Cemented sands: from field deformation to inter-particle bonding

The occurrence and evolution of localized deformation in sands and sandstones can be affected, among other factors, by the size, shape, and mineralogy of the grains and also by the distribution, type and the overall chemistry of the cementation. The latter, in fact, characterizes the strength of a sandstone and definitely alters porosity and permeability. Cementation is relatively poorly researched and it is the topic of this study. In the present work we aim at understanding how the type, degree and bonding morphology of the cement affect the mechanical properties of artificially cemented sands. This is a multi-scale study, moving between centimeter sized samples and grain scale using laboratory cemented sands and pieces of naturally cemented materials.
The sand grains in our work have been collected from a quarry, in the southern France, consisting of a weakly cemented sand. Numerous deformation bands have been observed in the outcrops of this quarry. The grain size of the undeformed material is between 300 and 400 µm, whereas the deformation bands are generally characterized by angular sand grains of a smaller size. Scanning electron imaging of poorly cemented sand chunks, coming mainly from the undeformed region of the quarry, reveals a cementing action by clays mixed with water, formed during early diagenesis. The clay platelets partially rim the sand grain surfaces. In particular, the cement mixture forms menisci between adjacent grains that may or may not be touching and acts as bonds connecting single sand particles into an identifiable, although weak, structure.
Motivated by the above mentioned natural example, we focus on studying at the laboratory scale similar cement bonding morphologies obtained in artificial, laboratory-created cemented sands. To do so, we mix sand grains from the quarry with three different cements using a water-based solution of a) sugar; b) calcite; c) clay powders. The artificially cemented samples are then x-ray scanned in order to obtain a 3D understanding of the grain-cement structure. Moreover, the pore networks are extracted from the x-ray images allowing us to calculate flow properties within the artificially samples. Scanning electron imaging is also performed in some of these samples to capture a high resolution cement topology in 2D.
Once we understand the cement distribution and morphology, next steps will consist of performing triaxial compression experiments on the artificially cemented sand samples while scanning them in situ with x-rays. In this way we aim at gaining better insights of the different cement type behaviors when the samples are subjected to the same loading conditions. Moreover, we question whether the cement type and topology influence the actual deformation processes. Finally, we compare the resolved deformation micro-mechanisms that develop in the artificially cemented sand with those of the deformation features in the French outcrops.