No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Preparation and properties of high blending phosphogypsum-desulfurization ash-waste soil based functional prefabricated autoclaved aerated concrete slabs
... To ensure the best pore structure during mixing, it is recommended to adopt a mixing process that involves slow, then fast, and then slow mixing. Before pouring the actual foam concrete, it is necessary to conduct a test specimen test to evaluate the pouring quality [21]. The cutting of PVC pipes was performed at heights of 500, 1,000, 1,500, and 2,000 mm using a cutting machine. ...
This research conducted in Jiangning, Nanjing, China, aimed to examine the effectiveness of traditional and contemporary construction techniques for external wall insulation. The study compared thermal resistance, indoor air temperature, energy consumption, and potential energy savings. The results showed that new construction methods utilizing fiber-based thermal insulation materials displayed superior performance in various aspects leading to improved thermal insulation properties. Based on the comparative analysis presented in this study, it is suggested that incorporating organic insulation materials into new construction methods for external wall insulation can enhance thermal performance and energy efficiency when compared to traditional approaches. Current energy-saving design standards have been raised by more than 50%, highlighting the growing significance of designing buildings with energy conservation in mind within our country's industrial development context. A newly proposed construction program involving integrated insulation panel composite exterior wall systems demonstrates cost-effectiveness with lower costs per square meter when compared to traditional methods. This study provides an analysis of the thermal performance and energy efficiency associated with both traditional and modern construction methods for external wall insulation specifically within Jiangning, Nanjing area. The investigation highlights the importance of considering varying climatic conditions' impact on thermal performance and retrofitting existing structures for enhanced energy efficiency. It leverages insights from sustainable development studies to promote sustainability in the construction industry while prioritizing public safety and compliance with energy-saving regulations.
The autoclaved aerated concrete (AAC) is a porous lightweight material. Its internal pore structure is a crucial technical feature that affects its performance. The effects of different types of foam stabilizers (cellulose ether, polyvinyl alcohol, soap powder and calcium stearate) on the compressive strength, dry density, dry wet cycle resistance, insulation performance and microstructure of iron tailings autoclaved aerated concrete (ITS-AAC) were investigated. The hydration products and microstructures were analyzed by X-ray diffraction and scanning electron microscopy. The mechanisms of the actions of the foam stabilizers were then revealed. It was shown that among the four foam stabilizers, the polyvinyl alcohol-modified ITS-AAC had the highest proportion of pores, with a smaller internal area and no apparent pore connectivity. These features had the most significant impacts on ITS-AAC performance and microstructure. Furthermore, the modified ITS-AAC showed the best performance for a polyvinyl alcohol content of 1.0%, with a compressive strength increase of 30.2%, a dry wet cycle coefficient increase of 63.9% for the 15th cycle, and a dry density decrease of 7.7% (with respect to the unmodified ITS-AAC),the thermal conductivity decreased by 42.6%.This was due to the fact that many highly active hydroxyl groups in polyvinyl alcohol would bind to water molecules with hydroxyl hydrogen bonds, thereby directly competing with the host hydrate molecules. The water molecules that were adsorbed on the surface of the hydrophilic SiO2 formed hydrogen bonds with the hydroxyl groups in the formation of a stable polyvinyl alcohol polymer. In addition, due to the excellent film-forming properties of polyvinyl alcohol after dissolution, some parts of the hydration C–S–H gel product and the slurry, which did not fully participate in the reaction, were wrapped in the film-forming process. A water hardening slurry with a good network and dense spatial structure was formed.
The pressing demand for green buildings poses new challenges to the construction sector. In this context, an effective strategy to be pursued is represented by the use of construction materials with enhanced thermal properties, together with a reduced carbon footprint and adequate structural resistance. Autoclaved Aerated Concrete (AAC) masonry is one of the most interesting solutions. Several studies have been published so far to investigate the influence of raw materials and production processes on the macroscopic properties of AAC blocks. This paper aims to refine and extend the literature review on AAC, by collecting and processing experimental data related not only to the material itself, but also to masonry assemblages. In order to span different aspects of sustainable design, a comprehensive review on both physical and mechanical properties (related to safety issues) as well as thermal properties (related to environmental aspects) of AAC is provided. This study shows how constituent materials influence the macroscopic behavior of AAC masonry, also comparing experimental trends with relationships suggested in European design standards.
In the case of building demolition, the construction industry increasingly prioritizes sustainable waste management and environmental protection. Life cycle assessment (LCA) was used to assess two scenarios to evaluate the environmental impacts of four methods (demolition, transport, recycling, and landfills). To conduct an Attributional LCA, the SimaPro software suite and Ecoinvent v.3 Life Cycle Inventory database were applied. LCA showed that construction and demolition (CDW) management has major environmental effects on global warming, fine particulate matter formation, human carcinogenic toxicity, and human non-carcinogenic toxicity. The results indicate the beneficial environmental impact of two waste management scenarios. The extent to which recycling waste from building demolition can benefit the environment is dependent on the type of material being recycled. Copper recycling can reduce the building industry’s environmental impact. Human non-carcinogenic toxicity is the most important and beneficial factor. Previous research has largely neglected this stage. The findings indicated that environmental conditions have a significance impact and, accordingly, the optimal scenario. This study provides stakeholders with a road map for making informed decisions regarding the application of CDW management. It suggests that LCA studies should take multiple indicators into account, and sensitivity and uncertainty analyses were performed to evaluate the accuracy of the results.
The mining industry generates large amounts of tailings every year. The most common destination for the tailings is deposition in tailings storage facilities (TSFs), which can have enormous dimensions. The management and storage of such large volumes of material pose many challenges in terms of dam stability and immobilization of hazardous contaminants that represent human-health and environmental risks, particularly for sulfide-containing materials. In addition, considerable amounts of precious and base metals can be lost in the tail-ings. Due to the economic value and growing industrial demand for precious and base metals, tailings may therefore be potential sources of secondary raw materials. This contribution investigates the flotation of pyrite-rich tailings, containing residual chalcopyrite, galena, and sphalerite, and high amounts of ultrafine particles. Flotation was used to recover the sulfide minerals and generate tailings with low sulfur content. The Cu-Pb-Zn-rich product could go to further treatment (e.g. (bio)hydrometallurgy) to recover the metals, while the low sulfur fraction could be used in the civil construction industry. Automated mineralogy (MLA) was used to provide quantitative mineralogical and textural data. Bench-scale experiments were performed by combining classic flotation and floc flotation (flotation of flocs of targeted minerals). Flotation of the material as received, as well as after classification into two fractions was performed. The samples as received and the coarser fraction (+37 µm) underwent classic flotation, while the finer fraction (− 37 µm) was processed either by using the classic or the floc flotation approach. The flotation of the coarser particles provided higher sulfide recoveries, higher combined Cu-Pb-Zn grades in the concentrate (3.66 %), cleaner residues (1.6 % S), faster flotation rates, and reduced reagent consumption. Likewise, the results from the fine particle flotation allowed lower S content in the residues (3.4 % S) as compared to the flotation of the original material. The results of the use of floc flotation for the finer fraction show an increase in the mass pull with a slight increase in the recovery of sulfides. Overall, the development of a route to process the tailings proved to be promising and the use of a two-route approach indicates advantages as compared to a single route.
Autoclaved aerated concrete (AAC) is a widely used building material for masonry units, prefabricated reinforced components, and lightweight mineral insulation boards. Its low thermal conductivity and good fire resistance increase its popularity in residential buildings. Thus, post-demolition wastes are expected to increase in the future. However, post-demolition AAC (pd-AAC) is mainly disposed in landfills while landfill capacities decrease and legal framework conditions in Europe are tightening. This study performed life cycle assessments (LCA) of different pd-AAC recycling options and compared them to each other and to current landfilling to identify the best end-of-life handling of pd-AAC from an ecological perspective. The functional unit was 1 kg pd-AAC, and the system boundaries included pd-AAC at the demolition site, transports, pd-AAC treatment, and secondary production processes. Final products of the recycling process gained environmental credits/rewards for avoiding primary production using system expansion. Providing primary resources, primary production, and use phase were not in the scope of this study. Results show that especially closed-loop recycling of pd-AAC in AAC production has a high potential of improving environmental impacts. In the best recycling option (high substitution in AAC-0.35), potential savings per kg pd-AAC compared to landfilling reach up to 0.5 kg CO2-Eq, 7 MJ fossil resources, 0.005 mol H+-Eq (acidification), 0.17 CTU (freshwater ecotoxicity), 0.2 g P-Eq (freshwater eutrophication), 5.2 × 10-9 CTUh (carcinogenic effects), 4.4 × 10-8 CTUh (non-carcinogenic effects), 2.5 × 10-5 g CFC-11-Eq (ozone layer depletion), and 1.6 g NMVOC-Eq (photochemical ozone creation). Despite data uncertainties, recycling of pd-AAC is advantageous for several recycling options, including the production of AAC, light mortar, lightweight aggregate concrete, and shuttering blocks made from concrete without fine fractions (no-fines concrete). In Germany, up to 280,000 t CO2-Eq could have been saved in 2022 by pd-AAC recycling using different recycling options instead of landfilling.
The storage of phosphogypsum could bring huge burden to the environment, while the traditional method of washing and purifying phosphogypsum produced a large amount of waste water and colossal energy consumption. The novel colloidal-mechanical method as a new treatment technology of purifying phosphogypsum was benefit for the large-scale comprehensive utilization of phosphogypsum, but its purification effect and influencing factors needed to be further studied. Surfactants like Span-80, TX-100 and Tween-80 were one of major factors for colloidal-mechanical purification method about whiteness improvement and impurities removal of phosphogypsum. It created colloidal-mechanical microenvironments during the purification process of phosphogypsum, and helped to realize the best removal effects and purification technology of phosphogypsum. After characterization, it showed that the whiteness of phosphogypsum increased under the action of micelle and mechanical force, and the removal effects of soluble phosphorus and fluorine impurities were noticeable. The whiteness value of purified PAS could reach 51.9, which was 57.8% higher than natural PG. The phosphorus content in the slurry solution of PAS was 274.649 ppm, which was 208% higher than PG. The fluorine content in PAS was 0.289 wt%, which was 24.3% lower than PG. The purification effect of phosphogypsum was the best through the micro-environment created by Span-80 and mechanical force. This study provided a green design direction for the critical purification and its further utilization of phosphogypsum. This work provided a clear idea for solving the current purification dilemma of phosphogypsum with an outstanding application prospect.
The transformation of phosphate ore into phosphoric acid results in the generation of high volumes of phosphogypsum (PG), an industrial by-product largely stockpiled worldwide. This solution, considered as the least damaging to the environment, constitutes a risk for the receiving environment due to the presence of harmful impurities such as heavy metals and radionuclides which hinder its large-scale valorization. This paper presents an environmental characterization of Moroccan phosphogypsum and an investigation on the environmental performance of a new lime (L) - fly ash (FA) treated phosphogypsum based road material. The concentration of metallic trace elements (Cr, Pb, Ni, Zn, Cu) in raw phosphogypsum ranged between 0.2 and 243 ppm, while its radioactivity reached 970 Bq/kg for Ra-226. The environmental performance of the proposed new road material (40% PG, 42% FA, 18% L) was evaluated using radiological risk indices besides static and dynamic leaching tests. The results showed a radioactivity reduction up to 82%, and an immobilization of metallic trace elements ranging from 25 to 100%. The stabilization/solidification mechanisms involved in the lime - fly ash treatment would be responsible for the fixation of these contaminants within the newly formed matrix.
The phosphate industry produces a hazardous byproduct called phosphogypsum (PG). A rising global stockpiling of PG poses severe threats to public health and the environment. Fortunately, recycling this material may be a risk-free and environmentally beneficial answer to this problem. It is necessary to examine its current application and future growth prospects in civil engineering to increase the PG utilization rate. This paper summarizes previous research on reusing PG in building materials. This review article is divided into four essential sections. Firstly, the chemical properties of PG and their treatment methods before using PG in any application are reviewed extensively. Secondly, the strength properties of bricks made with PG and their microstructural characteristics through scanning electron microscope (SEM) and X-ray diffraction (XRD) were reviewed. The third part of this article is about using PG as a substitution for cement and studies the setting time, compressive strength, SEM and XRD of cement mortar. The final section is PG-based concrete and studies concrete specimens' workability, compressive strength, SEM and XRD. The last three sections extensively focused on the microstructural characteristics of the building materials. Following a comprehensive literature search, it was determined that PG could be utilized in the construction industry. Several studies have been performed on using PG in building materials such as bricks, concrete and other materials. However, PG utilization in large-scale practical applications needs better social and political awareness. More money is needed for research and development to free the economy and technology from their shackles. The findings of this review will serve as a foundation for further research and practical applications of PG in environmentally friendly processes.
With the acceleration of urbanization, the production of waste concrete is getting higher and higher. A large amount of outdoor accumulations of waste concrete will leach heavy metals, not only causing harm to the soil, but also posing a risk to human health. Based on this, this paper systematically studies the basic physical properties and microstructure (XRF, XRD, and SEM–EDS) of outdoor natural accumulation waste concrete, and analyzes the heavy metals in waste concrete from the aspects of existing state, leaching mechanism, human health risk analysis, and summarized the direction of resource utilization of waste concrete, calculated the carbon emission reduction during recycling. The study found that heavy metals in waste concrete mainly exist in hydration products in the form of precipitation, adsorption, and replacement, summarized the leaching mechanism from the micro- and macro-aspects. The leaching mechanism of heavy metals can be assigned to chemical (mineral dissolution and effective amount of components) and physical (advection, surface erosion, and diffusion) processes from the macro-perspective. From the micro-analysis, it can be assigned to the following five processes: acid migrates from solution to liquid–solid surface, acid migration through leaching layer, rapid dissolution reaction controlled by diffusion at leaching boundary, heavy metal through leaching layer, and heavy metals through the solid/liquid surface to the solution. In addition, the concentration and the leaching rate of heavy metals in waste concrete were analyzed. It was found that the concentration of Cr was the highest reached to 4.7 mg/kg and the leaching rate of Cd was the highest, its leaching coefficient was calculated as a result of 1.713 × 10–6. However, there was no obvious regularity in the leaching of heavy metals in different accumulate particle sizes. Through the establishment of risk assessment system was found the concentration of heavy metals in waste concrete will not cause significant harm to human health. The effective limit of heavy metals after 3 months of accumulation of waste concrete was calculated as: Cr < 0.09 mg/kg, Cd < 0.00715 mg/kg, As < 0.392 mg/kg, and Pb < 0.732 mg/kg. And the carbon emission reduction of waste concrete recycling was calculated to be 28.764kgCO2/t. All the results of this study can promote the safe and environmentally friendly utilization of waste concrete.
Phosphogypsum is an industrial waste generated in the production of fertilizers causing environmental problems. Calcined phosphogypsum (CPG) is obtained by low-temperature calcination and dehydration of phosphogypsum. CPG is mainly used to produce building materials. However, mechanical strength properties and water resistance of CPG materials is extremely poor, which leads to its relatively low utilization rate in construction industry. To address this issue, various amount of Ordinary Portland cement (OPC), fly ash (FA), rice husk (RH) and modified admixtures (AD) were mixed with the CPG to obtain CPG-based composite cementitious materials, which enhanced the water resistance and mechanical strength of the corresponding hardened materials. The influence of different contents of cement on the setting time, hydration heat, hydration products, mechanical properties, water absorption, heavy metal leaching and deterioration coefficient of the composite cementitious material is investigated. The results showed that the optimal mixing ratio of CPG: OPC: FA: RH was 57: 20: 10: 13 considering the water resistance and mechanical strength properties of CPG-based composite cementitious materials. On these conditions, the compressive strength, the flexural strength, water absorption and deterioration coefficient of the specimens at 28 days are 6.1 MPa, 4.6 MPa, 29.4% and 0.66 respectively, which were in line with the national standard of phosphorus building gypsum wallboard. It was demonstrated that two skeleton types are present in the samples prepared using CPG-based composite cementitious materials, which had a synergistic support effect on the structure. The first skeleton was formed by gypsum dihydrate crystals (CaSO4·2H2O) and provided strength to the specimens at an early stage, while the second skeleton was formed by ettringite (AFt) and C–S–H gels, which provided strength at a later stage. The skeletons make the hardened materials denser and more waterproof. Moreover, the composite materials exhibited a positive solidification effect on heavy metals present in CPG.
When acid phosphogypsum (APG) is recycled as aggregate of cemented backfill in mines, pretreatment is usually needed to reduce its adverse impact on the backfill. In this study, a new approach of using a neutral salt (CaCO3) to pretreat APG has been proposed to enhance the strength development of the cemented PG backfill. The role of CaCO3 in cemented PG backfill acting as a reactant is explored. In the pretreatment stage, CaCO3 reacted with the residual acid in PG, and the generated calcium ions could solidify the impurities, such as soluble phosphate and fluoride, with solidification rates reaching 90% and 99%, respectively. This neutral modifier buffered pH values of PG aggregate from 1.8 to about 5.0, and excess dosage of CaCO3 did not adjust the pH value of aggregate to an excessively high level. Hence, there was no interference with the hydration reaction of the binder (cement). Using PG pretreated with CaCO3 as aggregate for cemented backfill effectively accelerated the setting process of backfill and increased the unconfined compressive strength (UCS) of the backfill matrix from 0.14 MPa to 2.27 MPa at 28-day age. The present results suggest that CaCO3 pretreatment is an effective way to improve the accessibility of PG as aggregate, thus significantly increasing the utilization rate of PG and enhancing the backfill performance.
Gasification fly ash (GFA) is a hazardous solid residue generated in the slagging-gasification of municipal solid waste (MSW). GFA contains higher amounts of heavy metals such as Pb and Zn than incineration fly ash (IFA), which increases the difficulty of heavy metal immobilization but simultaneously makes it a potential feedstock for metal recovery. Water washing and acid washing are conventional and economic methods to treat wastes with high heavy metal and chloride contents. However, the research on the effects of such methods in treating GFA is still blank. Hence, in this study, water washing and acid washing of GFA were investigated in detail. Heavy metal behaviors at different time points during the washing processes were studied in a wide pH range and comprehensive characterizations of washed GFAs were also conducted. The results show that different re-precipitates could be identified in washed GFAs depending on different pH conditions. After water washing for 24 h, more than 60% of Zn in GFA would dissolve and re-precipitate into calcium zincate. It is also revealed that the precipitation effect could in turn influence the pH during the washing process. After acid washing with a low-concentration acid, heavy metal leachabilities were found reduced due to the pH and precipitation effect. High-concentration acid washing could effectively extract Zn and Cd with extraction ratios exceeding 90%. Applying 1.2 M-HCl washing, a short washing period of 15 min could realize a Pb extraction ratio of 81.2%, much higher than 53.2% when extending the washing period to 24 h.
This paper presents an experimental study on the effects of calcined gangue and wet co-milling on the properties of phosphogypsum-based excess-sulphate cementitious materials (PESCMs), including setting time, rheology, heavy metal solidification and strength development. For PESCMs containing 0%-50% calcined gangue, with and without wet co-milling, we examined hydration and microstructural evolution. Results indicate that the addition of calcined gangue shortens the initial and final setting time of fresh PESCM pastes, and impairs stability, fluidity and strength development. After wet co-milling, the rheological and mechanical properties of PESCM pastes are improved, where the binders with 10–20% calcined gangue show a 40% increase in 28-d compressive strength. Hydration of calcined gangue is rapid and weakens the retarding effect of impurities, providing more active aluminates and silicates for further hydration. Wet co-milling promotes physical dispersion, chemical dissolution and thus hydration, which helps refine microstructure for a better development of strength and heavy metal solidification capability.
In China, the amount of phosphogypsum (PG) accumulated has been increasing rapidly with the development of the phosphate fertilizer industry. PG exists mainly in the form of dihydrate gypsum, which can replace natural gypsum to produce hemihydrate gypsum and is used in the building materials industry. However, it contains impurities, such as phosphate, fluoride, sulfate, and natural radioactive elements, which limit its application and seriously pollute the surrounding environment. To promote the application of PG, it is crucial to understand how pretreatment affects the properties of hemihydrate phosphogypsum (HPG). In this research, the mechanical properties and microstructure of HPG after pretreatment was investigated for improving the reuse of PG as a promising resource. The harmful phosphorus impurity in PG was efficiently removed with different concentrations of by H2C2O4. Then the hydration process and the hardening characteristics of HPG were studied in detail. The findings indicate that H2C2O4 alters the particle characteristics of HPG and has an impact on hemihydrate gypsum dissolution and dihydrate gypsum precipitation. According to studies, small amounts of H2C2O4 can remove impurities, promoting hydration and enhancing strength by opening the closed orifice of HPG, which would promote the dissolution of HPG and alter the morphology of gypsum from a tiny crystal to a thickened columnar structure. However, the addition of excessive H2C2O4 impacts the compression strength of HPG plaster and reduces the rehydration of HPG due to the formation of calcium oxalate on the gypsum crystal surface. These new findings may give insight into how H2C2O4 affects the performance of HPG. Therefore, to ensure the efficient utilization of PG, quality stabilization and improvement are crucial.
Large amounts of coal gangue and municipal sludge are generated from coal mining and municipal sewage. Some scholars have studied their use in ceramics, concrete, land reclamation and so on. However, in current study, heavy metals in coal gangue and municipal sludge in utilization have not been safely controlled, and there are potential risks to the ecological environment and human health. Therefore, this study systematically investigated the basic physical and chemical properties of coal gangue and municipal sludge samples, the ecological environment safety risk of heavy metals and other harmful substances (soluble phosphorus and fluorine). The results showed that the main compositions of coal gangue and municipal sludge samples were cristobalite, hematite and anorthite, which can be used as raw material for ceramsite preparation. The characteristic heavy metals in coal gangue and municipal sludge were As, Cd, Cu, Ni and Zn, which had certain environmental risks. When the optimum proportion of coal gangue: sludge = 70: 30, adding clay and blowing agent as auxiliary materials to make ceramsite, its properties, including bulk density, cylinder pressure strength and other physical properties, meet the Chinese standard “light aggregate and its test method” (GB/T 17431). The heavy metal environment and human health assessment model of the product was constructed, in which the risk assessment results are, that the content of characteristic heavy metals in ceramsite was significantly reduced, and this content conformed to the requirements of heavy metals in the Chinese standard “technical specification of cement kiln cooperative disposal of solid waste” (GB/T 30760), riskless to the human body and external environment. The compressive strength of the two kinds of ceramsite concrete can reach LC35, and the density grade was 1400–1650 N/m³, which meets the requirements of Chinese standard “technical specification for lightweight aggregate concrete” (JGJ 51). All the properties of this product meet the corresponding standards and there was no risk to the external environment, to provide theoretical guidance for the harmless and resource utilization of urban sludge and coal gangue.
Industrial solid waste (slag, desulphurization gypsum, fly ash) and construction soil architecture residue soil are used to solidify dredged silt to develop a novel cover material. Through the application of shear, compressive, and permeability tests, shear strength parameters, unconfined compressive strength, volume shrinkage and hydraulic conductivity of the cured silt were assessed under the different curing times and wet-dry cycles conditions. The result shown slag played the most significant role in the silt consolidation process, followed by FGD gypsum and fly ash. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and nuclear magnetic resonance (NMR) detection methods were employed to determine the hydration products, chemical characteristics, microscopic morphology and pore structure of the solidified sludge, and identify the solidification mechanisms of the industrial waste and slag. After the wet and dry cycles, micropores and foramen (0.1–0.5 µm) continued to dominate pores, accounting for 64–76% of pore space. Hydration products in the solidified sludge primarily consisted of C-S-H, C-A-S-H, and N-A-S-H gelled products, along with ettringite and zeolite.
Disposal of flue gas desulphurization residue (DR) and red mud (RM) has been a focused problem due to the huge amounts and complicated compositions. In this work, an efficient co-disposal method was developed, through which DR and RM can be converted into solid precursors for the synthesis of high strength alkali activated materials (AAM). The effect of DR/RM mass ratio and calcination temperature on mineralogical composition of obtained precursors were investigated, together with the mechanical properties, microstructure, chemistry of reaction products of the synthesized AAM. Results show that the synthesized AAMs achieved excellent mechanical properties, with 3 d and 28 d compressive strength of 7.7–25.1 MPa and 30.1–58.8 MPa, respectively. Main reaction products include gypsum, hydrotalcite and C-ASH gel.
The potential harm to the environment and human body caused by heavy metal elements such as Cd, Cr, Pb in fly ash cannot be ignored. In this study, microscopic analysis and heavy metal hazard analysis were performed on fly ash samples from four regions in China. XRF, XRD, FTIR and SEM/EDS test results showed that the main components of fly ash are Si, Ca, Al, O and other elements, and the main mineral components are mullite, quartz and amorphous aluminosilicate. The results showed that six heavy metal elements of As, Hg, Cd, Cr, Pb and Ni from the EDS test charts of these four types of fly ash. Based on the detection of the six heavy metal contents in fly ash, the results of the pollution evaluation of these four types of fly ash were consistent according to the single factor index, potential ecological risk index, and index of geo-accumulation. This indicated that Cd/Hg in FA 1, FA 2 and FA 3 has a high risk of environmental pollution. In the meantime, Cr and As in fly ash need to be controlled, because the human health risk assessment showed that they will bring carcinogenic risk to the human body through oral intake. The study on the migration law of characteristic heavy metals by taking FA 4 as an example showed that the leaching rate of Cr, Hg and Cd decreased with the increase of grinding particle size. Among the three characteristic heavy metals, mercury had the highest leaching rate. From the XPS detection results of FA 4, it was found that the valence states of these three heavy metals in FA 4 are Cr³⁺, Cr⁶⁺, Cd²⁺, Hg²⁺ respectively. These harmful heavy metal ions gradually enter the deep soil and groundwater through osmosis. Finally, based on the control model constructed in this study, the contents of these three heavy metals in fly ash in different application scenarios were limited.
In the previous studies, chemical, physical, and heat treatment methods are commonly used to treat waste phosphogypsum (PG). This study reports a new method that uses biowashing to remove phosphorus, fluorine impurities, and 12 kinds of heavy metals in PG. Raw phosphogypsum (RPG) and calcined phosphogypsum (CPG) were treated by biowashing, and the potential of treated PG used for cement paste preparation was investigated. The effects of biosolution on the stability of PG and the growth of bacteria in RPG and CPG were analyzed by zeta potential and optical density. A series of characterization tests were used to analyze the removal of impurity in PG after biowashing. Additionally, the removal ratio for the impurities, compressive strength, and leaching behavior of PG-incorporated cement paste (PGC) were discussed, and the mechanism was revealed according to the test results. The results showed that, compared with RPG, the growth of bacteria was better in CPG, and the phase of CPG after biowashing was more stable. Biomineralization could effectively remove 74-77% of phosphorus and fluorine impurities in PG, but the removal ratio in CPG was 45-55% higher than that in RPG. The PG after biowashing contained a large amount of calcium carbonate, which improved the early strength of the PGC by 15-20%. Compared with PGC, the leaching concentration of heavy metals in biowashed-PG incorporated cement (APGC) decreased by 50-90%. Biowahing removes the impurities mainly by turning the insoluble impurities into soluble H3PO4 and HF, which are filtered out with bacterial solutions. This study provides a new idea for the management of PG and expands the application field of microbially induced carbonate precipitation (MICP).
Phosphogypsum (PG) is a solid waste discharged from the industrial wet production of phosphoric acid. In order to improve the utilization rate of PG and understand the performance of PG mixed with calcium aluminate cement (CAC), it is necessary to determine the effect of different dosages of PG on the performance of CAC. therefore, this study investigated the hydration properties and compressive strength of PG and CAC mixtures. PG was added into CAC in an equal proportion, and the hydrated products were studied by XRD, TG-DTG, SEM and MIP methods. The research demonstrate that with the increase of PG content, the setting time and flowability of CAC pastes gradually decreased, and the compressive strength increased gradually. However, when the content of PG is greater than 15%, the growth rate of compressive strength of CAC becomes slower. The presence of PG resulted in the complete disappearance of the hydration products CAH10 and C2AH8 of CAC. With the increase of PG content, the pore size distribution of CAC paste was changed, and its main hydration products were transformed into ettringite, AFm and AH3, which became the main source of affecting the compressive strength of CAC.
High-silica phosphogypsum (PG) is a kind of industrial by-product with great utilization potential. However, it is difficult to reuse PG directly due to the related gangue minerals (e.g., SiO2), and thus efficient purification is required to allow its further applications. Herein, a typical high-silica phosphogypsum waste was purified by a new “reverse-direct flotation” method. The organic matters and fine slimes were removed by reverse flotation, and then, the silica impurity was removed by direct flotation. Via the closed-circuit flotation process, the whiteness of the PG concentrate is improved from 33.23 to 63.42, and the purity of gypsum in the PG concentrate increases from 83.90% to 96.70%, with a gypsum recovery of 85%. Additionally, the content of SiO2 is significantly reduced from 11.11% to 0.07%. In-depth investigations suggest that the difference in the floatability of gypsum and quartz is prominently intensified by flotation reagents at pH = 2–2.5, and thus leads to good desilication performance. Further characteristics of the PG concentrate prove that impurities have been well removed, and the PG concentrate meets the requirement of related standards for gypsum building materials. The flotation method reported here paves the way for the purification of high-silica phosphogypsum, which can be extended to the purification and value-added reutilization of other industrial solid wastes.
In this study, the phosphogypsum-based cementitious materials were modified by adding raw lime, fly ash and cement into hemihydrate phosphogypsum. Then the modified phosphogypsum-based cementitious materials were foamed by different foaming methods. Finally, the synergistic strengthening effect of mechanical and thermal insulation properties of foamed phosphogypsum-based cementitious materials was studied. The results show that when the content of phosphogypsum is 60 %, the water-binder ratio is 0.300, the content of fly ash is 30 %, and the content of quicklime is 8 %, the mechanical and thermal insulation properties of phosphogypsum-based cementitious materials are the best. Although the porosity of physical foaming specimens is lower than that of chemical foaming specimens, the mechanical and thermal insulation properties of physical foaming specimens are better than those of chemical foaming specimens. The larger the pore diameter and the more connected the pore, the more serious the stress concentration of the specimen under compression, resulting in the decrease of the compressive strength. In addition, the porosity increases with the increase of foaming agent content, but the thermal conductivity does not decrease significantly with the increase of porosity. The reason is that with the increase of foaming agent content, the pore diameter also increases, which reduces the thermal resistance of the specimen and affects the further reduction of the thermal conductivity. This study can provide a theoretical basis for the practical engineering application of phosphogypsum as thermal insulation building materials.
Cement retarder preparation is one of the main avenues for the resource utilization of phosphogypsum, and appropriate modification technology is the key for ensuring that the phosphogypsum is adjusted for achieving the desired setting time. This study uses carbide slag, circulating fluidized bed (CFB) fly ash, and other solid wastes to comprehensively modify phosphogypsum required for preparing the cement retarder; in addition, it discusses the influence of material ratio and aging time on the modification effect of phosphogypsum. The effects of modified phosphogypsum, undisturbed phosphogypsum, and natural gypsum on the properties of Portland cement were comparatively studied. The modification mechanism of carbide slag and CFB fly ash on phosphogypsum and the mechanism of influence of the modified phosphogypsum on the hydration characteristics of Portland cement were revealed by XRD, SEM, and hydration heat analyses. The results show that adding 6% carbide slag and 4% CFB fly ash can effectively reduce the content of soluble phosphorus and soluble fluorine in phosphogypsum, increase the pH value, enhance the strength of modified phosphogypsum, and enable its use as a cement retarder after aging for 7 d. Compared with the undisturbed phosphogypsum, the setting time of Portland cement made from modified phosphogypsum is significantly shorter; the early-strength is higher, and the compatibility with water reducing agent is better. The microscopic test results showed that the CFB fly ash has good self-hardening properties. The CFB fly ash also showed better hydration hardening performance and curing effect on soluble phosphorus and soluble fluorine under the synergistic excitation of alkali-sulfur by carbide slag and phosphogypsum, which is beneficial for the modification of phosphogypsum and improves the retardation effect of Portland cement.
While traditional disposal of phosphogypsum either occupies excessive amounts of land or causes serious pollution, novel recycling of phosphogypsum is desirable. Making phosphogypsum-based cold-bonded aggregates (PCBAs) through granulation technology is a potential recycling strategy. In this study, the quality of the recycled PCBAs is assessed, considering physical properties, mechanical strength, impurity stabilization ability, and microstructure. With the increase of phosphogypsum content from 60% to 90%, 28-day bulk density of the PCBAs decreased from 1080 to 950 kg/m³, cylinder compressive strength in over dry (OD) condition decreased from 16.5 MPa to 3.9 MPa, and water absorption increased from 5.9% to 13.6%. SEM, XRD, and MIP analyses showed that as the phosphogypsum content increased, hydration products in the aggregates diminished and proportion of harmful pores (greater than200 nm) increased. Leaching test on 28-day PCBAs showed a reduced concentration of phosphorus and heavy metals in the leachate compared to the leaching of the original phosphogypsum. The study concluded that the PCBAs with the phosphogypsum content up to 80 % are qualified lightweight aggregates (LWAs) for concrete.
As a kind of solid waste, molybdenum tailings are used to prepare low density autoclaved aerated concrete (AAC) (dry density ≤450 kg m⁻³), which greatly improve their utilization rate and reduce stockpiling. Several studies showed that hydrophobic agents could further increase the performance of AAC by reducing the water absorption and improving the thermal insulation properties. The present study investigates the impact of different water-repellents such as powdered silane, zinc stearate, water dispersible zinc stearate, and osmotic crystalline on the compressive strength, thermal conductivity, and water absorption of AAC. The amount of water-repellent to be added to the AAC samples is calculated by the mass of total siliceous and calcareous materials and the following concentrations of water-repellents are mixed with AAC: 0.05%, 0.1%, 0.15%, 0.2%, 0.25% and 0.3%. Spectroscopic and microscopic analyses revealed the content and morphology of the hydration products of AAC, and the changes in the pore structure. The obtained results clearly suggest that all the tested hydrophobic agents can reduce the water absorption and improve the compressive strength and the thermal insulation properties of AAC samples. Interestingly, powdered silane has the best effect when compared to other water-repellents tested in this study. The improved performance of powdered silane is due to the formation of a hydrophobic film on the surface of the pore wall, which, in turn, increases the content of tobermorite crystals, and reduces the pore size of AAC samples.
Cool cement blocks are used for pavement construction to mitigate the heat island effect. Cement block pavements in contact with water will produce leachates, which may affect soil, surface water and groundwater quality. Therefore, the objective of this work was to determine and compare the leaching behavior of heavy metals (Cd, Co, Cr, Fe, Mn, Ni, Pb, Zn) from conventional and reflective cool cement blocks using the mass transfer tank leaching test, according to USEPA method 1315 and the batch leaching test as a function of pH, according to method CEN/TS 14429. Comparison of the results of the two tests indicates that equilibrium was not reached during the mass transfer test. In all cases, the Leachability Index values are higher than 8, indicating limited mobility. Observed diffusivity values differ by several orders of magnitude (10⁻¹¹ – 10⁻²⁰ m²/s) and are very small, indicating very slow diffusion. Leaching curves as a function of pH were very consistent and close to each other, indicating that the leaching mechanisms were the same for all cement blocks. Α two-domain one-dimensional mathematical model describing heavy metal diffusion from the pavements to the underlying soil was developed and tested. Simulations with this model showed that emissions from reflective cool cement pavements to the underlying soil are not different from the conventional ones in the long run.
CO2 is proposed as an oxidant in a two-step phosphogypsum decomposition process, to oxidize the intermediate product of CaS into CaO. Isothermal and non-isothermal tests are conducted at a thermogravimetric analyzer combined with a Fourier transform infrared spectroscopy to reveal the reaction mechanism of CaS oxidation with CO2. During the course of non-isothermal process, several side reactions have been observed at low temperatures with CaCO3 and CaSO4 formations. CaS cannot be fully converted into CaO owing to the liquid melt formed by the mixture of CaSO4 and CaS. Full conversion into CaO is observed in the isothermal tests. The reaction order model fits the description of CaS oxidation into CaO with CO2, which is validated by both isothermal and non-isothermal data. In comparison with pure CaS, the CaS prepared from PG has a relatively lower reaction temperature with CO2, indicating the presence of impurities may have a catalytic effect.
Phosphogypsum is a by-product from the phosphoric acid industry following the wet process. Known for its low cost, this process generates large amounts of phosphogypsum, the management of which has so far remained problematic due to the impurities it contains including heavy metals, radionuclides and acidic residuals. It is above all, its acidic nature that hinders its use as a road material since it increases its solubility which in turn makes it sensitive to water variations. This paper presents an investigation on the stabilization of Moroccan phosphogypsum, through neutralizing acidity, while considering both techno-economic and environmental constraints. To this end, mixtures of phosphogypsum and fly ash were prepared then activated with varying quicklime additions (beyond Initial Consumption of Lime) to develop different combinations of phosphogypsum (40–80%), fly ash (20–60%) and lime (4–20%). These have been tested for their mechanical and mineralogical properties at different curing periods (7, 28, 90, 180 days) besides durability against Wetting/Drying cycles, in order to evaluate the stabilizing effect of fly ash and lime. Results from Unconfined Compressive Strength (UCS) tests showed a strength increase with fly ash and lime addition with increasing the curing time. The UCS at 180 days varied between 1 and 3 MPa for formulations containing 4% lime (ICL) while those with 12–20% lime reached 4–7 MPa (12 times higher than UCS at 7days). As for durability, these showed weight losses of up to 10% upon W/D cycles, a strength loss of up to 56% and high porosity compared to formulations with higher lime content which rather gained up to 264% in strength. The influence of curing time and mixture composition on the mineralogy of the prepared composites was assessed through XRD and TGA analyzes highlighting the formation of ettringite as main hydration product responsible for strength development.
Semi-dry flue-gas desulfurization (FGD) processes abate 99% of atmospheric emissions of sulfur dioxide from coal-fired power plants at the expense of producing daily tones of solid FGD residues containing sulfites, sulfates, carbonates and hydroxides of calcium and magnesium, besides fly-ashes. In this work, a fluidized-bed reactor pilot plant was used for experiments of dry-oxidation of FGD residues aiming at converting sulfites into sulfates in order to upgrade such residues for utilization as raw material to the cement industry. A two-dimensional design of experiments on the plane of feed air temperature and reactor time-on-stream was conducted in the pilot plant generating sulfite conversion data and transient reactor temperature profiles. These data were used for estimating the first-order kinetic parameters of sulfite conversion via non-linear regression following the Maximum Likelihood Principle. The optimized Arrhenius factor and Arrhenius activation energy obtained via the Nelder–Mead Flexible Simplex method were, respectively, 0.001 mol/kg.s.bar and 14146.5 J/mol. This kinetic model allows designing large-scale plants for treatment of semi-dry FGD residues in order to beneficiate it for utilization in the cement industry, avoiding the disposal and environmental costs of landfilling such residues.
A potential industrial waste-waste co-treatment process was proposed and verified for the recovery of the valuable metals Co, Ni, and Cu from copper smelting slag by utilizing high temperature SO2 off-gas. Sulfation roasting followed by water leaching under designed thermodynamic conditions was conducted to facilitate the selective formation of Co, Ni, and Cu sulfates while separating iron as oxide. Several parameters were studied such as roasting temperature, roasting time, the addition of Na2SO4, and leaching agent. Under the optimized sulfation roasting conditions (Gas flow: 500 mL/min, 5% SO2 + 20% O2 + 75% Ar; Roasting temperature: 650 °C; Roasting time: 4 h; Addition of Na2SO4: 30%) followed by water leaching (Leaching temperature: 80 °C; Leaching time: 5 h; solid to liquid ratio: 0.05 g/ml), the extraction yields of Ni, Co, and Cu were shown to reach 95.8% and 91.8%, 81.6%, respectively. Furthermore, the sulfation roasting – water leaching process was confirmed on lab-scale as a feasible and efficient way to recover valuable metals and the mechanism was determined and verified from the microstructural evolution. Finally, a potential environmentally friendly industrial process in terms of the energy flow and material flow was presented based on preliminary assessments for environmental benefits, economic benefits, and heat recovery.
The production of by-product phosphogypsum is very problematic. In order to recycle the phosphogypsum into a building material, its harmful acidic impurities (especially the soluble phosphate) should be purified, and the mechanical properties improved. Therefore, in this study, a novel method of processing the phosphogypsum is proposed. It consists in the addition of waste zeolite to adsorb the phosphate impurities, and in the application of press-forming processing to improve the compressive strength of the hardened specimens. The zeolite additive has proven to be an efficacious adsorbent, which reduced the soluble phosphate content by 3.2-7.5 times and surprisingly improved the hydration degree of the phosphogypsum. Moreover, the press-forming of phospho-gypsum by applying 15 and 20 MPa pressure produced specimens with a compressive strength over 50 MPa.
In this research, the effect of curing temperature on the Arsenic (As(III)) leachability of cemented paste backfill (CPB) that contains blast furnace slag (OPC/Slag-CPB) are studied. ASTM C 1308 leaching protocol is adopted to assess the leachability of OPC/Slag-CPB specimens cured at various temperatures (2 °C, 20 °C and 35 °C). Furthermore, various microstructural techniques are used to relate the temperature-induced changes in the microstructural properties of the OPC/Slag-CPBs to their leaching characteristics. The results show that leaching of arsenic from OPC/Slag-CPB is temperature dependent. Higher curing temperature leads to a decrease in the amount of arsenic released from the OPC/Slag-CPB. The As leachability-decreasing factors are: (i) faster cement hydration rate and pozzolanic reaction with higher curing temperatures, which increase the volume of As immobilizing phases (C-S-H, CH, ettringite) in the OPC/Slag-CPB; (ii) lower volume of connective pores (smaller connective pore surface area) with higher curing temperatures, which decreases the area of reaction sites, thereby reducing the mobility of arsenic. Leaching of As from OPC/Slag-CPBs is dominated by diffusion, irrespective of the curing temperature. Results also reveal that CPBs with ordinary Portland cement (OPC) leach significantly less than CPBs with OPC and Slag (50/50) for all curing temperatures. However, curing temperature has the opposite effect on the OPC/Slag-CPBs compared to OPC-CPBs. OPC is suggested as a binder in situations where the principal design parameter is immobilizing As(III) in CPB subjected to various thermal curing conditions. The results indicate that curing temperature is a critical variable to consider when assessing the ability of CPB structures to release arsenic into the underground environment.
This study has investigated the impact of foaming and expansion of municipal solid waste incinerator (MSWI) bottom ash (BA) during alkali reaction. Furthermore, the activity enhancement of the BA has been investigated. Based on the microanalytic appearance and compressive strength of the samples, the results show that the reaction between the metallic aluminium in the BA and alkali in the activator, which produced large quantities of hydrogen, is the primary cause for both foaming and expansion. The residual organic matter and unburnt substances in the BA are the main factors that affect the activity. However, the alkali defoaming and calcination-melting pretreatments completely prevented the expansion and calcining further improved the BA activity. The alkali activated materials prepared with the BA calcined at 700 °C, granulated blast furnace slag, and slaked lime followed by alkali defoaming pretreatment produced the best mechanical properties.
Solid waste phosphogypsum (PG) generated during phosphoric acid production was used as the sole raw material to prepare a new paper-free and fiber-free plasterboard. Dehydrated PG was granulated with water, press-formed and hydrated with intermittent pressing into paper-free and fiber-free plasterboards with bending strength of 14.7 MPa. The semi-hydrate gypsum is hydrated into dense and interlocking dihydrate gypsum crystals, thereby achieving a high mechanical strength. Due to the free of paper, fiber and additive, the plasterboard is effective to recycle PG and has potential to be a new-type wall material with favorable fire resistance, cost-effectiveness and environmental friendliness.