Strengthening Cementitious Composite Using Boron Nitride Nanosheets


Student thesis: Doctoral Thesis

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Awarding Institution
Award date4 Aug 2023


Cementitious materials with excellent ductility and load-carrying capacity are in high demand in various applications of concrete structures (e.g. the beam/column joint and the link slab for continuous bridges). However, the fine cracks in concrete can develop into coarse cracks due to the quasi-brittle nature of cementitious materials, which results in a low ductility for concrete and causes the sudden collapse of concrete structures. Hence, the application of cementitious materials in such structures is restricted. Nano-fillers are added to cementitious materials to improve their ductility because these nano-fillers yield better compactness and more efficiently restrict the development of nano-crack in cementitious composites compared to traditional fibers. Boron nitride nanosheets (BNNSs), one of the most widely used nano-fillers, can effectively improve the mechanical properties of cementitious materials. When compared with graphene, the friction coefficient between BNNSs and water is three times larger than that between graphene and water. Hence, the presence of BNNSs in capillary pores shows a higher sliding resistance in cementitious composites than in graphene. Besides, BNNSs exhibit robust interfacial interaction with cementitious materials due to their polarized boron-nitrogen bonds, which do not exist in nonpolarized carbon-carbon bonds in graphene. The robust interfacial interaction between BNNSs and cementitious materials provides sufficient load transfer, which yields significant enhancement in the mechanical properties of cementitious composites.

Molecular dynamics (MD) simulation is capable of accurately predicting the interfacial properties between nanosheet and tobermorite (representing cementitious material), given that the forcefield and corresponding parameters are appropriate. By employing MD simulation, the energetic, structural, and dynamic properties of the interface between nanosheet and tobermorite can be explored at the atomistic level, which gives insights to the stability of the BNNS-tobermorite composite from the atomistic perspective. MD simulation is useful to study the interfacial properties between BNNS and tobermorite and to quantify the improvement of BNNS in the shear properties of the BNNS-tobermorite composite from the atomistic scale, which is of benefit to the crack resistance because shear failure is the dominant mode in concrete columns and beams. Due to the deformation of the cementitious composite, the folded BNNS is unfolded. This unfolding behavior of BNNS is investigated by MD simulation. The simulation results show that unfoldings of the self-folded BNNSs initiate at the interface between the interlayers of the self-folded BNNSs because this interface is weaker than the BNNS-tobermorite interface under individual tensile and shear deformation. The interfacial microstructure shows that the ends of the self-folded BNNSs are embedded into the rugged surfaces of tobermorite, which exhibits a mechanical interlocking effect on the BNNSs-tobermorite interface. Furthermore, the existence of the hydrogen bonds at the BNNS-tobermorite interface yields a robust interfacial interaction between BNNSs and the tobermorite. The critical peeling forces required to peel the adhesion between BNNS and tobermorite and between interlayers of self-folded BNNSs are obtained from a continuum model, which gives insights into the unfolding initiation of the self-folded BNNSs. The progressive peeling at the BNNS-tobermorite interface allows the self-folded BNNSs to maintain mechanical reinforcement in tobermorite under a large deformation at the atomistic scale. The atomistic scale reinforcement of the self-folded BNNSs in the deformation capacity and shear properties of cementitious composites significantly improves the ductility of cementitious composites. The folded BNNS deposits on the surface of the crack wall in the cementitious material when BNNS is pulled out from the substrate (i.e. cementitious materials). When external free water molecules penetrate the folded BNNS through the cracks, the water-driven unfolding of folded BNNSs on the surface of the tobermorite is studied by MD simulation. It shows that the unfolding of folded BNNSs is significantly dependent on the deterioration of the interfacial interactions between the folded BNNSs and tobermorite and between interlayers of the folded BNNSs. The interface deteriorations due to water penetration allow the relative sliding between folded BNNSs and tobermorite and between interlayers of folded nanosheets. Afterward, the vibration of the monolayered BNNS induces unfolding due to its flexibility. To quantify the deterioration of the adhesion between folded BNNSs and tobermorite and between interlayers of folded BNNSs, steered molecular dynamics (SMD) simulations are performed to calculate the energy barrier between them. It indicates that the presence of water molecules significantly reduces the energy barrier, resulting in the release of strain energy in folded BNNSs and the unfolding behavior of BNNSs. Due to the presence of water, the hydration of unhydrated cement particles occurs and releases heat. Hence, the effect of temperature on the unfolding degree of BNNS is studied, which shows an acceleration in the unfolding degree of BNNS and envisions the self-healing of cracks in the tobermorite composite when the temperature is increased. Hence, the addition of BNNSs to cementitious composites can effectively bridge fine cracks and prevent the fine cracks from developing into coarse cracks and deteriorating the mechanical properties of concrete.

The robust interfacial interaction between BNNS and cementitious materials shows excellent load transfer capacity and improvement in the mechanical properties of cementitious composites at the nanoscale, which is a promising reinforcement for the structural performance of engineering structures. In view that BNNSs are expensive, it is necessary to provide the guideline for manufacturing BNNSs reinforced cementitious composites so that the expected mechanical improvement in the structural performance of engineering structures can be obtained. To link the material properties of the nanocomposites to the macroscale mechanical performance of engineering structures, the multiscale models are constructed to describe BNNSs reinforced cementitious composite from microscale to macroscale. The hierarchical representative volume element (RVE) models are constructed to mimic the microstructural characteristics of BNNSs reinforced cement paste. By changing the sizes of RVE models, finite element analysis (FEA) shows insensitivity to the sizes of the RVE models at the microscale. The interfacial properties between BNNSs and cement paste are described by the cohesive zone model (CZM) developed by MD simulations. Based on the RVE models and the MD-based CZM, the compressive strength of the macroscale cement paste are obtained by the FEA. To validate the accuracy of the MD-based CZM for describing the load transfer between BNNSs and cement paste, the compressive strength of BNNSs reinforced cement paste from the FEA is compared to those from measurements. The strain rate is 0.1/s, determined by adopting three different strain rates and having convergent results. The FEA shows that the compressive strength of BNNSs reinforced cement paste is close to that of the measurements. The multiscale modeling envisions designing engineering structures starting from the nanoscale material properties to obtain the expected structural performance at the macroscale.