Mineralogical and microstructural characterisation of rammed earth and earthen mortars from 12th century Paderne Castle

Abstract

Abstract In many countries, earth was a very common construction material until the middle of the 20th century, being rammed earth one of the most widespread construction techniques. Nowadays, there are still a large number of constructions with this traditional technique, from vernacular dwellings to monuments. Therefore, rammed earth constructions integrate cultural, historical, technical and architectonic recognised value and the constant neglect of this type of constructions, without maintenance and conservations measures over time, or with inadequate interventions, presents a worrying reality. This paper aims to contribute towards increasing the knowledge on materials from 12th century Paderne castle, located in Algarve, south of Portugal. Data from interventions performed on the castle in the 20th and 21st centuries were gathered. Samples from the rammed earth castle tower and walls and mortar from a more recent chapel were collect during the 21st century interventions and characterised, mainly by microstructural and mineralogical techniques. Most of them present calcareous and siliceous aggregates in their composition, except for samples collected from the wall where no siliceous aggregates were detected. Rammed earth samples present a brownish colour being the binder a mixture of clayey earth and lime, and the coarse aggregate mostly limestone. Iron oxide/hydroxide and clay minerals, very common in earth composites, are detected and a lime + clay binder/aggregate ratio of 1:1.5 in volume is the most frequently found. The large amount of calcite in all rammed earth samples is an indication of the use of carbonate gravel mixed with a high content of air lime. The presence of white nodules in the earthen matrix induces that the lime was probably applied as quick lime and, afterwards, hydrated by mixing with the moistened earth. The lime was used as a stabiliser (technique called “military” rammed earth) to promote a hardening process by carbonation and increase durability to weathering. Therefore, characterisation results obtained in this study allow to draw conclusions about the materials and construction techniques used in the past, in order to support future compatible, effective and reversible interventions in this and in similar historic military rammed earth constructions.