Biomass is one of the most important sources of renewable energy in the energy industry. It is assumed that by 2050 the global energy deposit could be covered in 33–50% of biomass combustion. As with conventional fuels, the combustion of biomass produces combustion by-products, such as fly ash. Therefore, along with the growing interest in the use of biomass as a source of energy, the production of ash as a combustion by-product increases every year. It is estimated that approximately 476 million tons of ashes per year can be produced from biomass combustion. For example, the calorific value of dry wood mass tends to be between 18.5 MJ × kg⁻¹ and 19.5 MJ × kg⁻¹, while the ash content resulting from thermal treatment of wood is from 0.4 to 3.9% of dry fuel mass. However, biomass ash is a waste that is particularly difficult to characterize due to the large variability of the chemical composition depending on the biomass and combustion technology. In addition, this waste is, on the one hand, a valuable fertilizer component, as it contains significant amounts of nutrients, e.g., calcium (Ca), potassium (K) and microelements, but on the other hand, it may contain toxic compounds harmful to the environment, including heavy metals and substances formed as a result of combustion, such as polycyclic aromatic hydrocarbons (PAHs) or volatile organic compounds (VOCs). PAHs and VOCs are formed mainly in the processes of incomplete combustion of coal and wood in low-power boilers, with unstable operating conditions. However, it is important to remember that before the fly ash is used in various industries (e.g., zeolite synthesis, recovery of rare earth metals or plastic production) as an additive to building materials or fertilizers for cultivation, a number of analyses are to be conducted so that the by-products of combustion could be used to allow the by-product of combustion to be used. It is important to conduct tests for the content of heavy metals, chlorides, sulphates, microelements and macroelements, grain and phase composition and organic compounds. If such ash is characterized by low pollution levels, it should be used in agriculture and reclamation of degraded land and not directed to landfills where it loses its valuable properties. The purpose of this review is to present the properties of ashes generated as a result of biomass combustion in Poland and the world, to discuss factors influencing changes in its composition and to present the possibilities of their reuse in the environment and in various branches of industry.

Landfill sealing layers protect groundwater and soil against contamination from waste leachate. Fly ash, which is a coal combustion by-product, might be used as a substitute material for natural soils to build sealing layers. To improve its properties, different additives can be used. In this study, bentonite in the amounts of 5%, 10% and 15% of the dry mass of fly ash was used as an addition. Compacted fly ash and fly ash-bentonite mixtures were tested in order to verify how bentonite and the amount of addition affect the properties of the materials. Therefore, hydraulic conductivity, as the most important parameter, which determines the suitability of a material for construction landfill sealing layers, were tested. Results indicate that bentonite changes the properties of fly ash and allows the achievement of smaller hydraulic conductivity values. To provide a comparison to the natural soil, sandy silty clay that meets the criteria for natural soils for sealing layers was tested and the differences are presented. Fly ash with 15% of bentonite addition reached a value of hydraulic conductivity for the material suitable to build compacted landfill sealing layers.

Stabilisation with cement is an effective way to increase the stiffness of base and subbase layers and to improve the rutting of subgrade. The aim of the study is to investigate the effect of different percentages of cement additives (1.5%, 3.0%, 4.5% and 6.0%) on the resilient modulus of coarse-grained soil used on road foundations. The influence of the compaction method, the standard Proctor and the modified Proctor, as well as the sample curing time is analysed. The cement addition significantly increases the resilient modulus and reduces the resilient axial strain. Extending the curing time from 7 to 28 days also improves the resilient modulus. The change in the compaction energy from standard to modified does not increase the resilient modulus of the stabilised gravelly sand due to its compaction characteristics. The test results of the resilient modulus of the gravelly sand stabilised with cement indicate the possibility of using it as a material for the road base and subbase due to meeting the AASHTO requirements. However, the non-stabilised gravelly sand does not meet the above requirements. It has been sheared during cyclic tests at the first load sequence, regardless of the compaction method.

The behavior of soils under tensile stress is of interest to geotechnical engineers. Tensile strength of soils is often associated with tensile fractures that can generate a privileged flow path. The addition of bentonite improves the plastic properties of the soil, therefore the study was conducted for the compacted class F fly ash and fly ash with various bentonite additions. An amount of bentonite was: 5, 10 and 15%, calculated in weight relation to dry mass of samples. The tensile strength of compacted clay was also established, for comparison. Laboratory tests were carried out using the direct method (breaking) on cylindrical samples and the indirect method (the Brazilian test) on disc-shaped specimens. For this purpose, a universal testing machine with a frame load range of ±1 kN was used. It is stated that bentonite considerably influences the tensile strength of the fly ash evaluated with both methods. The tensile strength values obtained with the Brazilian method are comparable or higher than those obtained with the direct method. The achieved tensile strength values of compacted fly ash, improved by 10−15% of bentonite addition, are comparable with the results obtained for clay used in mineral sealing, while the strain at maximum tensile strength is similar in the direct test and lower in the indirect test.

Leachates from landfills constitute a very complex environmental matrix with potentially toxic properties for both the environment and humans. Due to this fact, toxicological tests were carried out on landfill leachate (LL) obtained from the local landfill in Hryniewicze (Poland). The analyses included physicochemical studies of leachates and their impact on soil and plants, and studies conducted in bacterial models and human cell models. The results obtained indicate high contents of metals and organic matter, especially dangerous PAHs, in the tested leachate. This results in the influence of LL on changes in the content of assimilation pigments and oxidative stress observed in plants grown in soil fed with leachates. The effect of leachate on the growth of Sporosarcina pasteurii, Staphylococcus aureus, Lactobacillus rhamnosus, Saccharomyces boulardii and Candida albicans varied depending on the strain and LL dose. A particularly significant increase in proliferation after exposure to LL was noted for S. aureus. In studies conducted on human cancer cell lines representing three types of glioblastomas and one type of colorectal adenocarcinoma, a particularly significant increase in the viability of cells treated with LL was noted for the DLD-1 cell line. The results obtained, especially the stimulation of the growth of cancer cells and an increase in the number of pathogenic bacteria, indicate the potential toxic properties of the tested leachates. This is confirmed by the high level of oxidative stress in plants. The results indicate the need for continuous monitoring of waste landfills and leachates generated there.

The aim of this study was to compare the stiffness of gravelly sand under various load conditions—static conditions using the CBR test and cyclic conditions using the resilient modulus test. The tests were conducted on natural soil and soil improved by the addition of polypropylene fibers and/or 1.5% cement. The impacts of the compaction and curing time of the stabilized samples were also determined. The soil was sheared during the $M_r$ tests, even after fiber reinforcement, so the resilient modulus value for the unbound sand could not be obtained. The cement addition improved $M_r$, and the curing time also had an impact on this parameter. The fiber addition increased the value of the resilient modulus. The CBR value of the compacted gravelly sand was relatively high. It increased after adding 0.1% fibers in the case of the standard compacted samples. The greater fiber addition lowered the CBR value. For the modified compacted samples, each addition of fibers reduced the CBR value reduced the CBR value. The addition of cement influenced the CBR increase, which was also affected by the compaction method and the curing time. The addition of fibers to the stabilized sample improved the CBR value. The relationship $M_r=f\left(CBR\right)$ obtained for all data sets was statistically significant but characterized by a large error of estimate.