Search Results (15)

This study aimed to improve the mechanical properties and water resistance of calcined gypsum from phosphogypsum (CGP) by incorporating organic additives and inorganic admixtures. The effects of the dosage of these additives—including kaolin, nano-SiO₂, polycarboxylic acid superplasticizer, and sodium methyl silicate—on the properties (flexural strength, compressive strength, water absorption, and softening coefficient) of CGP composites (CGPCs) were investigated. A high water resistance of the CGPCs was achieved using nano-SiO₂ and sodium methyl silicate modification, superplasticizer addition, and the partial replacement of gypsum with mineral admixtures. The results showed that the flexural and compressive strength of the composites hit 4.61 MPa and 19.54 MPa, respectively, while the softening coefficient was 0.70 and the water absorption rate was 19.85%. Microstructural investigation confirmed that the combination of nano-SiO₂ and kaolin led to the formation of calcium silicate hydrate. Additionally, the superplasticizer played a crucial role in reducing the water-to-cement ratio, while unhydrated mineral particles had a filling effect, thereby enhancing the density of the hardened paste. The sodium methyl silicate formed a hydrophobic film on the surface of the hardened paste, increasing the contact angle to 109.01° and improving the water resistance of the CGPCs. Full article

To address the high carbon emissions and resource dependency associated with conventional ordinary Portland cement (OPC) production, this study systematically investigated the preparation processes, hydration mechanisms, and chemical properties of high-belite calcium sulfoaluminate (HBCSA) and calcium sulfoaluminate (CSA) cements based from industrial solid wastes. The results demonstrate that substituting natural raw materials (e.g., limestone and gypsum) with industrial solid wastes—including fly ash, phosphogypsum, steel slag, and red mud—not only reduces raw material costs but also mitigates land occupation and pollution caused by waste accumulation. Under optimized calcination regimes, clinkers containing key mineral phases (C₄A₃S⁻ and C₂S) were successfully synthesized. Hydration products, such as ettringite (AFt), aluminum hydroxide (AH₃), and C-S-H gel, were identified, where AFt crystals form a three-dimensional framework through disordered growth, whereas AH₃ and C-S-H fill the matrix to create a dense interfacial transition zone (ITZ), thereby increasing the mechanical strength. The incorporation of steel slag and granulated blast furnace slag was found to increase the setting time, with low reactivity contributing to reduced strength development in the hardened paste. In contrast, Solid-waste gypsum did not significantly differ from natural gypsum in stabilizing ettringite (AFt). Furthermore, this study clarified key roles of components in HBCSA/CSA systems; Fe₂O₃ serves as a flux but substitutes some Al₂O₃, reducing C₄A₃S⁻ content. CaSO₄ retards hydration while stabilizing strength via sustained AFt formation. CaCO₃ provides nucleation sites and CaO but risks AFt expansion, degrading strength. These insights enable optimized clinker designs balancing reactivity, stability, and strength. Full article

Red mud (RM), a highly alkaline waste from alumina production, poses severe environmental threats due to massive stockpiling (>350 million tons in China) and groundwater contamination. This study evaluates three scalable strategies to repurpose RM: iron recovery via magnetic separation, sintered brick production using RM–fly ash–granulated blast furnace slag (6:1:3 ratio), and non-calcined cementitious binders combining RM and phosphogypsum (PG). Industrial-scale iron extraction achieved 23.85% recovery of iron concentrate (58% Fe₂O₃ grade) and consumed 3.6 million tons/year of RM, generating CNY 31 million annual profit. Sintered bricks exhibited 10–15 MPa compressive strength, meeting ASTM C62-23 standard while reducing material costs by 30%. The RM–PG binder achieved 40 MPa compressive strength at 28 days without cement or calcination, leveraging RM’s alkalinity (21.95% Na₂O) and PG’s sulfate activation. Collectively, these approaches reduced landfill reliance by 50% and CO₂ emissions by 35%–40% compared to conventional practices. The results demonstrate RM’s potential as a secondary resource, offering economically viable and environmentally sustainable pathways for the alumina industry. Full article

Supersulfated cements (SSCs) are one of the promising binders characterized by low CO₂ emissions. A significant advantage of SSC is the possibility of using phosphoanhydrite binders as a sulfate component, obtained by the calcination of phosphogypsum—a waste product of acid and fertilizer production. The utilization of phosphogypsum is a global problem. Differences in the properties of phosphogypsums from various industrial enterprises are determined by the difference in phosphate rock and the technological mode of production. This gives reason to believe that phosphoanhydrite binders (FABs) will also have differences in properties, which in turn will influence the process of structural formation of SSC. In the article, the effect of FAB produced at calcination temperatures of 600, 800, and 1000 °C using phosphogypsum of two different industrial enterprises was studied. It is established that the morphology and pH value of FAB particles, and the ratio of components in the binder have the greatest influence on the physical and mechanical characteristics of the SSC. The use of FAB with a high pH value (≈12) allows for obtaining free-of-cement SSC, with compressive strengths of up to 50 MPa at the age of 90 days. Full article

Phosphogypsum (PG) is a phosphate fertiliser by-product. This by-product has a low level of utilisation. Calcium sulphate is dominated in PG similar to gypsum and, therefore, has good binding properties (similar to natural gypsum). However, the presence of water-soluble phosphates and fluorides, an unwanted acidic impurity in PG, makes PG unsuitable for the manufacture of gypsum-based products. In this study, the binding material of PG (β-CaSO₄·0.5H₂O) was produced from β-CaSO₄·2H₂O by calcination. To neutralise the acidic PG impurities, 0.5 wt% quicklime was added to the PG. In the construction sector, 3D-printing technology is developing rapidly as this technology has many advantages. The current study is focused on creating a 3D-printable PG mixture. The 3D-printing paste was made using sand as the fine aggregate and a binder based on PG. The results obtained show that, despite the low degree of densification, 3D printing improves the mechanical properties of this material compared to cast samples. The 3D-printed specimens tested in [u] direction reached the highest compressive strength of 950 kPa. The cast specimens showed a 17% lower compressive strength of 810 kPa. The 3D-printed specimens tested in the [v] and [w] directions reached a compressive strength of 550 kPa and 710 kPa, respectively. Full article

The acid co-production of cement is a prominent research focus for the large-scale, high-value utilization of phosphogypsum in the context of dual-carbon strategies. This paper builds on extensive research conducted by its authors on the co-production of sulphoaluminate cement clinker through acid production from gaseous-sulphur-reduced phosphogypsum. The solid mineral phase transformations occurring in the kiln during this process are systematically studied, and the effects of various calcination regimes (temperature, time, and atmosphere) on the evolution of clinker mineral phases are elucidated. This paper provides basic data support for the gas-sulfur-reduced phosphogypsum-acid cogeneration of sulfoaluminate cement clinker processes, and promotes the realization of the large-scale high-value utilization of phosphogypsum resources. The generation of the clinker mineral phase anhydrous calcium sulphoaluminate (C₄A₃S̅) begins at 1100 °C. Increasing the calcination temperature and extending the calcination time promote C₄A₃S̅ formation. However, when the calcination temperature exceeds 1350 °C, C₄A₃S̅ decomposes, leading to the formation of low-activity C₂AS. In a CO atmosphere, the main mineral phases in the clinker transform into C₂AS and 12CaO·7Al₂O₃, owing to the decomposition of CaSO₄, which inhibits C₄A₃S̅ formation. At calcination temperatures exceeding 1300 °C, a significant amount of C₂AS appears in the calcined material, and 12CaO·7Al₂O₃ begins to form. Full article

In an effort to appropriately address the insufficient mechanical properties of calcined phosphogypsum, this research intends to explore how to utilize basalt fiber and calcium carbonate whiskers as reinforcing agents. The study delves deep into their impacts on the flexural and compressive strength, toughness, water resistance, and tensile strength of calcined phosphogypsum. In the individual tests, basalt fibers with different lengths (3 mm, 6 mm, 9 mm, and 18 mm) were added at dosages of 0%, 0.5%, 1.0%, and 1.5%, respectively. As clearly demonstrated by the research findings, basalt fiber effectively reinforces the flexural and compressive strength, toughness, and tensile strength of calcined phosphogypsum, though compromising water resistance. Among the various fiber lengths, the 6 mm fibers impose the most advantageous influence on the performance of calcined phosphogypsum. Afterwards, a test was conducted to explore how cross−scale fibers affect the properties of calcined phosphogypsum by mixing 6 mm basalt fibers and calcium carbonate whiskers. As illustrated by the experimental findings, calcium carbonate whisker refines the pores, thereby elevating the flexural strength and toughness of calcined phosphogypsum. Furthermore, it compensates for the water resistance limitations associated with the sole utilization of basalt fiber while further augmenting the tensile strength and strain capacity. Nonetheless, it is particularly noteworthy that heightening the dosage of both calcium carbonate whiskers and basalt fibers concurrently gives rise to augmented porosity of phosphogypsum and lowered compressive strength. Full article

The proper reutilization of the phosphogypsum (PG) by-product derived from the production of phosphoric acid and recycled concrete aggregate (RCA) from waste concrete in roadbase materials is of great necessity and importance. This investigation tried seeking a new approach to reuse them to high quality, including turning PG into calcinated PG (CPG) via washing and calcination, as well as adopting sodium metasilicate nonahydrate (SMN) to strengthen the roadbase materials of cement-stabilized CPG and RCA. Upon the mix design, with a series of experiments including unconfined compressive strength, the wet–dry cycle, freeze–thaw cycle, and scanning electron microscopy, the comprehensive effects of PG treatment, the CPG to RCA mix ratio, SMN dosage, wet–dry cycle and freeze–thaw cycle on the road performance of roadbase materials were well evaluated, and the traffic bearing capacity and microstructure characteristics were also analyzed. The results demonstrate that the 7 d unconfined compressive strength of CPG/RCA roadbase materials can reach 5.34 MPa as the CPG and SMN dosage are 20% and 11%, respectively, which meets the requirements of an extremely and very heavy traffic grade. After five wet–dry cycles and freeze–thaw cycles, the resistance of the CPG/RCA roadbase materials to moisture and frost was significantly improved as 11% SMN was added. Meanwhile, SMN contributes to the reduction in crack width and densifies the microstructure of CPG/RCA roadbase materials. The research results can be used to provide new guidance for building more durable roadbase materials. Full article

Phosphogypsum and red mud are bulk industrial solid wastes that trigger local environmental problems. In the present investigation, an efficient valorization process was developed through which phosphogypsum and red mud can be transformed into a precursor for the synthesis of high-strength, alkali-activated materials with a seawater-bearing sodium silicate solution as the alkaline activator. The effects of the activator modulus and liquid-to-solid ratio on the strength evolution of the synthesized AAMs as well as the microstructure and chemistry of the reaction products were investigated. The results showed that mineral reconstruction between PG and RM took place during calcination at 950 °C, forming ye’elimite, anhydrite and gehlenite, which then took part in the alkali-activation process and generated thenardite and C-A-S-H gel. The mechanical properties of the synthesized AAMs, ranging from 12.9 MPa to 40.6 MPa, were determined with the activator modulus and liquid-to-solid ratio. Results from the present investigation contributed to the facile and efficient valorization of phosphogypsum and red mud into cementitious construction materials. Full article

Preparing calcium sulfoaluminate cement (CAS) from solid waste phosphogypsum (PG) instead of natural gypsum is an effective way to utilize solid waste. In this paper, CAS clinker was successfully prepared from PG and the mineral content of calcium sulfoaluminate (${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$) in the sample was above 65%. The effects of raw material ratio, calcination temperature, and time on clinker composition were investigated. The mechanical properties of different samples were tested. The optimum ratio for preparing CAS using PG was 42.23% limestone, 17.43% PG, and 40.34% bauxite. The optimal calcination conditions are a high temperature of 1250 °C for 45 min. The 3-day compressive strengths of the laboratory-prepared CAS were all above 50 MPa. It was found that as the calcination temperature increased, the amount of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ produced gradually increased. Temperatures above 1300 °C resulted in the decomposition of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$. The calcination time did not significantly affect the mineral composition of the clinker or the strength of the cement. ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ was observed to be rounded and hexagonal platelets with crystal sizes of 1 to 2 μm, a relatively small size that is favorable to the hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, as observed by SEM images. In addition, the high calcination temperature affected the particle morphology of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, changing it from a well-defined polygonal structure to a molten state. The test results provide helpful information for improving PG utilization and applying PG in CAS production. Full article

The geopolymer concept has gained wide international attention during the last two decades and is now seen as a potential alternative to ordinary Portland cement; however, before full implementation in the national and international standards, the geopolymer concept requires clarity on the commonly used definitions and mix design methodologies. The lack of a common definition and methodology has led to inconsistency and confusion across disciplines. This review aims to clarify the most existing geopolymer definitions and the diverse procedures on geopolymer methodologies to attain a good understanding of both the unary and binary geopolymer systems. This review puts into perspective the most crucial facets to facilitate the sustainable development and adoption of geopolymer design standards. A systematic review protocol was developed based on the Preferred Reporting of Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist and applied to the Scopus database to retrieve articles. Geopolymer is a product of a polycondensation reaction that yields a three-dimensional tecto-aluminosilicate matrix. Compared to unary geopolymer systems, binary geopolymer systems contain complex hydrated gel structures and polymerized networks that influence workability, strength, and durability. The optimum utilization of high calcium industrial by-products such as ground granulated blast furnace slag, Class-C fly ash, and phosphogypsum in unary or binary geopolymer systems give C-S-H or C-A-S-H gels with dense polymerized networks that enhance strength gains and setting times. As there is no geopolymer mix design standard, most geopolymer mix designs apply the trial-and-error approach, and a few apply the Taguchi approach, particle packing fraction method, and response surface methodology. The adopted mix designs require the optimization of certain mixture variables whilst keeping constant other nominal material factors. The production of NaOH gives less CO₂ emission compared to Na₂SiO_3, which requires higher calcination temperatures for Na₂CO₃ and SiO_2. However, their usage is considered unsustainable due to their caustic nature, high energy demand, and cost. Besides the blending of fly ash with other industrial by-products, phosphogypsum also has the potential for use as an ingredient in blended geopolymer systems. The parameters identified in this review can help foster the robust adoption of geopolymer as a potential “go-to” alternative to ordinary Portland cement for construction. Furthermore, the proposed future research areas will help address the various innovation gaps observed in current literature with a view of the environment and society. Full article

Phospho-gypsum is an industrial solid waste discharged from the phosphate production process. The waste includes complex impurities such as phosphoric acid and its salts, fluoride, and organics. Usually, retarders are mixed in gypsum-based building materials to extend setting time. Although the effects of the impurities on hydration properties and the mechanical strength of calcined gypsum have been analyzed, the impact and mechanism of soluble phosphorus on the phospho-gypsum under retardation is yet to be defined. In this study, we employed thermogravimetry (TG), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to evaluate the hydration kinetics, phase transformation, structure, and morphology of the calcined gypsum. The data showed that the retarder or soluble phosphorus prolonged the setting time. A single retarder considerably shortened the initial setting time from 95 min to 60 min, even at the lowest dosage of 0.1 wt.% soluble phosphorus. In addition, drying flexural and compressive strengths were markedly decreased. On the other hand, the induction period was advanced with extension of acceleration and deceleration stage. SEM results indicated that the crystal morphology of the gypsum changed from a long to short column or block. An EDS analysis showed that phosphates were concentrated on the surface of gypsum crystals. Full article

Carbon dioxide (CO₂) emissions associated with Portland cement (PC) production is ranked as the highest among the construction materials and it is estimated that 8% of the worlds CO₂ discharges is due to PC production. As an average, the production of PC clinker including calcination process generates 0.81 kg of carbon dioxide per one kg of cement. Hence, new approaches which limit the negative environmental impacts of cement production and are aimed at the development of advanced methodologies are introduced. Implementation of lower energy consumption materials in production, which could moderately substitute PC in binders, can be addressed as one of the probable methods in mitigating environmental risks. Therefore, alternative binders fit into the most promising solutions. Present research investigates the environmental impact of the building sector, if an alternative to PC binder is used. Life cycle assessment (LCA) was used in this research to assess the environmental impact of the alternative ternary gypsum-PC-pozzolan binder in the production of mortar, and the environmental benefits were calculated and compared to traditional cement-based building materials. Phosphogypsum was considered as a secondary raw material, as in the current approach it is collected in open stacks bringing environmental concerns. SimaPro LCA software with the Ecoinvent database was used for most of the calculation processes. Results indicate that with alternative binders up to 30% of energy can be saved and 57 wt.% of CO₂ emissions can be reduced, bringing positive impact on the construction industries contribution to the environment. Full article

Phosphogypsum (PG) is not only a solid waste discharged from the phosphate fertilizer industry, but also a valuable resource. After high-temperature heat treatment, it can be decomposed into SO₂ and CaO; the former can be used to produce sulfuric acid, and the latter can be used as building materials. In this paper, the catalytic thermal decomposition conditions of phosphogypsum were optimized, and the effects of the reaction temperature, reaction atmosphere, reaction time and carbon powder content on the decomposition of phosphogypsum were studied. The research shows that the synergistic effect of carbon powder and CO reducing atmosphere can effectively reduce the decomposition temperature of phosphogypsum. According to the results of the orthogonal test under simulated suspended laboratory conditions, the factors affecting the decomposition rate of phosphogypsum are temperature, time, atmosphere and carbon powder content in turn, and the factors affecting the desulfurization rate are time, temperature, atmosphere and carbon powder content in turn. Under laboratory conditions, the highest decomposition rate and desulfurization rate of phosphogypsum are 97.73% and 97.2%, and the corresponding reaction conditions are as follows: calcination temperature is 1180 °C, calcination time is 15 min, carbon powder content is 4%, and CO concentration is 6%. The results of thermal analysis of phosphogypsum at different temperature rising rates show that the higher the temperature rising rate, the higher the initial temperature of decomposition reaction and the temperature of maximum thermal decomposition rate, but the increase in the temperature rising rate will not reduce the decomposition rate of phosphogypsum. Full article

An experimental study was conducted to investigate the potential use of phosphogypsum (PG) to produce self-leveling underlayments. The study was designed in two stages. Initially a phosphoanhydrite (PA) was produced by heating phosphogypsum at temperatures of 350 °C, 450 °C, 550 °C, and 650 °C. Two periods of heating were applied (2 and 4 h). The formation of anhydrite was determined by thermogravimetric analysis (DTA-TG) and confirmed by X-ray diffraction (XRD). The results show that anhydrite II was obtained at temperatures above 450 °C, and at higher calcination temperatures the PA solubility was lower. In the second stage of this research, the PA was used in self-leveling underlayments as the main binder in the ternary system comprised of calcium sulfate, calcium aluminate cement, and Portland cement. Self-leveling mortar screeds produced using PA (550 °C/4 h) and PA (650 °C/4 h) showed the best performance in terms of mechanical strength and no degradation was observed after immersion and immersion-drying tests. The formation of ettringite, identified by scanning electron microscopy (SEM), may have contributed to these results. Morphological changes were studied using the scanning electron microscopy (SEM) technique. Full article