The role of ink-bottle pores in freeze-thaw damage of oolithic limestone

Many natural building stones are altered when subjected to freeze-thaw (FT) cycles. On the one hand, the sensitivity of a material to FT damage has been quantified in the past by the existence of a material-specific critical water saturation. On the other hand, it was noticed that natural stones with a high volume of ink-bottle pores, normally hold a relatively large FT resistance. The relationship between the pore structure, the saturation and the FT resistance is however not well understood. In this paper, the influence of water content and the porosity on FT behaviour is investigated macroscopically with temperature and strain measurements, established techniques in FT related research, and X-ray computed micro-tomography (µCT). Strain measurements performed on Savonnières limestone samples clearly show an increasing expansion with increasing water content, with the existence of a critical water saturation level between 70 and 80%. Differential X-ray imaging on differently saturated limestone samples in an unfrozen and frozen state then aided in explaining the origin of this critical saturation degree. By draining water from surrounding micro-pores through cryo-suction, ink-bottle ooid voids served as expansion reservoirs. When the majority of the ooid voids is water saturated prior to freezing, these voids lose their expansion reservoir ability and damaging pressures arise when water freezes inside undrained micro-pores. These findings not only help to understand the FT resistance of this limestone, but also give insight in the general FT behaviour of materials with bi- or multimodal pore-size distributions.