Rock phosphate is a non-renewable primary source for mineral phosphorus (P) fertilizers that intensive agriculture is highly dependent on. To avoid P fertilizer shortages and limit negative environmental impacts, circular economy approaches are needed with recycling-derived fertilizer (RDF) applications. Here, a grassland field trial was established with two struvites (potato wastewater, municipal wastewater) and two ashes (poultry-litter ash, sewage-sludge ash) at a P application rate of 40 kg P ha(-1) (replicates n = 5). The impact of these RDFs on the soil microbial P cycling community was compared to conventional mineral P-fertilizer and a P-free control. Topsoil samples were taken directly after Lolium perenne grass cuts at months 3, 5 and 15. Cultivable phosphonate and phytate utilizing bacteria, potential acid and alkaline phosphomonoesterase activity, and phoC and phoD copy numbers responded stronger to seasonal effects than treatment effects. No significant overall effect of the fertilizer application was detected in the beta-diversity of the bacterial and fungal communities after 15 months, but individual phylogenetic taxa were affected by the treatments. The ash treatments resulted in significantly higher relative abundance of Bacillota and Rokubacteria and lower relative abundance of Actinomycetota. Sewage-sludge ash had significantly lowest abundances of genera Bacillus and Bradyrhizobium that are well known for their P cycling abilities. The struvite RDFs either positively influenced the P cycling microbial community as demonstrated through higher tri-calcium phosphate solubilizing capabilities (month 3), or were similar to the superphosphate and P-free treatment. From a soil-microbial health perspective, the presented findings indicate that struvites are a suitable substitute for superphosphate fertilizers.

This research is the result of work on implementing a closed-loop economy in geotechnics, which aligns with the broader concept of a circular economy in construction by promoting the use of waste materials and reducing environmental impact. The research presented in the article focuses on the use of fluidized bed furnace bottom ashes, a by-product of coal combustion in fluidized bed boilers, in the production of cement-soil jet grouting slabs. Samples were analyzed for their structural and mechanical properties to assess their suitability for geotechnical applications. The mixtures were distinguished between those using CEM I and those using CEM II. Mixes based on two types of cements had an additional division based on the amount of additives: reference mix, 5% ash, 15% ash, and 10% ash + 5% microsilica. The conducted experiments aim to determine the physico-mechanical parameters of the new mixtures, highlighting the potential of these materials in mining and geotechnical technologies. The research took into account the impact of time over a period of two years for mortars and 28 days for cement-soil. The authors' studies included determining the compressive strength, bending strength, and imaging using computed tomography. Computed tomography allowed imaging of the internal structure and porosity analysis. Employing CEM II as the primary binding material slows early strength gain, but adding microsilica significantly enhances strength, compaction, and durability. Despite improved properties, CT imaging revealed increased cracking in mixtures with CEM II, indicating reduced water tightness and highlighting areas for further study.

The present study has focused on stabilizing the soils of the embankments and improving the mechanical properties of gravel in subbases of pavements with different contents of bottom ash from thermal power plants and low percentages of lime. The density, humidity, simple resistance strength and bearing capacity of the new materials resulting from this combination have been studied. The results indicated that the optimal proportion of bottom ash added to the analyzed soil is 15%, while the optimal addition of lime is 1% for application in embankments and 2% for application in road subgrades. In clay soil that has a low simple resistance strength when 25% of bottom ash is added without lime, it can double the resistance. In the case of the gravel evaluated, it was found that the optimal ratio between the addition of bottom ash and lime is 6.5. In conclusion, it can be noted that soil that does not have any resistance when certain percentages of bottom ash are added, its properties are improved to be used in embankments.