Numerical modeling of moisture transport in sandstone: the influence of pore space, fabric and clay content

In the present study, a numerical modeling of moisture distribution under real climate conditions within sandstone monoliths is accomplished, based on detailed material-specific transport and storage functions. The impact of lithology and pore-radii distributions is modeled with consideration of (1) the single sandstone monolith; (2) the sandstone monolith with clay layers; and (3) the sandstone monolith with clay layers and hydrophobic treatment. The results reveal that the unimodal equal pore-radii distribution of the quartz arenite promotes quickly a (capillary) water uptake during driving rain (liquid stage), but due to its missing smaller capillaries a high drying velocity leads to an almost dry pore space, since moisture can only be absorbed via gaseous stage (e.g. during summer). On the contrary, the sublitharenite with a unimodal unequable pore-radii distribution is characterized by a distinctly higher water content, since in addition smaller pores also allow the absorption of moisture via sorption. Moreover, the high clay content promotes a retarded interaction with the environment, which is also reflected by the high vapor-diffusion resistance. The highest water content shows the feldspathic litharenite with highest clay content and bimodal pore size distribution. Here, over nine magnitudes of water transporting pores is involved at water transport and storage. Results also reveal that moisture accumulations during droughts trace the deterioration shape of rounding. For all sandstones highest annual fluctuations are observable within the rim zone of the monolith, while the center is characterized by more stable moisture content, which mainly depends on rising water content of the bedrock. The presence of clay layers has for each sandstone specific consequences. However, within the whole sandstone the stress index is increased and stress location is displaced to the boundaries of clay layers. Results of modeling the hydrophobic treatment reveal that this conservation strategy is only useful for sandstones where all moisture is absorbed in liquid stage, why then all water absorption is hindered. In case of sandstones with bimodal and unimodal unequal pore size distributions moisture uptake is possible also via sorption. Accordingly, moisture accumulates behind the zone of hydrophobic treatment. This finally will lead to stress transfer to the outer rim during salt- or ice crystallization and will be responsible for flaking.