Hexavalent chromium release over time from a pyrolyzed Cr-bearing tannery sludge

Pyrolysis in an inert atmosphere is a widely applied route to convert tannery wastes into reusable materials. In the present study, the Cr(III) conversion into the toxic hexavalent form in the pyrolyzed tannery waste referred to as KEU was investigated. Ageing experiments and leaching tests demonstrated that the Cr(III)–Cr(VI) inter-conversion occurs in the presence of air at ambient temperature, enhanced by wet environmental conditions. Microstructural analysis revealed that the Cr-primary mineral assemblage formed during pyrolysis (Cr-bearing srebrodolskite and Cr-magnetite spinel) destabilized upon spray water cooling in the last stage of the process. In the evolution from the higher to the lower temperature mineralogy, Cr is incorporated into newly formed CrOOH flakes which likely react in air forming extractable Cr(VI) species. This property transforms KEU from an inert waste to a hazardous material when exposed to ordinary ambient conditions

Multiscale Characterization at Early Ages of Ultra-High Performance Geopolymer Concrete

The main obstacle of using geopolymer as a construction repair material is its slow strength development rate, which is the most significant attribute of an early-age opening for traffic and striking-off formwork. Geopolymer technology has recently attracted huge interest as an alternative to traditional cementitious materials with low environmental impact. Thus, this study investigates the feasibility of developing an ultra-high performance geopolymer concrete (UHPGC) with the aim of achieving high early-age strength. For this purpose, UHPGC mixtures activated with different potassium hydroxide molarities and aluminosilicate material types were developed and examined being cured with different curing temperatures. The early strength and durability of the UHPGC after 8 and 24 h were investigated. Experimental results revealed that the optimal mix design of UHPGC corresponds to a KOH molarity of 16 M and a 30% silica fume content. Furthermore, former mixture cured at 100 °C gave superior 8 and 24 h early strength values of 79 and 134 MPa, respectively. Moreover, a superior interaction of slag, silica fume, and activator solution at early age for UHPGC is revealed by the microstructural characteristics examined by a field emission scanning electron microscope (FESEM) with energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy analysis, and thermogravimetric (TGA) techniques. It was also found that the compressive strength results and the results of the microstructure analysis are well coincided. The experimental results obtained in this study emphasize the feasibility of using developed UHPGC as an eco-friendly quick repair materials The development of one-part UHPGC as a quick, cost-effective, and high-strength product for all construction repair maintenance will lead to huge improvements in the structural capacity and durability of structural components

Durability, Microstructure, and Optimization of High-Strength Geopolymer Concrete Incorporating Construction and Demolition Waste

The incorporation of construction and demolition (C&D) waste in concrete production has gained great importance toward sustainability, especially in geopolymer concrete. In this study, ground granulated blast-furnace slag (GGBFS) and fine aggregate of normal geopolymer concrete were partially replaced by clay brick powder (CBP) and fine clay brick (FCB) derived from C&D waste, respectively, aiming to produce high-strength geopolymer concrete (HSGC). Fly ash (FA) was also used as a partial replacement for GGBFS in normal geopolymer concrete. Twenty HSGC mixtures were designed using the response surface methodology with three variables, including CBP (0–25%), FA (0–25%), and FCB (0–50%). The performance of the proposed HSGC mixtures was assessed by measuring several mechanical and durability properties. In addition, a variety of physicochemical methods, including X-ray fluorescence spectroscopy, X-ray diffraction, and scanning electron microscopy, were used to examine the mineralogical and microstructural characteristics of the control and the developed mixtures. The findings revealed that the compressive, splitting tensile, and flexural strengths of the HSGC made with C&D waste ranged from 38.0 to 70.3 MPa, 4.1 to 8.2 MPa, and 5.2 to 10.0 MPa, respectively. The results also indicated that the incorporation of FA is an essential parameter to eliminate the negative impacts of C&D waste addition on concrete workability. The optimal proportions for the HSGC were 5% for CBP, 5% for FA, and 40% for FCB, which were determined to generate the optimized HSGC with the highest mechanical performance, according to the verified models and optimization findings. The physicochemical analyses showed that the thick amorphous geopolymeric gel predominated the nonporous structure of the optimized HSGC, which had good mechanical characteristics. Furthermore, the anti-carbonation performance and freezing resistance of the optimal HSGC increased by 17.7% and 14.6%, respectively, while the apparent porosity decreased by 8.4%

Characterization and optimization of fresh and hardened properties of ultra-high performance geopolymer concrete

Due to the deterioration and environmental issues of cement-based ultra-high-performance concrete (UHPC), there is an increasing interest in utilizing geopolymer technology to develop UHPC. In this regard, a statistically based model using response surface methods is applied to predict the fresh and mechanical properties of ultra-high performance geopolymer concrete (UHPGC) mixtures. Twenty mixtures were designed and generated experimentally using the central composite design (CCD) concept. In this context, the effects of three principal factors—the effects of silica fume (SF), natural sand content, and different curing temperatures—on the flowability, setting times, bulk density, compressive, and flexural strengths were examined. The models presented herein reveal that all the inputs and outputs are perfectly correlated. Microscopic investigations were used to determine the morphological characteristics of the optimum mixture. The results revealed that the bulk density, flexural, and compressive strengths of the UHPGC ranged from 2461 to 2536 kg/m3, 6.48–9.39 MPa, and 104–152 MPa, respectively. According to the validated models and optimization findings, the optimum mixture of SF (22.5%) and sand content (1150 kg/m3) cured at 100 °C was found to develop the optimized UHPGC with the best microstructure performance. From the experimental results, the optimized UHPGC mixture possessed excellent mechanical properties (2521 kg/m3, 9.39 MPa and 152 MPa), and it had a non-porous structure primarily made of a dense geopolymer gel. Overall, the current study provides new insights into the design and production of UHPGC materials using the response surface method, which could contribute to the widespread use of this concrete in practical applications

Technological Potential Analysis and Vacant Technology Forecasting in Properties and Composition of Low-Sulfur Marine Fuel Oil (VLSFO and ULSFO) Bunkered in Key World Ports

Analysis of the very-low-sulfur fuel oil (VLSFO) and ultra-low-sulfur fuel oil (ULSFO) bunkered in key ports in Asia, the Middle East, North America, Western Europe, and Russia is presented. The characteristics of said fuels, including density, sulfur content, kinematic viscosity, aluminum and silicon content, vanadium and nickel content, as well as pour point are investigated. Furthermore, the main trends and correlations are also discussed. Based on the graphical and mathematical analysis of the properties, the composition of the fuels is predicted. The key fuel components in Asian ports, the most important of which is Singapore, are hydrodesulfurized atmospheric residues (AR) (50–70%) and catalytic cracker heavy cycle oil (HCO) (15–35%) with the addition of other components, which is explained by the presence of a number of large oil refining centers in the area. In the Middle East ports, the most used VLSFO compositions are based on available resources of low-sulfur components, namely hydrodesulfurized AR, the production facilities of which were recently built in the region. In European ports, due to the relatively low sulfur content in processed oils, straight-run AR is widely used as a component of low-sulfur marine fuels. In addition, fuels in Western European ports contain on average significantly more hydrotreated vacuum gas oil (21%) than in the rest of the world (4–5%). Finally, a mixture of hydrotreated (80–90%) and straight-run fuel oil (10–15%) with a sulfur content of no more than 2.0–2.5% is used as the base low-sulfur component of marine fuels in the ports of Singapore and the Middle East