Polymorphic foam clay (PFC) was proposed to retard the spontaneous combustion of coal. It is mainly synthesized using foaming clay and auxiliary solutions. When the ratio of foaming clay solution to auxiliary solution was (1) over tilde .0 wt. %, the generated PFCs had the best performance. The sol-gel foams that could be obtained as gas were injected into the synthetic solution, which had an excellent wettability on the coal surface due to the synergistic action of polyethylene oxide and sodium dodecyl sulfate. PFCs were obtained with the reaction of organic acids and sodium silicate, and their times of liquidity and self-hardening loss were 180 and 225 s, respectively. The mechanisms of PFC inhibiting the spontaneous combustion of coal were analyzed by a simultaneous differential scanning calorimeter/thermogravimetric analyzer, an XP-550C polarizing microscope linked to a CK 300 thermal station, scanning electron microscopy, a simultaneous differential thermogravimetric scanning calorimetric analyzer linked to a mass spectrometer and X-ray photoelectron spectroscopy. Before 239 degrees C, the PFC was always in an endothermic state, so it could adsorb significant heat produced by the low-temperature oxidation of coal to cool the coal body. The fused PFC also could form a thin whole colloid clay layer to cover the coal surface and effectively seal gaps and cracks in the coal from oxygen ingress, thereby preventing coal from contacting oxygen. The colloid clay could retain water to increase the moisture of the coal body, resulting in the prevention of the generation of new alkyl groups from cycloalkanes and bridge bond breakage, thereby decreasing the concentration of active groups in the coal. The colloid clay possessed organic acidity that could consume active free radicals. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.