Clay swelling mechanism in clay-bearing sandstones

Abstract

Swelling clays in stone can generate damaging stresses during a wetting or a drying cycle, which lead to deterioration of building stones such as Portland Brownstone. There are two primary types of swelling identified for clays: short-range, ordered intracrystalline swelling, and long-range, continuous osmotic swelling. Identification of the swelling mode is important for understanding and ultimately preventing swelling damage. Through comparison of XRD and swelling experiments with cationic pretreatments and organic solvents, we demonstrate that intracrystalline swelling is the primary mode of swelling present in three different stones, including Portland Brownstone. The results highlight the importance of the counterbalancing cation to the swelling process, and a method for characterizing the intracrystalline swelling in sandstones is developed. Finally, the implications of long-term swelling behavior for stones are discussed.

Appendix

To estimate the swelling layers per unit length, one must assume all swelling layers have the same interactions with the solvent. The scaling factor relating linear dimension change to interlayer spacing change is calculated by

$$ \frac{{\varepsilon _{{\text{s}},2} - \varepsilon _{{\text{s}},1} }}{{d_{001,2} - d_{001,1} }} $$

where ε _s is the swelling strain, d ₀₀₁ is the interlayer spacing, and the numerical subscripts refer to a particular solvent. This scaling factor gives the prescribed increase of the stone’s linear dimension with a particular interlayer spacing increase, so the number of swelling layers per unit length is simply the amount of individual interlayer spacing increases necessary to produce a particular change in the stone’s linear dimension. For example, Portland Brownstone will produce a 0.11 μm expansion per mm of stone with a 1 Å layer spacing increase, therefore there are (0.11 μm/1 Å) × 10,000 Å/μm = 1,100 interlayer expansions, or swelling layers.