Tropical earthen plastering mortars: Effects of earth mineralogy, earth-to-sand proportion, and moisture conditioning

This study investigates the physical and mechanical behaviour of tropical earthen mortars, focusing on the influence of clay content, mineralogy and moisture condition. To address the limited documentation on tropical earthen mortars, eleven mortars were produced using three tropical earth and three proportions of added sand. Two of the earth were of pedogenetic origin, whereas the third was derived from gneiss aggregate washing residue. The added sand consisted of crushed basalt fines generated during gravel production. Materials were characterised by particle-size distribution and mineralogical composition, and the mortars were evaluated for porosity, bulk density, shrinkage, flexural (FStr) and compressive (CStr) strengths. Moisture sensitivity was assessed under high-humidity conditioning through the degree of saturation at mechanical failure. The results show that clay content and mineralogical characteristics strongly influenced water demand, shrinkage, porosity and density. Porosity ranged from 30% to 48%, with higher values in clay-richer mortars. Sand addition substantially reduced shrinkage, reaching up to 79% in the most clay-rich mixtures. FStr ranged from 0.6 to 1.0 MPa, while CStr ranged from 0.5 to 1.0 MPa, with several formulations exceeding the minimum requirement of EN 998-1. Moisture had a marked effect on performance, with CStr generally decreasing more sharply than FStr as saturation increased. Overall, the results demonstrate that clay content, mineralogy and moisture state play a decisive role in the behaviour of low-embodied-energy tropical earthen mortars, supporting their wider application as building plasters. • Eleven mortars from three tropical earths, with varied sand contents, were tested. • Fluidity index captured workability–water demand trends across formulations • Basalt crushed sand correction reduced shrinkage by up to 79% in clay-rich mortars • Strength was evaluated versus degree of saturation to quantify moisture sensitivity • Compressive strength decreased more than flexural strength as saturation increased