Understanding the high variability of clays: a rheological approach

Abstract

The broader adoption of raw earth in construction requires overcoming its heterogeneities and high variability, arising from differences in clay mineralogy and texture. These variations control water absorption and, consequently, rheological properties, drying behavior, shrinkage, and mechanical properties, making earth less predictable than cement-based concrete. This unpredictability limits standardization and its use in modern construction. To enable pourable earth, a rapid and robust characterization of fresh properties is essential. This study demonstrates the use of small amplitude oscillatory shear (SAOS) rheology and mini-slump flow tests to characterize clay–water dense suspensions (e.g., pastes) across a wide range of solid volume fractions (ϕ). Clay-rich materials, each predominantly composed of kaolinite, illite, and montmorillonite (Ca- and Na-bentonite), their 50:50 blends, and one clay-rich sludge were tested. Distinct rheological trends emerged, reflecting the water adsorption capacity of each material. SAOS and mini-slump flow measurements can be used to distinguish clay-rich materials according to the swelling or non-swelling behavior of their constituent clay minerals. The good agreement between the two methods highlights the mini-slump as a practical, field-compatible tool, while SAOS provides a deeper micro-structural insight into clay pastes (i.e., particle interactions). Together, these approaches offer complementary tools to rapidly classify clays and predict their suitability for earthen construction applications. This paves the way toward optimized, pourable raw earth formulations as a sustainable alternative to high-emission building materials.