Influence of thermal cycling in the mild temperature range on the physical properties of cultural stones

Abstract

The thermoclasty of building stones caused by insolation is an increasingly crucial problem. Moreover, how changes in surface temperature and accumulated thermal cycles would affect cultural stones, such as stone monuments, stone-clad historical buildings, statues, reliefs on stone faces, remains unknown. As limited information is available regarding the thermoclasty of stone materials owing to insolation, this study analyzes experimentally whether repeated heating and cooling degrades or makes common forms of stone material more durable. The samples of granite, marble, and sandstone which are used as building stones of cultural heritages were repeatedly subjected to temperature changes from 4 °C to 84 °C at a change rate of ±2 °C/min for 144 cycles. Furthermore, physical changes were assessed via the measurements of Leeb rebound hardness, ultrasonic pulse velocity, and acoustic emissions at certain intervals. Changes in the pore size distribution owing to thermal cycling were also assessed using mercury intrusion porosimetry. The results showed a slight decrease in the rebound hardness of all samples, however, the pulse velocity in the granite and marble drastically reduced during the initial thermal cycle (≤ 4 cycles) and then recovered (> 4 cycles). The formation of new microcracks at these boundaries and inside the minerals can be attributed to the initial cycling, which resulted in thermal stress at the interfaces. Moreover, after four cycles, re-strengthening may occur due to the evaporation of adsorbed water at the boundaries and anisotropic mineral expansion, instead of the adsorption of mineral surface. In the sandstone, there was a slight change in physical properties after the thermal cycling, suggesting that the thermal strain generated by the thermal cycling was absorbed by large pores. The stone response to repeated thermal disturbances, with phases of both damage and healing, results in a more complicated internal change. Our findings indicate that the three various stone types demonstrated different physical changes in response to thermal cycles of a mild temperature.