Utilization of Fly Ash to Enhance Ground Waste Concrete-Based Geopolymer

Demolition of the deteriorating infrastructure produces a large amount of waste concrete. Storage of waste concrete involves costly transportation, consumption of limited landfill space, and serious environmental problems. Combining them, the recycling of waste concrete is mainly limited to the use of recycled aggregates in the nonstructural filler and base course. Considering the limited geopolymerization potential of waste concrete, this study investigates the feasibility of using fly ash to enhance ground waste concrete-based geopolymer to recycle and utilize waste concrete in high-specification applications. The unconfined compressive strength of the binders synthesized at different fly ash contents, NaOH concentrations, and curing time are evaluated. Scanning electron microscopy/energy dispersive spectroscopy imaging, X-ray diffraction analyses, Fourier Transform Infra Red analyses, and Magic Angle Spinning-Nuclear Magnetic Resonance Spectroscopy techniques are also conducted to investigate the microstructure and phase/elemental compositions of the synthesized geopolymeric binders. The results indicate that the addition of fly ash to ground waste concrete profoundly enhances the strength of ground waste concrete-based geopolymer up to 75% fly ash content. The simultaneous formation of geopolymeric gel and CSH gel is responsible for the strength increase. The geopolymeric binder with required strength can be produced by the appropriate addition of fly ash to ground waste concrete

Microscopic Mechanism of Cement Improving the Strength of Lime-Fly Ash-Stabilized Yellow River Alluvial Silt

Silt is a kind of soil with poor engineering performance. Lime-fly ash- (LF-) stabilized silt has the problem of low early strength. In this study, it is aimed to investigate the effect of cement on improving the strength of LF-stabilized silt and reveal the microscopic mechanism. A fixed percentage of LF (18%) plus different percentages of cement (0%, 2%, 4%, and 6%) were mixed with Yellow River alluvial silt (YRAS). Soil samples for tests were artificially made by compaction in the laboratory. Unconfined compressive strength (UCS) tests were performed on soil samples cured for 7 d, 28 d, 60 d, and 90 d. Scanning electron microscope (SEM) tests, energy dispersive X-ray spectroscopy (EDS) tests, and mercury intrusion porosimetry (MIP) tests were performed on soil samples cured for 7 d and 28 d. UCS results showed that the early strength of LF-stabilized YRAS developed significantly after adding cement. UCS also increased with the increase in cement content and curing time. SEM results revealed the differences in microstructure of LF-stabilized YRAS before and after adding cement. Before adding cement, the main microstructure characteristics included small soil particles, large number of pores, and loose particle arrangement. After adding cement, the main microstructure characteristics included large bonded particles, small number of pores, and dense particle arrangement. The EDS results showed that, after curing for 28 d, the elements of gels in stabilized YRAS had changed, mainly including appearance of C and a significant increase of Ca. MIP results showed that the pores with a size of 1 μm∼10 μm accounted for the largest proportion in stabilized YRAS. The product (mainly C-S-H) of cement hydration mainly filled the pores with a size larger than 10 μm at the early stage. Combining strength results and microresults, the micromechanism of cement improving the strength of LF-stabilized YRAS was discussed

Field test research on the application of double-liquid curing agent in deep mixing columns

Cement is an important building material with high energy consumption and high emission, while energy conservation and emission reduction has been the consensus over the world in the background of the global climate warming. Aiming at the problems of large cement consumption and long construction period of deep mixing column, a double-liquid curing agent composed of low thickness cement slurry and sodium silicate solution is proposed to replace cement slurry in deep mixing columns. The installation equipment is modified to adapt to the double-liquid curing agent. The effect of the double-liquid curing agent is researched through field​ tests, which verify the environmental and economic benefits in cement saving and construction accelerating. The double-liquid curing agent for deep mixing columns has great promotion potential

Green, Rapid, and Universal Preparation Approach of Graphene Quantum Dots under Ultraviolet Irradiation

It is of great significance and importance to explore a mild, clean, and highly efficient universal approach for the synthesis of graphene quantum dots. Herein, we introduced a new green, rapid, and universal preparation approach for graphene quantum dots via the free-radical polymerization of oxygen-containing aromatic compounds under ultraviolet irradiation. This approach had a high yield (86%), and the byproducts are only H₂O and CO₂. The obtained graphene quantum dots were well-crystallized and showed remarkable optical and biological properties. The colorful, different-sized graphene quantum dots can be used in fluorescent bioimaging in vitro and in vivo. This approach is suitable not only for the preparation of graphene quantum dots but also for heteroatom-doped graphene quantum dots