Artificially induced calcium oxalate on limestone in urban environments – New findings

Abstract

Calcium oxalate, known for its greater resistance to deterioration when compared to calcium carbonate, is the desired end product when ammonium oxalate treatment is applied to calcareous stone. Calcium oxalate can form naturally, on exposed limestone and marble architecture and statuary, although over long periods of time. Two mineral forms can be found, these being the monohydrate whewellite and the dihydrate weddellite. This paper focuses on the first detection of the formation of weddellite, following ammonium oxalate treatment, applied to an inland urban church in Malta built with porous limestone. This new finding is discussed within the context of the conditions presented by this particular site, and the resultant products that could potentially form between the treatment and soluble salts that may be present in the stone. Investigations first started in the laboratory with artificially salt-contaminated Globigerina Limestone, using individual sodium salts of chloride, sulfate and nitrate. The more stable whewellite was consistently found to have been produced in these situations without any by-product formation. This steered the research towards real buildings where salt mixtures would be present. Extending the chloride, sulfate and nitrate laboratory case studies to the field, a marine, garden and urban building were respectively selected to represent each scenario through expected salt predominance within existing salt mixtures: chlorides in the marine environment, nitrates in the garden location and sulphates in the urban setting. The calcium oxalate formed in the first two sites – a coastal bastion and a garden wall – was whewellite in both cases, as per results from the laboratory studies. Weddellite was only formed in the urban environment case scenario. This paper discusses the determining factors in ammonium oxalate treatment of porous salt-infested limestone in relation to the resulting form of calcium oxalate produced. These were found to be the calcium to oxalate ratio and the environmental conditions during treatment.