Durability of Built Cultural Heritage Materials under different climate conditions

The European project SCORE (Sustainable COnservation and REstoration of built cultural heritage, GA 101007531) deals with a two-way assessment of impacts of the environment on building materials and of materials on the environment. Indeed climate conditions and atmosphere composition determine built cultural heritage (BCH) materials’ behaviour and, at the same time, BCH conservation processes (products and techniques) impact greenhouse gases emissions and eventually climate change.

This contribution presents a characterization of the effects of different climate conditions on several traditional restoration mortars, bricks and calcareous stones. A common strategy plan was designed in order to compare the results of weathering exposition experiments in several location: North of Spain, North-West of France, Denmark and Calakmul Biosphere Reserve in Mexico. Some of the presented results correspond to real conditions experiments started before the common exposition campaign but they will be presented because of their interest.

The exposition support is a frame with a plate surface of 1mx1m, placed at 1m height under two exposure conditions: inclination of 10° and at horizontal position. The exposition frame is oriented facing the predominant wind direction. For comparison we chose to expose two kinds of hydraulic lime mortars from Saint Astier company, bricks from Denmark and Mayan calcareous stone from Campeche area treated with Ca(Zn (OH)₃)₂·2H₂O nanoparticles (CZ) in order to improve their behaviour under the exposure conditions.  Mortar coupons had dimensions of 20 x 10 x  3 cm, while the stone samples ones were 5 x 5 x 3.5 cm. A first set of mortars consisting of two-layer sample was tested with a base of a salt protection mortar and a 1 cm upper layer with a finishing mortar. A second set of mortars consists of a monolayer of a masonry restoration mortar. In each specific site, local or recipe mortars have been also exposed. 

Samples have been characterized before exposition and at regular time intervals during the exposition period, that is not finished. Weight, hardness, deterioration patterns, colour and P wave velocity have been measured at different sample locations. Results indicate that in French locations commercial mortars become better than home-made ones, probably due to the absence of any additive in the home-made recipes. First results for the Spanish exposition site show that velocity measurements start detecting some points where layers begin to separate from each other. Some microcracks start to develop at the surface of one type of mortar respecting the monolayer one.  Finally colour changes have been detected in the two layers masonry mortar.

Such gradual (also visible) degradation has previously also been documented in laboratory examination of fired clay bricks submerged in liquids with varying pH (3,5,7,9,11) and varying duration up to more than one year (432 days). Increasing submersion duration resulted in increased degradation, whereas the different pH values representing exposure to various conditions (acid rain, traditional rain, connection with alkaline mortar) revealed different degradation patterns.

The stone coupons improve some of their properties due to CZ addition at initial periods, but they tend to decrease over time.