The context of the present study is the gasification of dried sludge at high temperature. The aim of this work is to shed new light on the impacts of sludge ashes on gasification process. The purification sludge can contain up to 50% inorganic matter. The objective of this study is to understand the role of these inorganics, especially phosphorus, during the heat treatment. Several techniques are used to solve this problem. First, using thermodynamic calculations (FactSage), the evolution of the volatility of the inorganics is observed and the condensed phases formed during the heat treatment are determined. The simulations are done under atmospheric pressure, from 500 to 1500 degrees C. Second, to compare with the calculus, XRD and TGA experiments are carried out on model samples, with composition close to the real sludge, to determine which species volatilize and to compare the experimental results with the thermodynamic calculations.