Breathing stones: developing laser-based spectroscopic methods to study moisture transport by historic limestone
Keywords: Cavity ring-down spectroscopy
heritage
Limestone
Drying kinetics

The deterioration of porous building materials occurs via physical, chemical and biological decay processes which are facilitated by the presence of water. Current methods of measuring moisture content in building stones have inherent limitations, with many requiring laboratory calibration and providing only limited spatial and temporal resolution, thus offering minimal insight into the water transport kinetics. The aim of this thesis is to develop and apply a laser-based spectroscopic technique as a novel method to measure the moisture dynamics of porous building stones, in order to enhance understanding of durability and assess the efficacy of conservation treatments for limestone deterioration. In particular, this work applies the highly sensitive and selective technique of cavity ring-down spectroscopy (CRDS) to measure the release of water and its artificially enriched isotopologues from a selection of commonly used building limestones, with a focus on those used in the historic centre of Oxford, and for comparison, a sandstone.
This thesis begins with an overview of the deterioration processes and conservation treatments of porous building stones, with particular focus on limestone and the application of Ca(OH)2 nanoparticles, known as nanolimes. Following this, a review of the current methods of measuring and monitoring moisture content, considering their advantages and disadvantages, is presented. In order to achieve the research aim, an understanding of the material and the technique is required, and thus, the fundamental theory of gas and liquid transport within porous media, and that of laser absorption spectroscopy and optical techniques is detailed. The development of the near-infrared cavity ring-down spectrometer and the prerequisite measurements required are also described.
The application of the developed instrument to the research aim is presented in the form of three peer-reviewed publications. The first paper describes the first measurements of moisture content and flux from limestone samples using CRDS, with the determined drying kinetics consistent with the well-established two-phase drying behaviour exhibited by porous media. The CRDS measurements are benchmarked against the traditional method of gravimetric analysis, and the spectroscopic mass determined is in excellent agreement with the gravimetric mass. The selectivity and sensitivity of the CRDS instrument is then shown in measurements of isotopically labelled water, and the drying kinetics are shown to be self-consistent when monitoring HDO and H2O. Following this, the flux of isotopically labelled water is measured under varying relative humidities, and a departure from the distinct two-phase drying behaviour with increasing humidity is observed.
Building on this work, the second paper investigates the drying behaviour of five commonly used building limestones, and a sandstone for comparison. The relationship between the pore size distribution and the drying kinetics is investigated, and while an increase in the phase I drying time is observed with increasing porosity, no clear link with the phase II behaviour is determined. Further investigations into the relationship between the drying kinetics and environmental conditions is achieved through measurements under varying temperatures. An activation energy associated with phase II drying behaviour is determined for a subset of the stones investigated and is found to be comparable with others reported in the literature.
Finally, while the effectiveness and compatibility of conservation treatments has been assessed, a surprisingly small number of studies have investigated their impact on the water transport kinetics. The final paper presents a first investigation into the effect of an ethanol dispersed nanolime on the drying kinetics of a limestone sample, conducted at a variety of temperatures. The presence of nanolime is confirmed in the samples through an increase in the dry mass, observable colour change and decrease in surface roughness following treatment. No changes in the total mass of water released, the phase I drying time or the phase II behaviour are measured, however, small but reproducible increases in the phase I evaporation flux are observed following treatment. The phase II activation energy also remains unchanged, indicating that there is little-to-no change in the internal surfaces.
This thesis presents proof-of-principle, laboratory-based measurements of water transport kinetics in porous building materials using the highly sensitive, selective and quantitative technique of cavity ring-down spectroscopy. The high temporal resolution and versatility of the instrument having thus been demonstrated under varying environmental conditions, this thesis provides the groundwork for future studies concerning water transport kinetics, other degrading species of interest such as pollutants, and conservation treatments.