of early hydration of tricalcium silicate (C₃S) is of great importance to cement science and concrete technology. However, traditional characterization methods are incapable of providing morphological and spectroscopic information about in situ hydration at the nanoscale. Using soft X-ray spectromicroscopy, we report the changes in morphology and molecular structure of C₃S at an early stage of hydration. In situ C₃S hydration in a wet cell, beginning with induction (~1 h) and acceleration (~4 h) periods of up to ~8 h, was studied and compared with ex situ measurements in the deceleration period after 15 h of curing. Analysis of the near-edge X-ray absorption fine structure showed that the Ca binding energy and energy splitting of C₃S changed rapidly in the early age of hydration and exhibited values similar to calcium silicate hydrate (C-S-H). The formation of C-S-H nanoseeds in the C₃S solution and the development of a fibrillar C-S-H morphology on the C₃S surface were visualized. Following this, silicate polymerization accompanied by C-S-H precipitation produced chemical shifts in the peaks of the main Si K edge and in multiple scattering. However, the silicate polymerization process did not significantly affect the Ca binding energy of C-S-H.
        Cover page: In Situ Soft X-ray Spectromicroscopy of Early Tricalcium Silicate Hydration
        • Multimedia
        • Peer Reviewed
        UC Berkeley Previously Published Works (2014)
        The predominant phase of calcium sulfoaluminate cement, Ca 4(Al6O12)SO4, was investigated using high-pressure synchrotron X-ray diffraction from ambient pressure to 4.75 GPa. A critical review of the crystal structure of Ca4(Al 6O12)SO4 is presented. Rietveld refinements showed the orthorhombic crystal structure to best match the observed peak intensities and positions for pure Ca4(Al6O 12)SO4. The compressibility of Ca4(Al 6O12)SO4 was studied using cubic, orthorhombic, and tetragonal crystal structures due to the lack of consensus on the actual space group, and all three models provided similar results of 69(6) GPa. With its divalent cage ions, the bulk modulus of Ca4(Al6O 12)SO4 is higher than other sodalites with monovalent cage ions, such as Na8(AlSiO4)6Cl2 or Na8(AlSiO4)6(OH)2·H 2O. Likewise, comparing this study to previous ones shows the lattice compressibility of aluminate sodalites decreases with increasing size of the caged ions. Ca4(Al6O12)SO4 is more compressible than other cement clinker phases such as tricalcium aluminate and less compressible than hydrated cement phases such as ettringite and hemicarboaluminate. © 2013 The American Ceramic Society.
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