Search Results (46)

Supersulfated cement (SSC) is an environmentally friendly cementitious material with a low clinker content, in which industrial byproduct gypsum serves as the sulfate source, thereby enabling the valorization of solid waste. The hydration process, pore structure, microstructure, and hydration products were investigated using paste samples by means of isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG–DTG), Fourier transform–infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), while compressive strength was evaluated using mortar specimens. Compared with ordinary Portland cement (OPC), SSC offers clear advantages in reducing energy consumption and greenhouse gas emissions. In this study, the effects of titanium gypsum (TG) and flue gas desulfurization gypsum (FGD) on the hydration behavior, fluidity, mechanical properties, and microstructural evolution of an anhydrite (AH)–phosphogypsum (PG)-based SSC were systematically investigated. The results indicate that the incorporation of 11% TG and FGD mitigates the strong sulfate environment caused by the rapid dissolution of soluble AH, thereby regulating the hydration process. As the proportion of TG and FGD increased, the cumulative heat release within 72 h gradually decreased. When AH was completely replaced, the cumulative heat release of TG4 and FG4 decreased by approximately 19.7% and 28.6%, respectively. TG and FGD exhibited opposite effects on the fluidity of SSC while both promoting strength development. Among all mixtures, TG2 and FG2 showed the best performance, with the highest 28-day compressive strengths of 50.15 MPa and 51.95 MPa, respectively. Microstructural analysis reveals that differences in particle size distribution and dissolution kinetics among gypsums governed the sulfate release characteristics and slag activation mechanisms, thus leading to distinct hydration pathways, pore structure evolution, and microstructural densification. This study provides a theoretical basis for the efficient utilization of various industrial byproduct gypsums and offers important guidance for the controllable design of SSC performance. Full article

The rational utilization of industrial solid waste is an effective way to reduce environmental pollution. This study investigated the potential application of fluorogypsum (FG), flue gas desulfurization gypsum (FGD), phosphogypsum (PG), and titanium gypsum (TG) in the production of excess-sulfated slag cement (ESSC). It further investigated the effects of different types of gypsum on the performance and hydration process of ESSC through a wet grinding process. The results showed that as the pH value of the gypsum increased, the setting time of ESSC decreased, and hydration heat release occurred earlier. Phase analysis and microstructural characterization indicated that the type of gypsum affected the hydration rate, microstructure, and quantity of hydration products of ESSC, thereby influencing its compressive strength. To further improve the performance of ESSC, a wet grinding process was employed to enhance particle activity and promote hydration reactions. PG, due to its high solubility, demonstrated a better activation effect; after wet grinding, the 28 d compressive strength reached 40.03 MPa. Meanwhile, ESSC pastes prepared with high-pH FG exhibited not only good early strength (3-day strength of 21.93 MPa) after wet grinding but also excellent water resistance, with a softening coefficient of 0.96. This study clarifies the impact of gypsum type on ESSC performance and provides valuable insights for enhancing its properties. Full article

Mineral expansive from FDG—flue-gas desulfurization—blended with calcium aluminate cement CAC was analyzed as mitigation shrinkage of alkali-activated residual mortars AAM. The AAM mortars were composed of red mud (RM) and bottom ash (BA), as precursors of a metakaolin MK-based system. MK replacement (0, 50, 70%) in alkaline solution (10M) and ratio 1:2 (binder/sand) was studied. Engineering properties were performed, and included mechanical strength, setting times, and dry shrinkage (HR 60%), as well as the microstructure formed at 7 d and 28 days. A total of 10% CAC-FGD dosage was the most efficient, reducing drying shrinkage by 23% and autogenous shrinkage by up to 30%. The findings showed that this addition also improved mechanical strength by approximately 16% at 28 days. Under the addition of CAC-FGD, the results suggest the presence of aluminosilicate gels of the (Na,C)-(A)-S-H type and the formation of ettringite, which are possibly responsible for ensuring good performance and a controlled expansion that, in turn, compensates for the shrinkage of the activated mortars. Full article

After Germany’s planned withdrawal from coal-fired power generation by 2030, the by-product known as FGD gypsum will no longer be available. As an alternative, loam can be utilized as a building material for non-load-bearing interior walls. Recycling loam is advantageous as it is readily available in large quantities. However, its unique properties, such as moisture retention and drying shrinkage, are crucial for its usability. Loam samples are modified with various additives and molded into prisms to investigate and optimize these aspects. These prisms are tested for drying shrinkage and strength behavior. The most effective mixtures undergo further evaluation of their long-term behavior when subjected to changes in moisture—the addition of 20 wt.-% brick dust results in a reduction of the drying shrinkage by 25%. In long-term tests, swelling deformation has been reduced by 35%. This article demonstrates the effectiveness of additives in minimizing moisture-inducted deformations while maintaining the same compressive strength. Additionally, it compares various measuring methods for recording length changes in loam blocks. Full article

The massive stockpiles of Bayer-process red mud (BRM) severely compromise soil integrity, necessitating the urgent development of efficient large-scale utilization strategies. BRM contains large amounts of calcium, silicon, and aluminum. Theoretically, water glass and flue gas desulfurization gypsum (FGD) can increase the active substances in BRM, making it a cementitious raw material capable of replacing cement. This study pioneers a novel activation strategy utilizing water glass–FGD synergism to amplify the BRM reactivity, enabling an increased dosage in construction materials through enhanced pozzolanic activity. They were blended into the cement at different ratios to prepare a grouting material (BF-C) for fissure sealing in mine rock strata. The hydration mechanism of BF-C was analyzed from a micro perspective by XRD, FTIR, ICP-OES, and SEM-EDS, and combined with the Ca/(Si + Al) ratio to reveal its hydration synergy. The results showed that the 3 d and 28 d strength of 70% BRM-FGD reached 8.94 MPa and 13.71 MPa, respectively. At this ratio, the hydration synergy of BF-C was the strongest. The addition of water glass and FGD can directly modulate the Ca/(Si + Al) ratio of the system to an optimal value of 0.94, which promotes the formation of early hydration products. C-S-H gel, calcite, and C(N)-A-S-H are the main hydration products of BF-C. C-S-H gels are encapsulated on cancrinite, and their three-dimensional network structures are dense. Meanwhile, C(N)-A-S-H crystals are interspersed between C-S-H gels, making the structure more stable. This achievement introduces an innovative method for the large-scale utilization of Bayer red mud, providing an effective solution in grouting technology using solid waste as raw material. Full article

To improve the mechanical and durability properties of low liquid limit soil, an eco-friendly, all-solid, waste-based stabilizer (GSCFC) was proposed using five different industrial solid wastes: ground granulated blast-furnace slag (GGBS), steel slag (SS), coal fly ash (CFA), flue-gas desulfurization (FGD) gypsum, and carbide slag (CS). The mechanical and durability performance of GSCFC-stabilized soil were evaluated using unconfined compressive strength (UCS), California bearing ratio (CBR), and freeze–thaw and wet–dry cycles. The Rietveld method was employed to analyze the mineral phases in the GSCFC-stabilized soil. The optimal composition of the GSCFC stabilizer was determined as 15% SS, 12% GGBS, 16% FGD gypsum, 36% CS, and 12% CFA. The GSCFC-stabilized soil exhibited higher CBR values, with results of 31.38%, 77.13%, and 94.58% for 30, 50, and 98 blows, respectively, compared to 27.23%, 68.34%, and 85.03% for OPC. Additionally, GSCFC-stabilized soil demonstrated superior durability under dry–wet and freeze–thaw cycles, maintaining a 50% higher UCS (1.5 MPa) and a 58.6% lower expansion rate (3.16%) after 15 dry–wet cycles and achieving a BDR of 86.86% after 5 freeze–thaw cycles, compared to 65% for OPC. Rietveld analysis showed increased hydration products (ettringite by 2.63 times, C-S-H by 2.51 times), significantly enhancing soil strength. These findings highlight the potential of GSCFC-stabilized soil for durable road sub-base applications. This research provides theoretical and technical support for the development of sustainable, cost-effective, and eco-friendly soil stabilizers as alternatives to traditional cement-based stabilizers while also promoting the synergistic utilization of multiple solid wastes. Full article

Due to changes in the German government’s energy concept, the amount of gypsum produced in flue gas desulfurisation plants (FGD gypsum) will fall from 5 million tons per year to 1 million tons or less by 2038 at the latest. As of 2016, FGD gypsum accounts for 55% of German gypsum mix. The resulting raw material gap must be closed through innovative recycling concepts, such as the processing of existing mine dumps. The process development aims to achieve a calcium sulfate dihydrate content of 85% and a reduction in the stockpile volume by 50%. The main components of the stockpiles are calcium sulfate in the form of gypsum stone as well as clay minerals and organic matter. Successful laboratory tests were transferred to a pilot scale jigging machine with dewatering screening. The process water is circulated throughout the entire process. The gypsum content in the heavy fraction is 76% when measured with ICP OES and 87% when measured via thermogravimetric methods. Furthermore, pilot-scale dry screening on the stockpile took place, and up to 1500 tons of material could be processed. Due to fluctuating weather conditions, the screening quality was subject to significant variations. Under optimal conditions, up to 60% of the feed could be recovered as gypsum stone; however, the screening process was nearly impossible during rain; therefore, a process combination of screening and a downstream jigging machine is recommended. Full article

This study proposes an innovative method for the harmless treatment of barium slag using the industrial by-product Flue Gas Desulfurization Gypsum. Barium slag is a by-product of the barium carbonate production process, and due to its high content of barium ions and corrosive properties, it poses a significant threat to the environment and human health. It is classified as barium-containing hazardous waste (code HW47) in China. In this study, barium slag was optimally combined with FGD gypsum, utilizing a synergistic precipitation mechanism to solidify the easily leachable barium ions and form stable sulfate minerals. Mechanical and heavy metal leaching tests showed that the harmlessly treated barium slag had a certain compressive strength, and the concentration of barium ions in the leachate was below the national hazardous waste identification standards (100 mg/L) and the drinking water quality standards (0.7 mg/L). Microstructural analysis using X-ray diffraction, Fourier Transform Infrared Spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy indicated that FGD gypsum promotes the solidification of barium slag, and through the synergistic precipitation mechanism, low-solubility barium sulfate minerals are formed. This treatment method also has a low cost and good potential for resource utilization, providing effective technical support for the green treatment of industrial waste. Full article

In recent years, the implementation of CO₂ capture systems has increased. To reduce the costs and the footprint of the processes, different industrial wastes are successfully proposed for CO₂ capture, such as gypsum from desulfurization units. This gypsum undergoes an aqueous carbonation process for CO₂ capture, producing an added-value solid material that can be valorized. In this work, panels have been manufactured with a replacement of (5 and 20%) commercial gypsum and all the compositions kept the water/solid ratio constant (0.45). The density, surface hardness, resistance to compression, bending, and fire resistance of 2 cm thick panels have been determined. The addition of the waste after the CO₂ capture diminishes the density and mechanical strength. However, it fulfills the requirements of the different European regulations and diminishes 56% of the thermal conductivity when 20%wt of waste is used. Although the CO₂ waste is decomposed endothermically at 650 °C, the fire resistance decreases by 18% when 20%wt. is added, which allows us to establish that these wastes can be used in fire-resistant panels. An environmental life cycle assessment was conducted by analyzing a recycling case in Spain. The results indicate that the material with CO₂ capture waste offers no environmental advantage over gypsum unless the production plant is located within 200 km of the waste source, with transportation being the key factor. Full article

The use of waste to capture CO₂ has been on the rise, to reduce costs and to improve the environmental footprint. Here, a flue gas desulfurization (FGD) gypsum waste is proposed, which allows us to obtain a CaCO₃-based solid, which should be recycled. The CO₂ capture stage has primarily been carried out via the direct carbonation method or at high temperature. However, a high energy penalty and/or long reaction times make it unattractive from an industrial perspective. To avoid this, herein an indirect method is proposed, based on first capturing the CO₂ with NaOH and later using an aqueous carbonation stage. This allows us to capture CO₂ at a near-ambient temperature, improving reaction times and avoiding the energy penalty. The parameters studied were Ca²⁺/CO₃²⁻ ratio, L/S ratio and temperature. Each of them has been optimized, with 1.25, 100 mL/g and 25 °C being the optimal values, respectively, reaching an efficiency of 72.52%. Furthermore, the utilization of the produced CaCO₃ as a building material has been analyzed. The density, superficial hardness and the compressive strength of a material composed of 10 wt% of CaCO₃ and 90 wt% of commercial gypsum, with a water/solid ratio of 0.5, is measured. When the waste is added, the density and the mechanical properties decreased, although the compressive strength and superficial hardness are higher than the requirements for gypsum panels. Thus, this work is promising for the carbonation of FGD-gypsum, which involves its chemical transformation into calcium carbonate through reacting it with the CO₂ of flue gasses and recycling the generated wastes in construction materials. Full article

Gypsum–urea is a kind of urea product with substantially reduced aqueous solubility and lower hygroscopicity that increases the soil retention time of urea and thus enhance its environmental sustainability. Here, gypsum–urea was prepared using bulk industrial solid waste flue gas desulfurization (FGD) gypsum as a raw material in a saturated urea solution via immobilizing urea molecules into the crystal lattice. The preparation process was achieved through a dissolution–recrystallization mechanism during which FGD gypsum dissolved into Ca²⁺ and SO₄²⁻, which then recrystallized with CO(NH₂)₂ to form gypsum–urea. The preparation process was almost completed within 10 min, and the formed gypsum–urea presented a uniform size distribution of 30–90 μm and a much lower hygroscopicity and nitrogen release efficiency than that of urea. With a high efficiency of synthesis, and sustainable features, and the recyclability of the saturated mother urea solution, the dissolution–recrystallization-based urea immobilization approach is highly promising regarding the preparation of gypsum–urea with the desired environmental sustainability and contributes to the realization of the sustainable reutilization of FGD gypsum. Full article

Over the last 20 years, flue gas desulfurization gypsum (FGD gypsum) has become a valuable and widely used substitute for a natural raw material to produce plasters, mortars, and many other construction products. The essential advantages of FGD gypsum include its high purity and stability, which allow for better technical parameters compared to natural gypsum, and, until recently, its low price and easy availability. This FGD gypsum is obtained in the process of desulfurization of flue gases and waste gases in power plants, thermal power plants, refineries, etc., using fossil fuels such as coal or oil. The gradual reduction in energy production from fossil raw materials implemented by European Union countries until its complete cessation in 2049 in favor of renewable energy sources significantly affects the availability of synthetic gypsum, and forces producers of mortars and other construction products to look for new solutions. The gypsum content in commonly used light plaster mortars is usually from 50 to 60% by mass. This work presents the results of tests on mortars wherein the authors reduced the amount of gypsum to 30%, and, to meet the strength requirements specified in the EN 13279-1:2008 standard, added Portland cement in the amount of 6–12% by mass. Such a significant reduction in the content of synthetic gypsum will reduce this raw material’s consumption, thus extending its availability and developing other solutions. The study presented the test results on strength, density, porosity, pore size distribution, and changes in the microstructure of mortars during up to 180 days of maturation in conditions of increased relative humidity. The results show that decreased porosity and increased mechanical strength occur due to the densification of the microstructure caused by the formation of hydration products, such as C-S-H, ettringite, and thaumasite. Full article

Heavy metals in flue gas desulfurization (FGD) gypsum from coal-fired power plants are at risk of releaching during the processes of stockpiling and resource utilization. In this study, the effects of organosulfur chelators dithiocarbamate (DTC) and trisodium trithiocyanate-15 (TMT-15) on the solidification characteristics of heavy metals in desulphurized gypsum under different mass fractions, pH values, water contents and reaction times were investigated. The chemical composition and morphology were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES) and scanning electron microscope (SEM). The experiments showed that both DTC and TMT-15 were effective at stabilizing the heavy metals in the FGD gypsum, with more than a 50% curing effect for all the heavy metals except Pb. DTC showed a better stabilization for Pb, Hg, Cu, Zn, and Cr, and TMT-15 showed a better curing effect for Cd. The solidified gypsum had good heavy metal stability in low-water-content environments. Increasing the mass fraction, reaction time, and pH decreased the heavy metal leaching, and the mass fraction had the greatest effect on the total heavy metal leaching concentration, followed by the reaction time and pH value. Full article

Silty soil performs poorly when used in roads. Cement is generally used as a stabilizer to treat silty soil and enable it to meet the requirements for roadbed filling. However, cement is an environmentally unfriendly material and can cost much. Meanwhile, solid wastes of ground granulated blast furnace slag (GBFS), fly ash (FA), and flue gas desulfurized (FGD) gypsum are produced in large quantities annually. Therefore, stabilizer A (cement:ground GBFS:fly ash:FGD gypsum = 30:44:15:11) and stabilizer B (cement:ground GBFS:fly ash:FGD gypsum = 40:38:13:9) were investigated in this study by reducing the cement content in the stabilizer and improving the utilization rate of solid wastes. The compressive strength development, California bearing ratio (CBR), temperature shrinkage, mineral composition, and micro-morphology of the stabilized silty soil were measured. The main findings are as follows: firstly, the addition of solid wastes can mitigate the adverse effect of delay time on compressive strength development. Secondly, the proposed stabilizers can significantly improve the CBR, which can reach 60% with a 4% dosage. Additionally, Stabilizer B is believed to improve the resistance to temperature shrinkage, and a higher stabilizer dosage can reduce the rate of decrease in water stability coefficient. Both X-ray diffraction analysis and scanning electron microscope observations show that the main hydration products that contribute to the stabilization are C-S-H and ettringite. Full article

Studying the distribution and transport dynamics of cations in plants is crucial for understanding their response mechanisms to saline–alkali stress conditions. However, our current understanding of how restoration measures affect cation distribution and transport in plants is surprisingly limited. To address this gap, we conducted a split-plot experiment using Medicago sativa L. cv. “Zhongmu No. 1” to investigate the combined effects of biological and chemical restoration measures—with bio-fertilizer as the primary zone and flue gas desulfurization (FGD) gypsum and with humic acid as the secondary zone—on soil properties, plant growth, and the content, distribution, and transport of cations in plants. The results revealed that bio-fertilizers exhibited positive effects on the plant growth, yield, and translocation of key ionic components to leaves. On the contrary, FGD gypsum with humic acid reduced the soil’s pH level, exchangeable sodium percentage (ESP), and sodium adsorption ratio (SAR) while increasing the contents of K⁺, Ca²⁺, and Mg²⁺ in the soil. The combination of bio-fertilizer, FGD gypsum, and humic acid increased the biomass and enhanced the translocation of Mg²⁺ to leaves. The distribution and transport of Mg²⁺ within the plant constituted pivotal elements for enhancing plant growth through restoration strategies. The application of bio-fertilizer, FGD gypsum, and humic acid reduced Na⁺ transport in M. sativa by enhancing the selective absorption of beneficial ions in leaves and by facilitating the transport of Ca²⁺ and Mg²⁺ from stems to the leaves. This, in turn, increases the salt tolerance of plants and promotes their growth. Our results offer new insights into the interactions among measures, soil, and plants in saline–alkali land restoration, providing practical solutions for the restoration of saline–alkali soil. Full article