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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2017)

Ordinary portland cement (OPC) has been used as the primary binding material in concrete for construction of buildings and other infrastructure over the last century due to its low-cost and the widespread geographical abundance of its raw materials. The manufacture of OPC accounts for approximately 3% of primary energy use and 9% of anthropogenic CO2 emissions globally. Such energy consumption and CO2 release is mainly attributed to the calcination and clinkering of raw materials (i.e., limestone and clay) in the cement kiln at high temperatures. Therefore, there is great need to reduce the CO2 footprint of cement, and secure alternative solutions for cementation as required for building and infrastructure construction. On the other hand, space conditioning consumes nearly 20% of annual energy consumption in the United States, which is still increasing with the increasing demand for thermal comfort in the context of climate change. The embedment of phase change materials (PCMs) in concrete is an effective means to improve its thermal inertia for building envelope applications, and can thus improve the energy efficiency of buildings. However, the viability of employing PCMs to enhance thermal performance of concrete depends on the stability and durability of PCMs in the highly caustic cementitious environment, and the durability of the PCM-added concrete. To address these limitations on sustainability and energy efficiency of current cement-based building materials, this dissertation mainly examines:

• PCM survivability during fabrication of PCM-mortar composites with respect to damage and/or rupture of the PCM microcapsules that may occur during mechanical mixing, as well as chemical durability of PCM within cementitious matrices, and the potential interactions between the PCM and the pore-fluid that result in enthalpy alteration,

• Cementitious matrix durability, with emphasis on assessing how dosage of PCMs alters water absorption, drying shrinkage, and restrained shrinkage cracking behaviors of cementitious composites containing PCMs, and

• The feasibility of developing sustainable building material through clinkering-free cementation by fly ash carbonation, with emphasis on the effects of CO2 concentration and processing temperature on the progress of carbonation reaction, the development of microstructure, and the strength evolution of the material.

The results of research on PCM embedded cementitious composites show that a reduction of around 25% in the phase change enthalpy is observed, irrespective of PCM dosage and aging. Such reduction in enthalpy is mainly caused by chemical interactions with dissolved sulfate ions. Examination of the influence of PCM additions on water absorption and drying shrinkage of PCM-mortar composites reveals that PCM microcapsules reduce the rate and extent of water sorption due to their non-sorptive nature and diluting effect. PCM inclusions do not influence the drying shrinkage of cementitious composites due to their inability to restrain the shrinkage of the cement paste. Assessments of free and restrained shrinkage, elastic modulus, and tensile strength also show that the addition of PCMs enhances the cracking resistance of cement paste because PCMs as soft inclusions offer crack blunting and deflection, and improved stress relaxation. In an effort to synergize the utilization of fly ash and CO2 in fly ash, the study shows that Ca-rich fly ash paste can readily react with dilute concentrations of CO2 in moist environments to produce cemented solids with sufficient strength (35 MPa) for use in structural construction. Detailed results from thermodynamic modeling, XRD analyses, and SEM observations suggest that fly ash carbonation results in the formation of reaction products including calcite, hydrous silica, and C-S-H, which collectively bond proximate particles into a cemented solid.

    Cover page: Phase Change Materials and Clinkering-Free Cementation for Sustainable Building Materials
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2024)
    Architected materials design across orders of magnitude length scale intrigues exceptional mechanical responses nonexistent in their natural bulk state. However, the so-termed mechanical metamaterials, when scaling bottom down to the atomistic or microparticle level, remain largely unexplored and conventionally fall out of their coarse-resolution, ordered-pattern design space. Here, combining high-throughput molecular dynamics (MD) simulations and machine learning (ML) strategies, some intriguing atomistic families of disordered mechanical metamaterials are discovered, as fabricated by melt quenching and exemplified herein by lightweight-yet-stiff cellular materials featuring a theoretical limit of linear stiffness-density scaling, whose structural disorder-rather than order-is key to reduce the scaling exponent and is simply controlled by the bonding interactions and their directionality that enable flexible tunability experimentally. Importantly, a systematic navigation in the forcefield landscape reveals that, in-between directional and non-directional bonding such as covalent and ionic bonds, modest bond directionality is most likely to promotes disordered packing of polyhedral, stretching-dominated structures responsible for the formation of metamaterials. This work pioneers a bottom-down atomistic scheme to design mechanical metamaterials formatted disorderly, unlocking a largely untapped field in leveraging structural disorder in devising metamaterials atomistically and, potentially, generic to conventional upscaled designs.
      Cover page: De Novo Atomistic Discovery of Disordered Mechanical Metamaterials by Machine Learning.Creative Commons 'BY' version 4.0 license
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