Towards low carbon earthen plasters: mechanical performance and Life Cycle Assessment of soil, sand and straw mixtures

Earth-based plasters represent a low-carbon, locally sourced alternative to conventional cement-based renders, contributing to reduced embodied CO₂ emissions, circular use of raw materials, and minimal processing energy in the building envelope. Despite these advantages, wider adoption remains limited due to insufficient systematic characterization and quantified environmental performance. The lack of standard-compliant datasets makes rigorous testing and standardized classification essential to validate their reliability.

This work investigates the influence of soil:sand ratio and straw additive on the mechanical properties, and embodied carbon of earth plasters in order to identify optimized formulations suitable for sustainable building applications. Earth plaster mixes (1:1,1:1.5,1:2,1:2.5,1:3,1:4) were prepared with standardized soil, locally sourced siliceous river sand and wheat straw at 1% of the combined dry mass of soil and sand. Specimens were tested for bulk density, linear shrinkage, flexural, compressive, and adhesion strength, following DIN18947, EN1015-11, EN1015-12, and EN1542 methodologies.

The results indicated that doubling sand content reduced density by roughly 3.2% and linear shrinkage by up to 66%, while flexural, compressive, and adhesion strength dropped by approximately 21%, 28%, and 45%, respectively. Based on DIN18947 classification, almost all mixes met SI requirements across all properties, but the 1:4 mix and 1:1 mix which had excessive shrinkage. The 1% straw addition improves the shrinkage of 1:1 mix making this proportion an acceptable solution. However, adding straw decreases strength, but still keeps it in acceptable classification.

A clear trade-off between mechanical resistance and embodied-carbon was observed. Life-cycle assessment (EN15804+A2) showed that global warming potential (GWP) slightly decreased with increasing sand content, while straw mixes achieved 11% lower GWP, driven by a strongly negative contribution from carbon sequestered in the straw fibers. Earth plaster formulations with straw reduce embodied carbon and improve dimensional stability while satisfying DIN18947, supporting bio-based materials in circular construction.