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. © 2018 Elsevier Ltd