Research and Comparative Analysis of Earth Mortars for Conservation and Restoration of Archaeological Structures: The Case of the Sant Jaume Site (Alcanar, Catalonia, Spain)

This paper characterises the original earth mortars of the fortified settlement of Sant Jaume (Early Iron Age, ca. 750–650 BC; Alcanar, Catalonia, Spain) and comparatively evaluates earth-based repair mortars stabilised with vegetable oils for its conservation. Previous analyses identified clay as the only binder, with no evidence of lime, so the intervention required compatible, locally sourced formulations. The original mortars and the excavated collapse-layer sediment were characterised by dry-sieving granulometry, ATR-FTIR spectroscopy, and acid digestion. Forty-two experimental mortars were then prepared from the sieved sediment, combining olive or linseed oil (3, 10, and 18 vol%) with linen threads or sisal fibres (25, 50, and 75 vol%), and tested for Shore D surface hardness, water drop absorption, and open porosity. Linseed oil clearly outperformed olive oil: it suppressed surface powdering, yielded absorption times up to the 60 min test limit, and was the only binder that withstood 24 h vacuum water saturation, whereas all olive oil mortars disaggregated. Linen threads provided better fibre–matrix adhesion and shrinkage control than sisal fibres, which induced surface cracking. Mortars combining 3–10 vol% linseed oil with linen reinforcement performed best (surface hardness up to 50.1 HD; open porosity 24–32%) and are proposed as compatible repair mortars for the site.