Abstract
With the rapid development of the construction industry, building materials are developing in the direction of high strength, good durability, intelligence, and functionality. Reactive powder concrete (RPC) is widely used due to its excellent mechanical properties and durability. It is used in high-rise buildings, special projects, marine engineering, etc. However, as a typical multi-scale building material, concrete has components ranging in size from nanometers to microscopes to centimeters and decimeters. Therefore, the destruction of concrete is a multi-stage process. Introducing reinforcement materials of different scales and forming multi-scale reinforcement can control the cracks of different scales of concrete materials.Nanomaterials have excellent mechanical properties on the nanoscale. Nanomaterials achieve enhancement effects through nucleation, improving pore structure, and controlling nanoscale cracks. On the micron scale, the micro steel fiber has a micro diameter, high aspect ratio, and large specific surface area, which can accelerate hydration reaction and enhance the compactness of concrete. In addition, the micro steel fiber has a thermal expansion coefficient comparable to the concrete. The micro steel fibers have good dispersibility in the concrete and form a widely distributed strengthening and toughening network. On a macro scale, macro steel fibers have good ductility, which can effectively hinder the formation of macro-cracks, significantly improving concrete tensile and flexural properties. This thesis uses low-carbon, durable sulphoaluminate cement (SAC). Reinforcement materials of different scales are used, including nanomaterials with 0-dimensional nanomaterial calcium carbonate (NC), one-dimensional nanomaterial carbon nanotubes (CNTs), two-dimensional nanomaterial multilayer graphene (MLG), micro steel fibers (mSF), and macro steel fibers (MSF), prepared low- carbon sulphoaluminate cement-based reactive powder concrete (SACRPC), the strength, toughness, hydration, micro-mechanism, and resistance to sulfate erosion was systematically researched. The main research contents and conclusions are as follows：
First, this thesis investigated the properties of SACRPC incorporated with 0-dimensional NC, including fluidity, strength, hydration, and microstructure. The results showed that adding NC reduced the fluidity of SACRPC mortar. The rate of heat flow and the cumulative heat of hydration increased along with the increased NC content, reaching the maximum at 2.5%. Moreover, adding NC enhanced SACRPC’s compressive and flexural strengths. When the NC content was 2.5%, the 90 d compressive and flexural strengths increased by 25.8% and 19.9%, respectively. The results of X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Inductively Coupled Plasma (ICP), and Isothermal calorimetry had a good consistency and revealed the mechanism of NC promotes the hydration process of SACRPC, i.e., more hydration products, such as AFt and AH3, were formed owing to the sufficient ion exchange in earlier ages, which filled the pores and made the SACRPC microstructure more compact and the mechanical properties better.
In addition, the ITZ (interfacial transition zone) also affects concrete's macroscale performances. One-dimensional CNTs with remarkable mechanical properties are good nanomaterials for reinforcing concrete. An effective method was proposed to enhance the interfacial transition zone (ITZ), involving the pre-saturation of sand particles in a dispersion solution of CNTs. The role of CNTs on the concrete’s mechanical property, hydration process, and microstructure was extensively explored using experiments and molecular dynamics (MD) simulations. The results indicated a 27.8% and 10.1% increase in 28-day compressive strength, along with a 16.7% and 6.3% rise in 28-day flexural strength for the sample with pre-saturated sand dispersed in CNTs (CS2), in comparison to samples without CNTs (C0) and those with dispersed CNTs (C2). The primary enhancement mechanism is ascribed to the CNTs’ adsorption and nucleation. The CNTs facilitated the generation of AFt (ettringite) and AH3 (gibbsite), enhancing the hydration degree. This mechanism was corroborated by MD simulations that analyzed the effect of CNTs on the mobility of calcium ions (Ca2+). When dispersed at the ITZ, CNTs had a seeding effect, leading to the generation of AFt and AH3 around the CNTs. This, in turn, reduced the ITZ’s width, thus effectively enhancing the ITZ’s microscopic structure.
Then, the influence of CNTs of four different aspect ratios with two dosages (i.e., 0.09% and 0.15%) on hydration, macro performance, and microstructure of low-carbon SACRPC was investigated. The SM2 (short-CNTs with 0.15% content) group with a small aspect ratio (50-100) had better flexural strength and compressive strength than M0 (plain group), which was increased by 32.7% and 37.0%, respectively, the hydration degree of C4A3Š (ye’elimite) and C2S (belit) were increased by 19.9% and 57.3% than that of the M0 group. At dosages (0.09%), LM1 (long-CNTs with an aspect ratio of 250-1500) and LM2 (long-CNTs with an aspect ratio of 125-750) improved flexural strength more than SM1 (short-CNTs with an aspect ratio of 100-200). However, LM1 (long-CNTs with a large aspect ratio of 250-1500) enhanced better at a dosage of 0.09% than 0.15%. CNTs mainly accelerated the hydration process of the matrix and promoted the generation of more AFt, AH3. The increase in content is beneficial to improve the toughness and flexural strength of the matrix. AH3 is closely connected to CNTs, which can improve the bond strength between the matrix and CNTs. It is observed that AFt is closely associated with AH3, filling the pores of the matrix, and the nanoscale AFt, AH3, and CNTs work together to improve the strength of the matrix, which differs from the effect of CNTs in OPC.
Next, two-dimensional MLG has excellent mechanical properties and a unique stacked structure. The effects of MLG on the hydration, microstructure, and mechanical properties of SACRPC were investigated, revealing the hydration mechanism and reinforcing mechanism of MLG on SACRPC. The hydration kinetic model and hydration kinetic equation of SAC was established, explaining the differences in cement hydration processes with and without MLG on SAC based on a hydration kinetic model. Revealing the multi-level reinforcing mechanism, namely hydration products (AFt, AH3) and MLG filling the pores; MLG prevented the extension of micro-cracks and changed micro-cracks development path through filling effect, deflection effect, pulling out, and bridging effect.
Finally, the effect of different scale steel fiber on macro performance (including rheological property, workability, strength, load-deflection curves, strength ratio, toughness index, and cracking behavior), durability, and micro/ nanostructure of SACRPC were explored. Results indicated that the flexural toughness highly increased owing to the synergistic effect of hybrid MSF and mSF. Various characterization methods showed that the addition of mSF improved the pore structure of the SACRPC and provided enough space for the growth of hydration products AFt and AH3. It was revealed that the multi-level and multi-scale reinforced mechanism, namely hydration products, contributed to micro or nanostructure, mSF bridged micro-cracks, and MSF prevented macro-cracks. In the sulfate erosion experiment, SO42- ions entered the matrix and occurred a chemical reaction; AFt regenerated on the surface and inside the SACRPC with a fibrous morphology and smaller crystals. The regenerated AFt filled the pores of the SACRPC and refined the pore structure. The whole process was similar to the self-healing effect of concrete. The unhydrated cement particles reacted with water, forming new hydration products that healed the cracks.
| Date of Award | 14 Aug 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jun Chang (External Supervisor) & Denvid LAU (Supervisor) |