New Insights into Cementitious Composites at the Molecular Level


Student thesis: Doctoral Thesis

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Awarding Institution
Award date14 Aug 2020


Cement has been used extensively in civil engineering since the beginning of the nineteenth century. To surmount some disadvantages of conventional cement, the strategy of introducing additional materials at the nanometre scale has been adopted in recent years. Meanwhile, growing environmental concerns have encouraged the application of cement matrices to immobilise various types of waste. Due to the complex morphology and characteristics of cementitious composites, their intrinsic structures and properties are not yet comprehensively understood.

In this thesis, a systematic atomistic study was carried out to investigate the complex physical and chemical phenomena and properties of a variety of cementitious materials blended with industrial waste and nanomaterials. First, the effects of aluminium (Al3+), following its dissolution from alumina-rich supplementary cementitious materials (SCMs), on the structural evolution and mechanical properties of calcium silicate hydrates (CSH; the principal binding phase in cement pastes) were studied. Structurally, Al3+ heals defects in the silicate chains, forms cross-links between calcium aluminosilicate layers and reduces interlayer water content. The chemical stability of the interlayer hydrogen bonds and Ca-O bonds is also improved. As a result, the diffusion of the interlayer water and Ca2+ ions is decelerated. Mechanically, the bridging role of Al3+ enhances the load-bearing capacity of aluminosilicate chains along the y direction, which results in ~57.1% and ~100.4% enhancement of the Young’s modulus and strength. In the z direction, the cross-links provide an ultra-strong connection between the layers by raising the Young’s modulus and strength by ~3.78 and ~5.84 times, respectively.

The effects of nuclear waste (Cs-137) on the structural, dynamical and mechanical properties of CSH gel were also investigated. Cs-137 enlarged the CSH structures via expansion of the CSH interlayer regions because Cs+ occupies more space than interlayer Ca2+. The interlayer water diffusion is hardly affected by Cs+ when the calcium-to-silicon (C/S) ratio of CSH was ≤ 1.3. However, the introduction of Cs+ into CSH at C/S ratios above 1.3 accelerates interlayer water diffusion due to interlayer expansion and the reduced ability of the ions to confine the movement of water. In addition, the Young’s modulus and strength of CSH were adversely affected by Cs+ because the load transfer efficiency of the interlayer components, including water and ions, is decreased. During the tensile process, crack formation and structural failure are accelerated, and the energy absorption capacity is degraded by Cs+.

The chemical reactions, mechanical behaviour and interfacial sliding of CSH incorporating graphene and graphene oxide (GO) containing epoxides (GO-Oo) and hydroxyls (GO-OH) were studied. Chemical reactions occur at the interface between GO and CSH because the alkaline environment allows CSH to interact with GO via Ca2+ coordination and hydrogen bonds. The Young’s modulus and strength of CSH are enhanced by 52.6% and 23.3%, respectively, with the incorporation of GO-OH, and increases of 31.6% and 17.5% in Young’s modulus and strength, respectively, were achieved by incorporating GO-Oo due to high interfacial interaction energy and mechanical interlocking. However, graphene can hardly enhance the mechanical properties of CSH because its 2D surface cannot interlock with the matrix. Mechanical interlocking plays a decisive role in the enhancement of the interfacial shear strength. During the pull-out process, functional groups are exfoliated from GO, which impairs its reinforcing ability.

The properties of silicon-doped graphene (Si-graphene) and the working mechanisms of Si-graphene in geopolymer (a novel type of cement) are also investigated. By studying the polymerization process of Al(OH)4/Si(OH)4 monomers on Si-graphene, it is found that Si-graphene can be chemically bonded with the geopolymer matrix by conventional condensation reactions. Interfacial bonding shows a beneficial effect on the density of the geopolymer composites due to a denser interphase region. The tensile test reveals that the Young’s modulus of 10% Si-graphene/geopolymer nanocomposites is twice that of graphene/geopolymer nanocomposites. Interfacial bonding can also strengthen the interfacial structure and arrest crack formation in geopolymer matrices.

Finally, the effects of chemical bonding at interfaces between carbon nanotubes (CNTs) and geopolymer were investigated. Interfacial bonding decreases the thickness of the interfacial van der Waals (vdW) excluded volume and interphase (inner) region and condenses the interfacial structure. Remarkably, interfacial bonding provides a new restrictive mechanism that limits the diffusion of the components in the geopolymer. A dynamical heterogeneous character is found, as the atom mobility of the inner region is slower than that of the outer region. With 10.0% interfacial bonding concentration, the interfacial shear strength is enhanced by about 15 times, which hinders crack formation under tensile loading. The tensile test reveals increases of about 60% and 80% in the Young’s modulus and tensile strength, respectively.