Role of petrography in durability of limestone used in construction of Persepolis complex subjected to artificial accelerated ageing tests

Persepolis historical complex hosts palaces and tombs which are constructed by stone blocks. These stones suffer from various weathering. Hence, in this study, physico-mechanical properties of Persepolis stone are investigated in relation to some important weathering factors including freezing–thawing, thermal shock and salt crystallization. This study focused on role of the stone petrography on its durability against the weathering factors. Petrography studies showed that the stone is a nonporous-impermeable bioclastic-packstone which suffered significant diagenesis pressures and contain fractures filled by clay. Based on the obtained results, most of the observed deterioration for the stone is related to freezing–thawing cycles. The freezing–thawing cycles induced significant losses in the stone mechanical properties. Similar variations were recorded after the salt crystallization and thermal shock cycles, although the changes in the stone properties were not to the extent of freezing–thawing. Assessing values of decay constant (λ) and half-time (N_1/2) obtained for the stone in relation to the ageing test cycles showed that freeze–thaw and thermal shock have more deteriorative effects on the stone durability than salt crystallization. It is suggested that porosity is the main factor that controls the Persepolis stone durability. Low permeability of the stone inhibits water percolation throughout the specimens, so their deterioration against salt crystallization is negligible. But in case of the freeze–thaw cycles, water uptake by clay minerals, which filled diagenesis fractures in the limestone, could be considered as the main factor for the observed deterioration during freezing–thawing. Also, effect of micro-cracks, caused by the thermal shock cycles, is the best explanation for reducing the mechanical properties of the stone.