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.

This research focuses on an evaluation of mineral phase and structure transformations in Class F fly ash-based geopolymer systems. The research also studies the strength response of geopolymers when exposed to temperatures between 25 and 800 °C. The purpose of this research is to understand the processes that occur in alkali-activated systems within a wide range of high-working temperatures. The XRD, SEM, and DTA/TG analyses performed for the alkali-activated compositions after exposure to different temperatures confirmed a direct correlation of structural transformations with strength performance. The detrimental effect of sodium hydrocarbonate Na₃(HCO₃)(CO₃) 2H₂O or trona contained in one of the fly ash products was observed for the corresponding alkali-activated composite under high-temperature exposure between 600 and 800 °C. It was also detected that a high-temperature interval of 400–800 °C created favorable conditions that helped to form nanosized nepheline crystals and an additional vitreous substance that also contributed to a denser alkali-activated matrix.

Geopolymers represent a new class of inorganic materials that have great potential for practical application due to the properties of used raw materials, as well as the peculiarities of the cementitious matrix structure formed during the geopolymerization process. Cellular geopolymer specimens were produced in this study using class F fly ash product, which is characterized by low reactivity during geopolymerization. Several standard methods, as well as microstructural studies were applied to evaluate the effect of the following factors on the physical-mechanical and thermophysical characteristics of cellular geopolymers: the use of various mineral modifying components for synthesis of geopolymer systems; high-temperature treatment; the introduction method of alkaline activator. It was observed that “ageing” an aqueous alkali solution for 24 h before mixing with fly ash and foam agent was able to provide a boost of compressive strength of cellular geopolymer specimens up to about 2.5 times, while decreasing the average density by about 28% for all experimental mixes, except for PC-modified mixes. Additionally, high-temperature treatment at 600 °C enables an enhanced strengthening effect of pore structure in cellular geopolymer matrix up to 1.5 times. This phenomenon is especially pronounced for the mixes with 24 h “aged” alkaline solution with exception for PC-modified mixes; for those, high-temperature treatment at 600 °C leads to strength decrease up to 40%. The introduction method of alkaline activator and high-temperature treatment showed a controversial effect on thermal conductivity coefficient depending on the mineral modifying component used for the synthesis of cellular geopolymers. The proposed method for calculation of total porosity of cellular structure of geopolymers as a polycomponent material demonstrated a high degree of correlation with the R² value of at least 0.96 between the average density and the calculated total porosity. However, a low degree of correlation with R² not exceeding 0.29 was observed for the measured nanoporosity, regardless of the introduction method of alkaline activator and high-temperature treatment.

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.