Influence of salt (NaCl) on hydric and hygric dilatation of clay-rich rocks

Abstract

• Hydric (liquid water) dilatation decreases with increasing salinity of pore water. • Salt contamination reverses hygric (water vapour) dilatation phases of clay-rich rocks. • Crystallisation of NaCl develops higher strain than hydration of clay interlayers. • Changes in volume occur more frequently once clay-rich rocks are exposed to salts. • Salt weathering of clay-rich rocks produces granular disintegration and scaling. The mechanisms through which crystallisation of salts and hydration of swelling clays cause deterioration of rocks are generally understood. However, how these two decay mechanisms operate synergistically is less clear. In this study, we investigated the impact of a salt (NaCl) on the deterioration of clay-rich basaltic scoria from which the churches forming the UNESCO World Heritage Site of Lalibela in Ethiopia are hewn. The basaltic scoria is rich in smectite (swelling clay) and zeolitic minerals which have micropores and an affinity for efficient water absorption. Hydric and hygric swelling experiments were carried out on 50 mm cube quarry samples and continuous measurements were taken with Linear Variable Displacement Transformers (LVDT). Hydric (liquid water) tests involved partially submerging the samples in deionised and saline solutions. Hygric (water vapour) tests were done in an environmental cabinet where the RH fluctuated between 60% to 90% RH with salt-free and salt-contaminated samples. The results showed that contamination with salts alters both hydric and hygric behaviour. Hydric swelling was reduced in most samples when contaminated with salt, while the expansion and contraction phases in hygric tests were reversed after contamination with NaCl. Continuous measurement of dilatation allowed us to quantify the strain developed during the phase transition of NaCl. Due to the crystallisation pressure, hygric dilatation increased significantly in salt-contaminated samples. Salt-contaminated samples also showed frequent episodic changes in volume that corresponded both with the hydration/dehydration of clay interlayers and crystallisation/dissolution of NaCl. These volume changes correspond with the damage profile on the surface of the samples, i.e., contour scaling and granular disintegration. The results indicate that the mechanism through which salt contamination accelerates the decay of clay-rich rocks is via an increase in the frequency and degree of hygric dilatation.