Pyrite-bearing unbound fills, widely used in eastern Ireland have heaved, causing serious structural damage to thousands of buildings. This study compares calcareous mudstones that degenerated rapidly, causing severe structural damage, with argillaceous limestones that did not. Framboidal pyrite in the mudstones is widely dispersed – every aggregate particle contains abundant framboidal pyrite. Oxidation of this produced sulfuric acid that reacted with calcite producing supersaturated solutions of CaSO4 and CO2 gas. It is suggested that the latter exerted pressures approaching 5 MPa within rock particles, creating micro-fractures into which gypsum crystallised. Antitaxial growth of gypsum continued expansion, a process analogous to the formation of mineral veins in rocks. Heave of the fill took place since all the loadbearing particles expanded. The limestones have a lower pyrite content, which occurs mainly in the shaly seams and is concentrated in the fines; limestones suffered similar oxidation, but the coarser aggregate remains unaltered, and gypsum is mainly pore-filling; little expansive force was generated. It is concluded that the actual amount of pyrite present is a less important factor controlling expansion of unbound fills than its crystal size, and its distribution throughout the aggregate.