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Title: Upcycling waste concrete aggregates by accelerated carbonation
Authors: Fang, Xiaoliang
Degree: Ph.D.
Issue Date: 2021
Abstract: Concrete has been the most important construction material for decades due to its good compressive strength, weathering resistance, excellent structural performance, and low costs. Rapid urbanization in recent years, particularly in developing countries (e.g. China) has increased the need to demolish old structures and replace them with new ones. The construction industry as a major consumer of natural resources and energy is thus eager to develop upcycling techniques for converting secondary resources derived from concrete waste into new and value-added products. This thesis is concerned with developing methods to improve the quality of both coarse and fine recycled concrete aggregates (RCAs) through accelerated carbonation. Being regarded as an efficient and economical approach to enhance the properties of RCAs, carbonation treatment of RCAs has been widely recognized and attracted a lot of research interests. However, the effect of carbonation is limited by the fast densification on the surface of RCA particles. Against this background, this thesis presents a systematic study on developing methods to enhance the effect of carbonation for both coarse and fine RCAs. Following a literature review that identifies the existing knowledge gaps and an introduction of the materials and methods, this thesis presents an experimental study on a series of pretreatment methods employed on both laboratory prepared cement pastes and real RCA samples to enhance the accelerated carbonation process. This involved the use of a laboratory prepared saturated Ca(OH)2 solution and wastewater generated from ready-mixed concrete plants (WW). The hypothesis was that with the additional introduction of Ca2+ and OH-into RCAs by either soaking or spraying, the amount of carbonation products can be enriched. The physico-chemical and microstructural properties of the samples were investigated by using mercury intrusion porosimetry (MIP), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) analysis, microhardness, scanning electron microscopy (SEM)-back scattered electron (BSE) imaging techniques. Moreover, the density and water absorption values of the samples were evaluated. The experimental results showed that the properties of both the cement paste and RCA samples were further improved with the use of the pretreatment method before carbonation. Having proven that the WW was effective in improving the carbonation, this thesis next presents a comparison study on four types of concrete which were separately prepared with natural aggregate, recycled concrete aggregate, carbonated recycled concrete aggregate, and wastewater pretreated and carbonated recycled concrete aggregate. A series of tests was performed to evaluate and compare the properties of the four types of aggregates and concrete: the density and water absorption of the aggregate; the compressive strength, density, water absorption, sorptivity, and the Cl penetration resistance of the concrete specimens. Moreover, the observation of the interfacial transition zone (ITZ) was conducted using scanning electron microscopy (SEM)-back scattered electron (BSE) imaging techniques to calculate and compare the porosity at the microscopic level. The results showed that the WW spray was able to enhance the effect of carbonation on RCAs and therefore improve the properties of the produced new concrete.
Sequentially, this thesis presents a novel two-step carbonation to upcycle the fine recycled concrete aggregate. Firstly, a technique was developed to convert recycled fine cement waste to a Ca-rich residue and a Si-rich gel by using a two-step carbonation process. The two-step process involves i) fine recycled cement powder reacting with a Na2CO3 solution to precipitate a calcium-rich residue and ii) after filtration, the filtrate containing Na2SiO3 and NaOH was subjected to flow-through CO2 gas carbonation to obtain a suspension with a silica-rich gel and the Na2CO3 solution. The physical and chemical properties of the precipitated products from both steps were analyzed by a range of techniques, including particle size distribution, Fourier-transformed infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), Thermogravimetric analysis (TGA) and Nuclear Magnetic Resonance (NMR). The results indicated that the proposed technique was able to successfully convert recycled cement paste powder to two new value-added reaction products, containing calcite and silica gel. Afterward, optimization of the above developed technique was carried out by systematically varying the carbonation parameters: i) Na2CO3 concentration, stir duration, and temperature of Step 1, and ii) CO2 flow-rate, CO2 concentration, and final pH of Step 2. A thorough investigation of the chemical and physical properties of the products In consideration of efficiency, the results suggested the optimal experimental conditions were i) Step 1: a 5-10 wt% Na2CO3 solution, a stir duration of 5-7 hours under ambient temperature, ii) Step 2: a CO2 gas with a concentration of more than 50%, and the carbonation ends at a final pH≤9.8. The supernatant of Step 2 can be reused as the Na2CO3 solution of Step 1 for another upcycling loop with a pH adjustment by NaOH. Finally, a fast (10 min) wet carbonation process was also tested for enhancing the quality of recycled fine aggregates for use as new aggregates for mortar production. A well-hydrated cement paste particles (RCP) and a real fine RCAs were used. The test results of RCP revealed that i) the carbonation products were noticeably increased after a short period of wet carbonation; ii) the surface layer of the particles were densified; iii) the pore volume were reduced after wet carbonation, especially the pores less than 10 nm. The new mortar specimens prepared with the carbonated RCAs showed improved compressive strength and reduced drying shrinkage, indicating the wet carbonation method was effective for fast enhancement of the properties of fine RCAs. The thesis concluded that the coarse, fine and powder fractions of the RCAs derived from C&D wastes can be effectively enhanced by different carbonation methods. The WW spray pretreatment improved the properties of coarse RCA by providing external Ca2+ .The fast wet carbonation improved the carbonation efficiency for fine RCAs by transforming the gas-solid reaction to the liquid-solid reaction. And the two-step carbonation upcycled the fine powder fraction of RCA into two value-added products. Further studies are still needed to understand the carbonation mechanism and ways to upscale the proposed carbonation methods for industrial applications.
Subjects: Concrete -- Recycling
Aggregates (Building materials) -- Recycling
Hong Kong Polytechnic University -- Dissertations
Pages: xxxi, 32-209 pages : color illustrations
Appears in Collections:Thesis

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