Phase reaction during thermochemical treatment of sewage sludge and biomass ashes with alkali compounds to increase nutrient plant availability Phosphorus is an essential element for plants animals and humans (e.g. storage and transfer of energy genetic materials bones and teeth). In agriculture fertilization of phosphorus is necessary to replenish the stock in soils. A promising phosphorus source to produce recycling fertilizers is sewage sludge ashes from mono sludge incineration plants which may have high phosphorus content (12%). A process to recover phosphorus from these ashes is thermochemical treatment in a rotary kiln (AshDec® process). A large scale plant located in Altenstadt (Bavaria) is expected to start operating in 2023. This process transforms poorly plant available phosphorus phases in the sewage sludge to highly plant available calcium alkali phosphate Ca(Na K)PO4 in the thermochemical product. To achieve this phase transformation sewage sludge ash is treated with sodium and or potassium additives at 800 1000 °C. In this thesis new thermodynamic data for calcium alkali phosphates were determined and used for thermodynamic calculations. Furthermore when sodium sulfate was used the formation of calcium sodium phosphates was complete at above 875 °C. The use of potassium sulfate required a reaction temperature above 1100 °C related to the melting points of these sulfate phases. In order to incorporate potassium sulfate into the process at low temperatures sodium sulfate and potassium sulfate were mixed before treatment. As a result the reaction temperatures could be reduced below 1000 °C. A fundamental finding was potassium is preferably incorporated into aluminosilicates instead of phosphates. Thus phosphorus compounds contain less potassium as expected. This could be a drawback for the production of phosphorus potassium fertilizers. Furthermore Magnesium and potassium can be incorporated into the known phase CaNaPO4 and lead to the formation of other phase modifications in the phase system (Ca Mg)(Na K)PO4.