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The global demand for resources is escalating in today’s rapidly evolving world. As traditional raw materials become scarcer and more expensive, alternative sources have to be found. One such emerging resource is oil shale ash. This article provides a comprehensive overview of the various fractions of oil shale ash generated in the oil shale industry in Estonia. The ash results from the direct combustion of oil shale with pulverised combustion (PC) and circulating fluidised bed combustion (CFBC) technologies, as well as from shale oil production processes. It offers detailed information about the proportions of ash derived from different technological processes and a thorough analysis of their mineralogical and chemical compositions, trace element content, and leaching characteristics. By examining these diverse characteristics, the study enhances understanding of the ash’s potential implications and applications.

Mesoporous silica is emerging as a nanomaterial with potentially huge applications in the industrial sector, particularly in the fields of chemical catalysis, host-guest encapsulation, and adsorbents. These materials possess well-defined honeycomb-shaped pores that can be fine-tuned to suit adsorbing moieties of different molecular sizes, with adjustable release rates. The most significant drawback that hampers the commerciality of mesoporous silica-based processes is that the inorganic precursors used in the synthesis of these materials are quite expensive. These costs can be offset somewhat by using low-cost, renewable precursors from natural sources and recycling industrial wastes. In this review, we have collected some of the more recent laboratory attempts to prepare mesoporous silica material from low-cost precursors. Scientometric analysis was performed to find the potential of mesoporous silica manufacturing from ash sources. Various sources of mesoporous silica preparation and works related to them were explained in detail. This article also explores various techniques used for the development and gives a brief insight into the possible applications for mesoporous silica. Graphical abstract Highlights Industrial waste ash and natural materials are promising sources for silica. An overall analysis of waste utilization was provided using Scientometric studies. Sustainable silica sources are used in the preparation of mesoporous materials. The use of low-cost and eco-friendly silica sources is of environmental importance. Current and future expectations were addressed on utilizing waste ash.

Fly oil shale ash (FOSA) is a waste material known for its pozzolanic activity. This study intends to investigate the optimum thermal treatment conditions to use FOSA efficiently as a cement replacement material. FOSA samples were burned in an electric oven for 2, 4, and 6 h at temperatures ranging from 550 °C to 1000 °C with 150 °C intervals. A total of 333 specimens out of 37 different mixes were prepared and tested with cement replacement ratios between 10% and 30%. The investigated properties included the mineralogical characteristics, chemical elemental analysis, compressive strength, and strength activity index for mortar samples. The findings show that the content of SiO2 + Al2O3 + Fe2O3 was less than 70% in all samples. The strength activity index of the raw FOSA at 56 days exceeded 75%. Among all specimens, the calcined samples for 2 h demonstrated the highest pozzolanic activity and compressive strength with a 75% strength activity index. The model developed by RSM is suitable for the interpretation of FOSA in the cementitious matrix with high degrees of correlation above 85%. The optimal compressive strength was achieved at a 30% replacement level, a temperature of 700 °C for 2 h, and after 56 days of curing.

Economic and technological factors necessitate the use of alternative fuels during oil shale combustion, a process that generates substantial amounts of solid waste with varying ash compositions. This study evaluates the potential of two such waste materials: (i) fly ash derived from the combustion of oil shale (a fine particulate residue from burning crushed shale rock, sometimes combined with biomass), and (ii) short carbon fibres recovered from the pyrolysis (a process of decomposing materials at high temperatures in the absence of oxygen) of waste wind turbine blades. Oil shale ash from two different sources was investigated as a partial cement replacement, while recycled short carbon fibres (rCFs) were incorporated to enhance the functional properties of mortar composites. Results showed that carbonate-rich ash promoted the formation of higher amounts of monocarboaluminate (a crystalline hydration product in cement chemistry), leading to a refined pore structure and increased volumes of reaction products—primarily calcium silicate hydrates (C–S–H, critical compounds for cement strength). The findings indicate that the mineralogical composition of the modified binder (the mixture that holds solid particles together in mortar), rather than the fibre content, is the dominant factor in achieving a dense microstructure. This, in turn, enhances resistance to water ingress and improves mechanical performance under long-term hydration and freeze–thaw exposure. Life cycle assessment (LCA, a method to evaluate environmental impacts across a product’s lifespan) further demonstrated that combining complex binders with rCFs can significantly reduce the environmental impacts of cement production, particularly in terms of global warming potential (−4225 kg CO2 eq), terrestrial ecotoxicity (−1651 kg 1,4-DCB), human non-carcinogenic toxicity (−2280 kg 1,4-DCB), and fossil resource scarcity (−422 kg oil eq). Overall, the integrative use of OSA and rCF presents a sustainable alternative to conventional cement, aligning with principles of waste recovery and reuse, while providing a foundation for the development of next-generation binder systems.

Secondary raw materials, such as ashes from the combustion of various fuels, are frequently used as alternatives to virgin raw materials. Among these, oil shale ash, a residue from oil shale power production and the shale oil industry, presents significant potential for use in sectors such as construction and agriculture. However, these materials might contain hazardous substances, such as dioxins, which are by-products of thermal treatment and other industrial processes. To date, the dioxin content in oil shale ash has been insufficiently examined. This article provides a comprehensive analysis of the dioxin content in oil shale ash from both a pilot unit and full-scale facilities. Additionally, the study compares the dioxin concentrations in oil shale ash with those in other types of ash and evaluates compliance with regulatory limits. The results showed that dioxin concentrations in the ash were below the limit of detection, regardless of the combustion technology, plant capacity, use of supplementary fuels, or utilisation of wastewater. The findings contribute new knowledge by highlighting the environmental advantages of oil shale ash as a secondary raw material, particularly due to its comparatively lower dioxin content relative to other types of ash.

In Estonian oil shale power plants, ash is transported to disposal sites using a wet transport method. Due to regional climatic conditions, where annual precipitation exceeds evaporation, part of the recirculated ash transport water has to be periodically discharged into natural surface waters. This discharge practice raises concerns about the potential environmental impact of the discharged water. This paper investigates the leaching behavior of several trace elements from oil shale ash, demolition wood ash, and their mixtures, with a focus on chromium. In particular, the occurrence, mobility, and oxidation state of chromium in the leachate are considered using the example of the mentioned ashes. The possible effect of ash content, influenced by fuel type and the liquid-to-solid (L/S) ratio, on trace element concentrations in the discharged water was studied. In leaching tests with 100% oil shale ash, the chromium concentration in the circulating water increases only slightly when circulating water from the oil shale ash field is used as the leaching medium. On the one hand, the amount of chromium and other trace elements leached from oil shale ash depends on the L/S ratio. At the same time, it is known from the literature that oil shale ash is also capable of binding chromium. The release and concentration of chromium in the leachate increase significantly as the proportion of waste wood ash rises, compared to tests using 100% oil shale ash. Consequently, the concentration of chromium in surface water, depending on the specific ratio of oil shale to waste wood used in co-combustion, can easily exceed the nationally permitted limit value.

Mining subsidence prevention has not been systematically addressed in Estonia. Despite the presence of numerous shallow and hazardous old mines, including some in urban areas, there are only a few practical examples of land stabilization. This article provides an overview of stabilization methods commonly used elsewhere. By considering the specific characteristics of shallow Estonian mines and existing infrastructure, we propose injectable backfilling through treatment boreholes as the most applicable land stabilization method in Estonia. The backfill typically consists of locally available byproducts or waste materials. We evaluate how Estonian oil shale ash, a residue from power and oil production, compares to the properties required for effective backfilling. Some properties, such as self-cementation, make it suitable for use in backfills. However, it also exhibits less desirable features – high water demand and fast setting. Despite extensive research on oil shale ash, certain material properties that critically impact its usability in backfills – such as its pumpability over longer distances – remain to be fully determined.

Oil shale ash generated from different locations in Jordan was investigated for rare earth elements (REE). The oil shale samples were combusted at 950 °C, and then milled to less than 74 μm. The resulting fine oil shale ash samples were acid-digested to remove minerals. The acid-digested residues were analyzed using inductively coupled plasma mass spectrometry. Fifteen REEs, along with yttrium and scandium, were detected, except for promethium. Among the detected metals, the highest concentrations were found for lanthanum and cerium, at 16.3 and 10.5 ppm, respectively, in the El-Lajjun deposits. The maximum concentration of REEs was 74.4 ppm in the Al-Shalaleh region, with a combined total of 47.06 ppm for light rare earth elements (LREE) and 29.31 ppm for heavy rare earth elements (HREE). The maximum calculated LREE/HREE ratio was 2.42 ppm in the Sultani region. The yttrium and scandium concentrations were 21.3 and 2.51 ppm in the El-Lajjun and Al-Shalaleh regions, respectively.

Estonia’s basic power supply is covered mainly by oil shale-fired thermal power plants. The pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies are used. The power plant exploitation has revealed the emission of gaseous pollutants, as well as ash handling problems. The hydro ash removal is used at large power plants in Estonia. An overview of the formation and properties of oil shale ash is given. The polluting impact of ash in contact with water is analyzed. Taking into account precipitation and evaporation conditions the amount of water bound by ash as well as ash field water balance is given. The leaching behaviour of oil shale ash is analyzed. The analysis of the ash field structure shows that the degree of water penetration of the ash field body meets the requirements for hazardous waste landfills. The water permeability through dense layers ranges from 0.15 × 10–9 to 16.1 × 10–9 m/s.

The article is devoted to the analysis of the influence of heat treatment of oil shale of the Leningrad field on the total surface area of nanopores and total porosity. Oil shale with particle size up to 0.125 mm in the form of powder and in the form of shale briquette was subjected to heat treatment. The change in the total porosity was studied in the temperature range (0÷1000) oC. The change in the total surface area of nanopores was studied by comparing the initial sample of oil shale with the oil shale ash obtained at 1000 oC. The data presented in this paper is indicative of a decrease in the oil shale nanopores total surface area under heat treatment, for example, for pore diameters (3÷4) nm the area decreases from 15.29 cm2/g to 2.563 cm2/g.