Porous mullite having needle-shaped interlocking microstructure is prepared from China clay and AlF3·3H2O by an in-situ vapour phase reaction. The effect of the amount of gases formed during the reaction on the phase evolution, microstructure and pore characteristics of the developed porous mullite has been studied. A significant enhancement in mullite crystal growth and apparent porosity is observed when the amount of gases maintained inside the reaction crucible is increased. During sintering, an optimum amount of sample (60 g/L) needs to be kept inside the crucible for obtaining better properties. © 2018 Elsevier Ltd and Techna Group S.r.l.