Porous mullite with interlocked needle shape microstructure was developed from China clay and aluminium fluoride trihydrate (AlF₃·3H₂O). The effects of various parameters like sintering temperature, sintering time and the amount of AlF₃·3H₂O on the phase evolution, microstructure and porosity have been studied. Quantitative analysis of mullite was carried out using X-ray diffraction combined with Rietveld-RIR method (Internal standard method). Porous mullite ceramics with 62% open porosity have been prepared at a relatively lower temperature of 1400 °C. The results show that the nucleation of mullite can be achieved from 700 °C onwards using hydrated aluminium fluoride without the formation of intermediate topaz crystals. The high amount of water vapour produced within the system during the in-situ reaction has a crucial role in deciding the reaction mechanism.