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Introduction. The use of building materials with photocatalytically active additives is considered as a promising solution to environmental and economic problems of the urban environment. In the field of building materials science the necessity of studying the microstructure of self-cleaning building materials and the influence of impurities on the efficiency of self-cleaning of materials with photocatalytically active additives is determined.Materials and methods. Red gypsum (production waste with photocatalytically active impurities), cement, building gypsum, microsilica, synthesized photocatalytically active titanium oxide – silicon oxide additive, and iron-based pigments were used. Tablet specimens of gypsum-cement-pozzolanic binder with various photocatalytically active components were made: photocatalyst additive, pigments, photocatalytically active oxide impurities. The microstructure of the specimens and the distribution of titanium and iron elements were studied using scanning electron microscopy. The efficiency of self-cleaning was determined by the change in the contact angle of a water drop on a surface coated with oleic acid.Results. The efficiency of self-cleaning of specimens with added and impurity photocatalytically active components was determined. The influence of added impurities on the structure of the material and the influence of the type and concentration of impurities on the efficiency of self-cleaning were revealed.Conclusions. Titanium oxide photocatalyst additive at a concentration of 4.4 % provides high self-cleaning efficiency, evenly distributed throughout the material without affecting the structure of the forming binder stone. Pigment (iron (III) oxide) provides sufficient self-cleaning efficiency at a concentration of 2–9 %, at concentrations of more than 2 % it is distributed unevenly, providing a small increase in self-cleaning efficiency indicators. With the joint introduction of titanium and iron oxides, deterioration in self-purification is observed due to the high degree of recombination of electron – hole pairs. Red gypsum with impurity oxides of titanium and iron has shown a high efficiency of self-cleaning, has a uniform distribution of impurities that do not have a clear effect on the structure of the material.

Red gypsum (RG) is an industrial waste generated from the titanium dioxide process produced. For the low-cost and high-benefit utilization of RG, calcium sulfate (CaS) was prepared from recycled RG. Rod-like gypsum crystals with a rough surface, uniform size, 1 µm in diameter and 5 µm in length, were obtained, when the as-prepared CaS was divided into three equal portions and one portion was added at each interval of 2 h to neutralize the simulated acidic wastewater. CaS was prepared from RG to neutralize acidic wastewater to produce silicon-enriched RG for the next cycle. High silica residue (HSR) was obtained after 6 cycles. The effect of calcined temperature on the desulfurization of HSR was investigated. The sulfur content in the residue decreased by 97% after calcined at 1200 ℃ for 2 h. According to GB/T 12957-2005 "Industrial waste residue activity test method for cement admixture", the activity index of desulfurized HSR was 90%, which it can be used as a cementing material. [ABSTRACT FROM AUTHOR]

Red gypsum is an industrial waste generated by the titanium dioxide industry. Currently, it is mainly used for environmental restoration, but this is not ideal due to problems of geographical availability of suitable sites and environmental issues. Different valorization routes have been proposed, including the utilization of red gypsum in the cement industry or its chemical recycling through thermal processes. These routes either lack the production capacity for an efficient disposal of red gypsum, or propose processes that need severe operating conditions and that generate a significant amount of CO2 emissions. This paper proposes a novel process for electrochemical recycling of red gypsum based on four fundamental steps: metathesis, electrodialysis, electrolysis, and carbonatation. Preliminary estimates on a process design basis are obtained and analyzed from a phenomenological point of view, leading to the development of mass and energy balances for the process. The advantage of such system is that an industrial plant capable of treating 80000 t/y of CaSO4 on an anhydrous basis leads to the production of 55234 t/y of concentrated H2SO4 and 52571 t/y of synthetic CaCO3, coupled with the mineralization of 23131 t/y of CO2. The process is completely electrified and has a power consumption of 8.5 MW that can be provided entirely from renewables-based power plants, leading to net-negative CO2 emissions. Tuscany region in Italy is taken as a practical example, where the TiO2 production industry could benefit from the ELEDGE (ELEttroDialisi di GEssi) process and couple it with a geothermal-based power plant. [ABSTRACT FROM AUTHOR]

The acidic wastewater and red gypsum generated by the titanium dioxide (TiO 2) industry pose a great threat to humans and the environment. In order to achieve low-cost and high-benefit resource utilization of acidic wastewater and red gypsum, a harmless process for the self-treatment of acidic wastewater and red gypsum as well as for the recovery of valuable resources such as Ti, Fe, Ca, and S is proposed in this study. Acidic wastewater was innovatively used to purify red gypsum while reducing its acidity. The results showed that Fe, Ti, Al, Mn, and Mg impurities in red gypsum could be leached efficiently by the acidic wastewater, resulting in the calcium sulfate products, which can be used as a substitute for natural gypsums in industrial production. Subsequently, phosphoric acid was directly added to the Ti-rich filtrate and stirred at 85 ℃ for 45 min, so that the titanium was converted into titanium phosphate precipitation, with an optimal titanium recovery ratio of 94.86%. After recovering Ti, the filtrate was used again to purify red gypsum. Finally, the pH of the low acidity solution was treated by oxidation and precipitation, and filtered to obtain the Fe-rich filter residue and ammonium sulfate solution. The Fe-rich filter residue could be used in the metallurgical industry, and the ammonium sulfate solution was evaporated and crystallized to obtain the (NH 4) 2 SO 4 product. The harmless self-treatment process not only facilitated the resource utilisation of acidic wastewater and red gypsum, but also solved the pollution problems caused by wastewater and solid waste in the TiO 2 industry. [ABSTRACT FROM AUTHOR]

Red gypsum comes from TiO2 production industry by sulfate method, which comes from the low acidic liquid stream treatment using lime or limestone. At present, RG main is treated by outdoor stacking, which induces additional costs and raises environmental concerns. It will be of interest to see the transformations of RG to obtain elemental sulfur and calcium cyclic utilization in the acidic liquid stream treatment. Calcium sulfide was prepared by calcining RG at 900 °C for 10 min and pure gypsum at 900 °C for 20 min, respectively. The decomposition of RG was easy than pure gypsum because of Fe2O3 catalysis. And srebrodolskite was observed in the samples from RG calcined at 800 to 1100 °C, respectively. Hydrogen sulfide was produced when CaS reacted with sulphuric acid wastewater and rod-like gypsum crystals were observed. And then, pellets of elemental sulfur, with 1.5–5 μm in diameter, were obtained by the H2S oxidation in Fe(III) chelate solution, which achieved elemental sulfur recovery and calcium cyclic utilization. [ABSTRACT FROM AUTHOR]

Concrete is the most commonly used building material. Even then, it has a drawback that it hasheavy density. Hence, researches are progressing everywhere to develop a new building material, which has low density, less water absorption as well as economical. And here lies the importance of foamed concrete. Foamed concrete is a type of lightweight concrete in which stable foam is used as the main ingredient. Stable foam is produced from standard foaming agent using a foam generator. The peculiarity of foamed concrete is that no coarse aggregate is used in it. An attempt is made in this research to use palm oil fuel ash as a partial replacement for cement and red gypsum as a partial replacement for manufactured sand. Both of these materials are waste materials and are abundantly available. The concrete made up of this will be more sustainable as well as economical. By adjusting the addition of stable foam, foamed concrete of various densities can be developed. Foamed concrete blocks made up of these materials can be used for load bearing as well as non-load bearing masonry works. It has been observed that the addition of Palm Oil Fuel Ash and Red Earth has improved the properties of foamed concrete.

The increasing production of wastes that are landfilled might contribute to sources of potentially toxic elements; this is the case of residual red gypsum tailings, a by-product of titanium dioxide extraction. Revegetation of such a site is essential, and Mn phytoextraction may render the operations economically profitable. This study aimed to apply phytomanagement techniques for increasing the plant development, tailings revegetation and an optimal Mn phytoextraction using silver birch, the most abundant plant species on this site. To enhance the nutrient availability from the tailings, amendments that reduce the pH, i.e., pine bark chips, Miscanthus straw, white peat, and ericaceous compost, were mixed with residual red gypsum and birches were allowed to grow for 3 months. The pine bark chips and ericaceous compost led to a maximum decrease in pH, allowing the accumulation of up to 1400 mg Mn kg–1 dry matter in the leaves silver birch leaves. However, some nutrient competition was found in the pine bark treatment, which halved biomass production as compared to control. Further amendment addition may be needed to take advantage of the pine bark capabilities as a soil conditioner and Mn solubilizing treatment in residual red gypsum.

[Display omitted] • Red gypsum as a catalyst for producing glycerol carbonate from crude glycerol. • Impact of the surface Lewis acidity and basicity of the active sites. • Red gypsum catalysts are resistant to the impurities that exist in crude glycerol. In this study, a heterogeneous catalyst was developed from red gypsum for the synthesis of glycerol carbonate through a glycerolysis reaction with urea. Under optimum reaction conditions, a simple heat treatment on bare red gypsum in a static air environment produces an 87% yield of the targeted glycerol carbonate product. The higher catalytic activity of the calcined catalyst is due to its basic characteristics and the presence of different phases of anhydrite CaSO 4 with additional elements, especially an active hematite (Fe 2 O 3) phase. The catalytic process was also found to be insensitive to the type of flowing gas that was used to remove the evolved ammonia gas, whereby, either inert nitrogen gas or reactive air containing oxygen did not significantly impact the glycerol carbonate yield. Indeed, with a promising yield, the developed catalytic system was able to directly synthesise glycerol carbonate from industrial crude glycerol from a biodiesel plant. The red gypsum-based catalytic system seems resistant to the presence of a certain level of moisture and other impurities in crude glycerol. The catalyst efficiency is being retained for consecutive reaction cycles, which are subsequently reused for the next batch reaction without requiring any pre-treatment. [ABSTRACT FROM AUTHOR]

• Red gypsum is an industrial waste rich in calcium favored for CO 2 sequestration. • Mono-ethanolamine absorbed more CO 2 in the aqueous solution rich in calcium ions. • Proposed accelerated carbonation consumed lower energy while forming calcite. This study provides a novel approach to sequester carbon dioxide (CO 2) using industrial waste red gypsum (RG) in an accelerated mineral carbonation process. In this study, RG samples and products were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), and transmission electron microscope (TEM). The XRD analysis of RG revealed that two main constitutions of RG are gypsum (CaSO 4.2H 2 O) and hematite (Fe 2 O 3). From XRF analysis, RG samples consisted of calcium oxide (CaO), sulfur trioxide (SO 3), and ferric oxide (Fe 2 O 3) as major components and titanium dioxide (TiO 2), manganese oxide (MnO), and europium oxide (Eu 2 O 3) as minor components. The accelerated mineral carbonation of RG was first performed using different concentrations of sulfuric acid (H 2 SO 4) from 0.5 M to 2 M to extract the calcium ions from the RG. Then, the mineral carbonation process was performed within an Autoclave mini reactor by preparing an aqueous solution containing RG, CO 2 , and 5 wt% 0.1 M to 1.4 M mono-ethanolamine (MEA). The results showed that using 1.4 M MEA could absorb the highest amount of CO 2 to precipitate the calcite. The significant advantages of using MEA were related to the further absorption of CO 2 than 2 M H 2 SO 4 in the slurry and the lower consumption of energy to precipitate the calcite. The findings presented in this study shed new light on precipitating carbonate minerals like calcite from industrial wastes rich in calcium as well as ones rich in iron, barium, and magnesium. [ABSTRACT FROM AUTHOR]

The massive accumulation of red gypsum has brought great harm to human and environment. In order to achieve low-cost and high-benefit resource utilization of red gypsum, a method of metal self-enrichment during the cycle and transformation of red gypsum was proposed. The carbon dioxide and ammonia water react with red gypsum to manufacture ammonium sulfate and solid phase products. And solid phase products neutralize pickling wastewater to produce metal-enriched red gypsum for the next cycle. After the cycle, solid phase product with one circulation (containing 39.45% Fe and 7.37% Ca) and two circulation (containing 45.79% Fe, 4.75% Ca) could be obtained, which can be used as the proportioning of metallurgical sinter. The self-enrichment process not only realizes the comprehensive utilization of red gypsum, but also solves the treatment of acidic wastewater. Unlabelled Image • The valuable metals in red gypsum can be enriched for utilization by self-enrichment process. • The comprehensive utilization of red gypsum and acidic wastewater can be realized. • The theoretical mechanism of cycle and transformation was researched. [ABSTRACT FROM AUTHOR]

• The present study aims to establish a flora inventory of a red gypsum technosol. • The site was characterized by a remarkably low availability of some nutrients. • A spontaneous tree succession has been established, dominated by Betula pendula. • This study aided the implementation of a revegetation strategy at the landfill. Red gypsum is the product of the neutralization of titanium dioxide (TiO 2) extraction residue from ilmenite and anatase. The disposal of red gypsum creates heterogeneous plots with layers that may include Fe, Ca, Al, Mg, Mn, S, and other elements and an alkaline pH that makes revegetation complicated and slow. The vertical and horizontal dispersion of the sediment particles are the main concern. Therefore, the establishment of precise vegetation cover is needed to address this issue. One of the aims of this study was (1) to explore the distribution of the spontaneous vegetation found along a red gypsum-formed landfill located at the Ochsenfeld site in eastern France. Additionally, (2) some pedological parameters were also studied to determine the most significant red gypsum chemical drivers influencing the occurrence and abundance of the vegetation within the site. The Braun-Blanquet scale was used to rate the species presence in the field contained in the spontaneous vegetation dataset. The vegetation survey revealed the presence of 59 species from 23 families. The most abundant species was Betula pendula, and a further cluster analysis enabled the differentiation of areas with this species. The CaCl 2 extractable concentrations of the nutrients and trace elements, as well as the pH and the organic matter (OM) present in the sampled substrate, were used to form the pedological parameters dataset. A multiple factor analysis (MFA) was performed to link the large datasets together and revealed 3 groups of plants. Group 1 was composed of pH-tolerant species such as B. pendula and S. caprea. Group 2 was formed by Cr-Zn-tolerant species, including Echium vulgare and R. pseudoacacia. Finally, group 3 was characterized by species such as Clematis vitalba and Artemisia vulgaris that tolerate the presence of Na. The MFA revealed a correlation between the Betula pendula distribution and the pH, CaCl 2 extractable P concentration and organic matter. On the other hand, A lack of relation of the CaCl 2 extractable concentrations of Fe, Mn, Na, Si and K were also found. This study aided in the selection of an adapted candidate for the implementation of the revegetation strategy of a red gypsum landfill in eastern France. Further tests were performed at the site using white birch for Mn accumulation and topsoil stabilization in situ. [ABSTRACT FROM AUTHOR]

Nowadays, the stability and the safe cover and closure of soft tailings has become one of the most challenging topics in geotechnical and environmental engineering. For appropriate analysis and modelling, geotechnical properties of tailings should be well defined. Due to low strength, viscous nature and poor trafficability, as well as due to the specific geometrical properties of fine particles and pore water chemistry in tailings, different test methods and their modification and improvements must be used for valid and reliable characterisation of tailings. This paper analyses and compares the results of different field test methods, piezocone penetration test (CPTu), flat dilatometer test (DMT) and field vane test (FVT), performed on the red gypsum sludge and complementary laboratory tests. CPTu and DMT measurements were evaluated using procedures developed for soils and the obtained mechanical properties were compared with those measured in the laboratory. The significant distinctions between different test methods and different empirical correlations were analysed taking into account differences in index properties of soils and the red gypsum sludge. Based on analysis, some modifications of conventional empirical correlations were proposed for field tests results interpretation and sludge characterisation. The obtained data also show the advantages and limitations of some conventional methods and prove the need for further development in that field of geotechnical investigation. [ABSTRACT FROM AUTHOR]

Highlights • Red gypsum is an iron and calcium rich source favored for mineral carbonation. • The morphology of red gypsum samples was changed during the carbonation process. • Mineral carbonation of red gypsum shows a distinct separation of reaction progress. Abstract In this research, the characterization of red gypsum (RG) and its reaction progress during the indirect mineral carbonation process were studied. X-ray diffraction results showed that the calcium sulfate and hematite are the dominant components in the RG samples. Three main phases of calcium in the RG were recognized: Ca (OH) 2 , Ca-S-O, and Ca-Fe-O. Element analysis by ICP-OES also revealed that the RG consists of three major constitutions: CaO (32.2%), SO 3 (31.6%), and Fe 2 O 3 (28.99%). To understand the reaction progress of RG in the carbonation process, the reaction model code PHREEQC-2.18 was found to be a very useful tool since it allows the exposure of all dissolved species of RG and presents its mineral phases at each time step. Furthermore, the mineral carbonation of RG delivered the reaction progress changes with Ca phases and, therefore, dissolved calcium (hydr)oxide starts to control the pH and the CO 2 uptake. In short, the findings of this study could be useful in resolving problem of greenhouse gases with less environmental issues. [ABSTRACT FROM AUTHOR]

The recycling of Ti-extracted residues (TRs) and red gypsum (RG), which are the industrial wastes from titanium dioxide manufacturing industry, will avoid the increasing impact on the environment due to their difficulty for disposal. In this paper, the ternary blended binder mainly consisting of ordinary Portland cement (OPC), TRs and RG for the preparation of foamed concrete (FC) was formulated in the laboratory. The hydration kinetics of blended cement, mechanical characteristics and microstructure of FC were investigated. The results indicated that quick lime and sulfoaluminate cement (SAC) evidently reduced the setting time of the complex binder, but mechanical activation for TRs could not substantially shorten those of blended binder. In the case of FC, the optimum technological parameters were proposed as follows: OPC: TRs: RG = 10:45:45 (by mass, the same below), quick lime 2%, SAC 4%, Na 2 SO 4 0.4%, water reducer 0.2%, foam 4.6% and W/B 0.60. The maximum compressive strength and specific strength of FC with density of 437 kg/m 3 reached up to 2.14 MPa and 4.91 kN·m/kg, respectively. Microstructure analysis revealed that the interconnected assemblages in FC are floccular and petal-like hydrated gels, need-like ettringite and unreactive plate-like gypsum. [ABSTRACT FROM AUTHOR]

In this research, the feasibility of CO 2 mineral carbonation by the use of by-product red gypsum (RG obtained from Huntsman Tioxide, Terengganu, Malaysia) to form iron carbonate (FeCO 3 ) was evaluated. Toward this aim, the wide-range conditions of key procedure variables such as reaction temperature, reaction time, and CO 2 pressure on the rate of mineral carbonation were studied. In addition, preliminary analyses on red gypsum were carried out to determine its physical and chemical characteristics. The feasibility of direct carbonation of RG to form FeCO 3 was assessed at different CO 2 pressures of 1 to 70 bar and different reaction temperatures of 25 to 200 °C to discover the effects of the key functions on overall direct carbonation. In general, increasing CO 2 pressure to 10 bar was found to increase the overall FeCO 3 purity and carbonation efficiency to 97% and 98%, respectively. Moreover, the reaction temperature affected the conversion rate at two different functions. First, FeCO 3 purity and its carbonation efficiency increased to the maximum values when the reaction temperature increased to 150 °C, and then they were slightly decreased to 91% and 92%, respectively, with increase of temperature to 200 °C. The results also showed that the maximum product purity is achieved at optimum reaction time of 150 min. [ABSTRACT FROM AUTHOR]

The objective of this study was to investigate the feasibility of indirect carbonation of red gypsum through pH swing process. The effect of CO 2 pressure as one of the major controlling parameters in carbonation process was investigated in details. The dissolution of red gypsum was performed using 2 M H 2 SO 4 at 70 °C for 60 min reaction time. The pH was regulated using NH 4 OH, which also precipitated out the impurities from the solution. Finally, the calcium rich solution was poured in an autoclave reactor and CO 2 was injected into the reactor for carbonate precipitation. The experimental results showed that the Fe, the main impurity can be effectively separated prior to carbonation stage (92–95%). At low pH the Fe was separated while at high pH Ca was precipitated as carbonate. The carbonation efficiency was directly affected by the CO 2 pressure and the maximum efficiency (100%) was achieved when 8 bar CO 2 pressure was used. In addition, carbonation experiments resulted in CaCO 3 production in the form of calcite, aragonite, and vaterite with 98% purity. [ABSTRACT FROM AUTHOR]

In this paper, the use of red gypsum (RG), waste from the naturally occurring radioactive materials industry that is devoted to the production of the TiO 2 pigment, was evaluated as a source of calcium for CO 2 sequestration by an indirect carbonation process. The main objective was to valorise this waste and, at the same time, analyse the reduction of greenhouse gas emissions (CO 2 ) emitted by industrial sources that use this process. In order to induce the carbonation process, the extraction of calcium from the sample was required beforehand. For this, two different extraction routes were applied (the NaOH and NH 4 OH pathways). The obtained results demonstrate that RG has high carbonation reactivity, depending on the extraction agent used at room temperature and pressure. The conversion of RG to calcium carbonate was 92% when using sodium hydroxide, whereas 64% was obtained with ammonium hydroxide extracting. The behaviour and fluxes of the radionuclides and trace elements, initially contained in the RG, were also evaluated during the full carbonation process. In general, the levels of pollutants in the final calcite (calcium carbonate) were comparable to the ones found for typical unperturbed soils. [ABSTRACT FROM AUTHOR]

Red gypsum can be considered as a long term storage of carbon dioxide through mineral carbonation. However, for the efficient carbonation, the calcium ions need to be extracted in the solution phase, followed by the carbonation reaction. Therefore, the extraction step is the key of successful mineral carbonation. In this study, solvent extraction of calcium and iron ions has been carried out using different types of acids and bases. In addition, the kinetic study of the extraction has also been performed. The study showed that the base solution was not capable of extracting significant amounts of calcium and iron, while acid solution was very efficient in extracting the ions. However, H 2 SO 4 resulted in higher calcium extraction efficiency as compared to HCl and HNO 3 . Increasing reaction temperature from 30 to 70 °C and also increasing reaction time from 5 to 120 min were found to be effective in enhancing the degree of extraction for all the three acids used. Kinetic analyses found that the dissolution rate of red gypsum is controlled by the combination of product layer diffusion and chemical reaction control. The calculated activation energy of calcium extraction was 44.02, 37.68, and 42.73 kJ/mol for H 2 SO 4 , HCl and HNO 3 , respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • The reduction of gas foaming caused by the weak acidity of RG was resolved by Na 2 CO 3 addition. • The addition of RG promoted the synergistic enhancement of the anhydrite and tobermorite system. • RG can be used as raw materials in the production of AAC. In this study, the utilization feasibility of red gypsum (RG) as a substitute for fly ash to prepare the autoclaved aerated concrete (AAC) was examined. Influences of RG replacement ratio on the gas foaming, mineral components, morphology and mechanical properties were investigated. In addition, analyses of microstructure evolutions, phase compositions and thermal changes were carried out to explore the mechanism of strength improvement for AAC blocks. Results show that the replacement of fly ash by RG reduced the gas-foaming rate, which was resolved by Na 2 CO 3 addition and it's optimum dosage is 0.75%. The main phases in AAC are C-S-H gel, katoite, tobermorite and anhydrite. A proper amount of RG replacement (5%–20%) promoted the increase of relative strength of AAC samples. The synergistic enhancement of the anhydrite system and tobermorite system ensured an increase in relative strength. The optimum RG replacement ratio in AAC is 20%. With this substitution ratio, density can be obtained around 617 kg/m3 and compressive strength can reach about 4.2 MPa, which satisfies the requirements of B06, A3.5 grade in Chinese national standard GB 11968-2019. [ABSTRACT FROM AUTHOR]

• Sewage sludge and red gypsum mixture was investigated on the k and compressive strength (UCS). • Various degree of compaction was tested for 5 months, and the leached was measured. • Compaction influences the initial k gradually stabilized at 10−6 cm/s after 75 days. • The leached metals and COD were within standard when used as intermediate landfill cover. • The recommended compaction is >80% to achieve 345 kPa compressive strength. This paper investigates the response of sewage sludge and red gypsum mixture under different compaction degree conditions in terms of hydraulic conductivity and unconfined compressive strength (UCS). The UCS and hydraulic conductivity assessments were undertaken using UCS machine and column test using the constant head method, respectively. Five samples were compacted to various degree of compaction (60, 70, 75, 80 and 85%) and percolated with distilled water for six months. The leachate samples from the column test were collected and analyzed at pre-determined days of 1, 3, 7, 14, 28, 42, 56, 75, 90, 120, 150 and 180. The UCS of samples increased with increasing degree of compaction, attributed to the increased number of contact and interparticle interaction between the particles. The degree of compaction appears to affect the initial hydraulic conductivity for the first three days of the experiment, however, gradually stabilize at 10-6 cm/s after 75 days for all samples. The concentration of leached metals and chemical oxygen demand (COD) is below the regulated limit that could affect the environment. The recommended degree of compaction is beyond 80% to achieve the required compressive strength of 345 kPa. [ABSTRACT FROM AUTHOR]

This study was focused on the historical evolution of the waste management policy carried out by the Spanish industry devoted to titanium dioxide pigments manufacturing for minimising its environmental impact. This challenge was achieved by modifying the original chemical process and converting the originally dissolved sulphate and sulphuric acid present in the final streams of the factory into sulphate minerals (melanterite FeSO4·7H2O, szomolnokite FeSO4·H2O, and gypsum CaSO4·2H2O). These by-products were physicochemically, mineralogically and radiologically characterised in order to gain basic information for its subsequent commercial use. Some of the uses summarised in this study for both ferrous sulphates are as a supplier of iron to prevent chlorosis, animal food, manufacture of cement (to reduce Cr VI), primary flocculants for ferrous sulphates, magnetite nanoparticle and nano-Fe2O3 formation, production of magnetite concentrate, remediation of polluted soils with metals, and treatment of wastewaters. Red gypsum was analysed as a substitute for natural gypsum in the manufacture of cement, construction materials, inhibitor in soil erosion, and the immobilisation of heavy metals in agricultural soils and carbonation processes. [ABSTRACT FROM AUTHOR]

One-step process of red gypsum leaching to produce anhydrite was proposed. The effects of sulfuric acid concentration, reaction temperature and reaction time on both the leaching rate of red gypsum, the phase and micro-morphology of the product were systematically investigated. The acid leaching solution was tried to use circularly. And the valuable components in the leaching solution were recovered by step precipitation. Anhydrous calcium sulfate powder with anhydrite crystal phase and smooth and serrate morphology was successfully produced. The leaching rate of red gypsum was 39.82% under optimized conditions. The optimum cycle number for the reuse of acid leaching solution was 5. Sulfate ions and iron ions in the leaching solution were recovered by adjusting the pH value. A preparation mechanism of anhydrite from red gypsum was also proposed by theoretical analysis. All of these indicated that the whole procedure setup showed potential application for red gypsum utilization. Image 1 • One-step process of red gypsum leaching to produce anhydrite was proposed. • Anhydrite II with smooth and serrate morphology was successfully produced. • The valuable components in the solution were recovered by step precipitation. • The optimum cycle number for the reuse of acid leaching solution was 5. [ABSTRACT FROM AUTHOR]

This study proposed the recycling of sewage sludge (SS) and red gypsum (RG) as potential temporary landfill cover materials. Mixtures with different SS and RG compositions were prepared and tested in determining the most suitable design mix based on the resulting physical, mechanical, and geotechnical properties, namely the hydraulic conductivity, compressive strength, and plasticity. A maximum compressive strength of 524 kPa was achieved for the optimum SS:RG composition of 1:1, corresponding to Ca:Si composition of 2.5:1, which was appropriate to form the calcium silicate hydrate (CSH) gel. The SS and RG compositions did not affect the hydraulic conductivity, which was in the order 10−5 cm/s for all mixtures. Mixtures with RG greater than SS in composition exhibited plastic behaviour due to the Fe content in the RG, which helped minimize the risk of cracking. The optimum mixture had compressive strength greater than the specified minimum of 345 kPa, medium hydraulic conductivity, and moderate plasticity, thus appropriate for application as an alternative material for the temporary landfill cover in the tropics. • A mixture of sewage sludge (SS), and red gypsum (RG) as temporary landfill cover was investigated. • Optimum mix at SS:RG ratio of 1:1 achieved 345 kPa and 10−5 cm/s. • SS:RG ratio of 1:1 corresponded close to Ca:Si ratio of 2.5:1for hydration to form CSH gel. • Fe in RG contributed to the plastic behaviour of the mix. • SS:RG ratio of 1:1 is appropriate for application as a temporary landfill cover material. [ABSTRACT FROM AUTHOR]