High-resolution insights into protohistoric construction: a micromorphological study of gypsum use in earthen architecture in Gebut (Lleida, Spain)

Abstract The technique of soil micromorphology has seen a significant increase in its use over the past few decades. However, this tool is still underutilized in protohistoric sites in the Iberian Peninsula, despite having shown promising results. This work examines the use of gypsum as a construction material in the protohistoric site of Gebut, (Lleida, Spain), through a geoarchaeological study based on micromorphology. The results challenge the belief that gypsum can only be used in interior spaces or for decorative purposes. Its use has been identified in floors, plaster, and mortars, both in interior and exterior spaces. Additionally, parallels are drawn with other protohistoric sites in the northeast of the peninsula, where gypsum was used in mortars, plaster, structures related to liquid production, and waterproof floors. Under environmental conditions, gypsum tends to dissolve/precipitate and can naturally occurring at the site through various mechanisms. The study incorporates micromorphology criteria to distinguish between pedogenic and anthropogenic gypsum based on its general spatial arrangement, percentage, or the morphology and size of its crystals. Furthermore, adding specific temper (well-classified ceramic fragments and rubified aggregates) and plant fragments has been documented as very likely to improve mechanical properties and durability. These findings reveal advanced technical knowledge of gypsum in the Protohistoric period, involving pyrotechnical skills, granulometric selection, and the use of natural additives to optimize its properties. The results encourage further physical-chemical studies to validate these hypotheses and reconsider gypsum’s role as a versatile structural material in protohistoric architecture.