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Silt is a common waste soil that is often treated by solidification with ordinary Portland cement (OPC). Geopolymer is a cementitious material formed by the polycondensation reaction of fly ash and other industrial waste materials in a non-high-temperature environment under alkali excitation. This study used fly ash-based geopolymer to partially replace OPC in silt solidification to reduce the use of OPC. The investigated replacement ratios of geopolymer for OPC were selected as 0% (control group), 5%, 10%, and 15%, and the water contents were selected as 28%, 30% (liquid limit of 29.6%), 32%, and 34%, respectively. To investigate the influence of curing time (7, 14, 28, and 60 d) on the mechanical properties of solidified silt, unconfined compressive strength tests and stress-strain curve study were conducted. To better understand how geopolymer and cement behave during the solidification of silt, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and mercury intrusion porosimetry were used to evaluate the morphology, elemental composition, mineral composition, and pore distribution of the solidified soils. The experimental results show that the silt solidified with geopolymer-cement had good performance. Under the same water content, as the proportion of cement replaced by geopolymer increased, the compressive strength of the solidified soil first increased and then decreased. When the silt had a water content of 30% and a curing period of 28 d, the strength of the solidified soil increased significantly (approximately 13.17%-51.42%). Microstructural analysis showed that more cementitious materials were produced in the soil solidified with geopolymer-cement. The geopolymer-cement wrapped around the soil particles and filled the voids, resulting in increased soil compactness and thus improving the silt strength. • Fly ash-based geopolymer partially replaces OPC to reduce the use of OPC. • The mechanical properties of solidified silt were studied, UCS test were conducted. • More cementitious materials are produced in solidified silt by geopolymer-cement. • It is beneficial to the reasonable application of fly ash in geopolymer field. [ABSTRACT FROM AUTHOR]

The ordinary Portland cement (OPC)-based solidification process is used extensively to reinforce soils due to its available and good bonding properties. Alternative products are used in cementitious materials to enhance the strength and to reduce OPC consumption. In this study, the effect of additive type and mass fraction on the microstructure and mechanical properties of solidified sandy silt are investigated. There are four types of additives (gypsum, lime, clay particles, and fly ash) at mass fractions of 2, 3, and 4% that are considered in order to study their mechanical properties (unconfined compression, indirect tensile, flexural strength, and compressive resilient modulus) at 7, 14, 28, 60, and 90 days. The optimal contents of additive gypsum, clay particles, and fly ash are determined to be 2%, 4%, and 4%, respectively. Such improvement of additive-modified OPC solidified sandy silt is due to the formation of the crystalline compound or the gradation composition improvement via field emission scanning electron and X-ray diffraction analysis. [ABSTRACT FROM AUTHOR]

Fly ash has been widely explored in alkali-activated concrete technology as a precursor; however, it suffers sudden brittle failure and requires heat activation for early-strength development. Previous studies have demonstrated that these shortcomings can be overcome by adding nanosilica and various types of fibers. However, they were mainly focused on improving the mechanical properties, but there is no clear understanding of the durability and microstructural changes. Therefore, this study systematically addresses the effects of nanosilica sol and hybrid fibers (steel and polypropylene fibers) on the workability, porosity, flexural/compressive strength development, flexural toughness, crack resistance, and durability (subjected to sulfuric acid of pH = 1, 5% sodium sulfate, and 3.5% seawater attacks) of fly ash-based mortars cured at ambient environments. The results demonstrate that nanosilica and hybrid fibers have acceptable simultaneous effects on the properties of mortar. Although they both reduce fluidity, nanosilica improves the pore enlargement caused by fiber materials; thus, it greatly improves the mechanical properties of mortar. The flexural toughness of the mortar with nanosilica and hybrid fibers was approximately 200% higher than that of the pure-fly ash. Only approximately 58.25 mm2 microcracks were observed during the early crack detection, and the residual strength also improved under chemical solution attack. The microstructures were studied by field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction/Rietveld method, and nanoscale 3D X-ray microscopy. The nanosilica enhanced the reactivity of the precursors and refined the pore structure of the mortar due to the changes in the crystalline and amorphous phases, and the hybrid fibers produced composites with good internal characteristics and strong fiber/matrix interfacial bonding. [ABSTRACT FROM AUTHOR]

• The geopolymers were investigated by a combination of experiments and molecular dynamics simulations. • The molecular model of Na 2 Si 2 O 5 is used as the basis to construct the structural model of different H 2 O/Al coal gangue-based geopolymers. • The effect of H 2 O/Al on the microstructure of geopolymers was investigated by XRD, RDF, elastic modulus, etc. methods. • The rationality of the proposed geopolymer model was verified, and it was demonstrated that the Universal force field is also applicable to the simulation and calculation of this geopolymer model. • It is beneficial to the reasonable application of coal gangue in geopolymer field. The effects of water on the structure and mechanical properties of a coal gangue-based geopolymer were investigated experimentally and compared with the results of molecular dynamics simulations. The chemical composition and crystalline phases of the raw materials were analyzed via X-ray fluorescence and X-ray diffraction (XRD) after 28 days of curing, and the mechanical properties were tested. Molecular structure models for H 2 O/Al ratios of 2.5, 3, and 3.5 were established using molecular dynamics simulation software, and structural optimization and molecular dynamics simulations were performed under the universal force field. Based on the obtained results, the energy and temperature change curves, simulated XRD spectrum, radial distribution function, hydrogen bonding, mean squared displacement, and mechanical properties of the amorphous geopolymer were analyzed. The rationality of the molecular model was verified through comparison with the results of the molecular dynamics simulations and experiments, and as the H 2 O/Al ratio increased within a given range, the energy and temperature stability time of the system are decreased, the area of the dispersion peak is decreased, the amorphous phase is increased, the diffusion degree of Si and Al in the system is decreased, the length of Si-O bond, Al-O bond, H-O bond and O-O bond is shortened, and the structure is more stable. The mechanical properties obtained from the experiment were also improved. [ABSTRACT FROM AUTHOR]