Interactions on site between powdering porous limestone, natural salt mixtures and applied ammonium oxalate

Tabitha Dreyfuss

doi:10.1186/s40494-019-0247-0

Abstract

The consolidation of powdering porous limestone with ammonium oxalate treatment is hereby being examined through its performance in the field. The literature to date reports this treatment’s potential use for consolidation and protection (from acid attack), while still retaining the water transport properties and supporting compatibility. This inspired a long-term research project which started in the laboratory and progressed to real buildings with a specific focus on the interactions in the stone taking place between the treatment and salt mixtures that may be present on site. This study focuses on the outcomes following a 24-h application of a 5% ammonium oxalate poultice on a historical soil-retaining wall located in a seventeenth century garden. In this case, besides calcium oxalate, the by-product formation of ammonium magnesium chloride was detected through X-Ray Diffraction (XRD). This is a first-time reported finding, which is important to consider within the environmental context of built heritage and ammonium oxalate treatment.

Highlights

Background to the study The subject of consolidation forms the basis of this research study where solutions for the mitigation of powdering limestone were considered
The incidence of gypsum is not considered to be a by-product of ammonium oxalate treatment but rather a pre-existing mineral within the stone’s depth and which was mobilised during treatment
Ammonium oxalate treatment was applied to powdering limestone on site in the presence of natural salt mixtures

Summary

Introduction

Background to the study The subject of consolidation forms the basis of this research study where solutions for the mitigation of powdering limestone were considered. The desirable intervention in this case would call for a reduction in powdering through consolidation of the loose grains to each other and to the parent stone beneath while slowing down further damage This would need to be achieved in the presence of naturallyoccurring salt mixtures, while still retaining the various properties of the stone including visual, physical and chemical aspects. Globigerina Limestone is a finegrained stone which is composed of calcium carbonate in the form of calcite crystals which are cemented together by micrite [1]. This limestone weathers readily in a salt-laden environment with powdering forming one of the main deterioration manifestations. Further testing on quarry Globigerina Limestone in the initial stages of this research project showed that the porosity accessible to water was 21.36% and the ratio of mesopores (6 nm < Ø < 50 nm) to macropores (Ø > 50 nm) was 1:7 [4]

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