Kseniya Levickaya

Phosphogypsum is one of the most widely produced gypsum-containing wastes. Therefore, researchers worldwide are exploring ways to recycle them. It is most often considered as an alternative to natural gypsum in the production of calcium sulfate hemihydrate. There are also isolated studies aimed at producing insoluble anhydrite (CaSO₄ II) from phosphogypsum. Compared to hemihydrate, anhydrite is characterized by greater strength and water resistance, and compared to Portland cement, it demonstrates lower energy consumption and CO₂ emissions during production. This study examined the possibility of phosphoanhydrite binder (CaSO₄ II) synthesis by calcination at 600, 800, and 1000 °C of phosphogypsum from four different industrial plants. Phosphoanhydrite binders capable of self-hardening, without the use of special additives, were synthesized. Their maximum strength at 28 days reached 57 MPa, and 69 MPa at 90 days. New data have been obtained regarding the influence of initial phosphogypsum characteristics and calcination temperature on the properties of CaSO₄ II and the hardened phosphoanhydrite paste.

Phosphogypsum, being one of the most large-tonnage gypsum-containing industrial wastes, has traditionally attracted the attention of research groups worldwide. The scope of application of this waste as an alternative to natural gypsum rock is quite broad; in particular, it can serve as a raw material for the production of high-temperature calcined anhydrite binders. Despite the technological challenges associated with the production of anhydrite binders, they possess certain advantages over calcium sulfate hemihydrates. Studying the possibilities of producing anhydrite from phosphogypsum will expand the scope of its practical application and increase the volumes of this waste recycling, which fully aligns with the priority objectives of the global resource conservation strategy. In this study, the effect of high-temperature calcination regimes of phosphogypsum – namely, temperature (500–1000 °C) and dwell time (60–120 min) – on the physico-mechanical properties of phosphoanhydrite binders (PAB) was investigated using the method of mathematical experimental design, with the aim of assessing their ability to self-harden without the use of activators. It was established that PAB are capable of hardening without activating additives. The optimal regime in terms of the balance between energy and material costs is calcination at 800 °C for 60 min, which allows obtaining a binder with a compressive strength of approximately 42 MPa. Further increase in temperature to 1000 °C leads to a decrease in strength characteristics and a significant rise in energy consumption for both calcination and grinding of the raw material.

Phosphogypsum, a byproduct of orthophosphoric acid production, is one of the large-tonnage wastes. Since phosphogypsum mainly consists of CaSO₄ 2H₂O, it can be considered as an alternative gypsum-bearing raw material in the production of gypsum binders. However, its features, such as particle morphology and the presence of impurities, can negatively affect the characteristics of phosphogypsum-based binders. Identification of these factors will allow us to develop methods for their minimization and increasing the efficiency of phosphogypsum use from the required source as a raw material for the production of phosphogypsum-based binders. In this regard, the manuscript contains a comprehensive and comparative analysis of phosphogypsum and natural gypsum, which makes it possible to establish their differences in chemical composition and structural and morphological features, which subsequently affect the properties of the phosphogypsum-based binder. It has been established that the key factor negatively affecting the strength of phosphogypsum-based paste (2.58 MPa) is its high water demand (0.89), which is due to the high values of the specific surface area of the particles and the presence of a large number of conglomerates with significant porosity in phosphogypsum. It has been suggested that preliminary grinding of phosphogypsum can help reduce the amount of water required to obtain fresh phosphogypsum-based paste with a standard consistency and improve its physical and mechanical properties.

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.

Gypsum binders and the products based on them are widely in demand in the construction materials market, which is due to their easy production technology, lower energy consumption, and low environmental impact in relation to Portland cement. Not only natural gypsum (NG), but also phosphogypsum (PG), which is a by-product of the synthesis of orthophosphoric acid from phosphorite rock, can be used as a source of raw materials for the production of gypsum materials. PG is produced annually in large quantities throughout the world. In chemical composition, PG mainly consists of calcium sulfate dihydrate CaSO₄·2H₂O, so it is a good potential analogue of natural gypsum, which is used as the main component of gypsum building materials. Thus, the useful recycling of PG as a technogenic resource with valuable properties will expand the raw material base for the production of gypsum materials. This approach to handling technogenic resources fits well with the principles of a circular economy. However, like any technogenic resource, PGs from different enterprises normally differ in their deposits of the original phosphate rock and production technologies. Therefore, PG contains a large number of undesirable impurities, the proportion and composition of which vary over a wide range. This feature does not allow for predicting the properties of PG-based materials without a preliminary detailed study of PG. This research was aimed at carrying out a comprehensive study of the characteristics of PGs from three different industrial plants to evaluate their relationship with the properties of gypsum materials based on them. It was found that PGs have significant differences in their structural and morphological characteristics both in relation to each other and in relation to NG. Also, binders based on PG and NG have significant differences in their physical properties. The average density, compressive strength, and flexural strength for the PG binders with equal workability are lower than those of NG binders. At a water/solid ratio (W/S) < 0.7, all PG binders exhibit comparable compressive strength to NG binders. Thus, PG can act as an alternative to natural gypsum in gypsum binders.

The search for ways to utilize and recycle industrial by-products is the basic principle that governs rational environmental management, synthesis of “green” materials, and appears as one of the main criteria for sustainable development in most countries of the world. Gypsum-containing waste (GCW) derived from industries, represents a large-tonnage product. The production of gypsum materials could be one of the ways to recycle GCW products. GCW from various industries can be used as an alternative to natural raw materials when producing gypsum binders. However, the features of GCW do not allow the production of a high-quality binder when traditional technologies are applied, so it requires the development of additional methods or the introduction of various modifiers to the binder system. One of the ways to increase the efficiency of GCW as a raw material for the production of gypsum binders is to apply a semi-dry pressing method, at reduced values of the W/S ratio of the binder. The objective of this research was to study the possibility of increasing the efficiency of GCW using citrogypsum for production of gypsum materials, by optimization of the mix design and by applying a semi-dry pressing method, using a lower pressure load at the molding stage. The mix design and technological parameters were optimized using mathematical planning of the experimental method. Parameters such as the amount of citrogypsum as an additive in the raw mixture, molding pressure, and water–solid (W/S) ratio were taken as input parameters of variation. To plot the relationship of the input–output parameters, the SigmaPlot software was applied, to analyze and demonstrate scientific and statistical data in the form of nomograms. It has been established that the use of the semi-dry pressing method with the optimal mix design and technological parameters, makes it possible to obtain gypsum samples with demolding strengths up to 2 MPa, and final compressive strengths up to 26 MPa. The incorporation of citrogypsum and the optimal W/S ratio of 0.25, results in positive effects, such as a reduction in the sticking properties of the mix during the demolding stage, and the homogeneity of compaction and visual appearance of the samples were also improved.