Water hyacinth (Eichhornia crassipes), an invasive aquatic plant with exceptionally high yield, is increasingly considered a renewable energy feedstock. This study examines its thermal decomposition and ash evolution between 600 and 900 °C using thermogravimetric analysis (TG/DTG), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX), and X-ray diffraction (XRD). TG/DTG identified three decomposition stages and a sticky-layer interval at 780–800 °C associated with alkali chloride melting. EDX results showed that K and Cl declined from 16.9 and 19.0 wt% at 600 °C to ≤ 2 wt% and ≤ 1.3 wt% at 900 °C, while Ca, Mg, and Fe increased. SEM analysis quantified a porosity maximum of 61.0% at 700 °C followed by a drop to 55.3% at 800 °C due to molten-phase infiltration. XRD revealed the progressive loss of sylvite and halite, the dominance of Fe₃O₄ at mid-range temperatures, and the emergence of MgFe₂O₄ spinel at 900 °C. The slagging index decreased steadily from 1.09 to 0.35 across the studied range. These results establish 780–800 °C as the sintering onset. For small-scale furnaces, maintaining bulk temperatures below 750 °C is recommended to minimize slagging and ensure stable operation.