Determining Water Transport Kinetics in Limestone by Dual-Wavelength Cavity Ring-Down Spectroscopy.

Understanding how water moves through porous materials is essential for preserving heritage structures. Prior research has shown that water influences physical and chemical decay in such materials. However, the specific drying mechanisms and transport rates remain unclear. Established methods like gravimetric analysis provide mass loss data but lack isotopic resolution. This gap motivated the development of new spectroscopic techniques. No prior work had resolved isotopic fluxes during drying. Existing models assume two-phase behavior but lack validation under humid conditions. The need for sensitive and selective monitoring tools persists. This study addresses these limitations by introducing a novel analytical approach.

The aim of this work is to evaluate the use of cavity ring-down spectroscopy for tracking water transport in limestone. The specific problem involves measuring drying kinetics and isotopic fluxes in porous materials. The motivation stems from the need for high-resolution monitoring of water movement. Traditional methods lack the sensitivity for isotopic analysis. This study tests whether CRDS can capture both mass and isotope data. The approach focuses on small limestone samples under controlled drying conditions. The goal is to determine if CRDS can provide insights into transport mechanisms. The results could refine models for porous media drying.

The study uses near-infrared cavity ring-down spectroscopy to monitor water release from limestone. Small limestone samples are dried under nitrogen flow with controlled humidity. The setup includes dual-wavelength CRDS to distinguish isotopologues like HDO and H2O. The drying process is observed under varying humidity levels at room temperature. Mass loss is tracked spectroscopically and compared to gravimetric data. The method captures both total water and isotopic fluxes simultaneously. The technique allows for real-time monitoring of drying phases. The approach validates CRDS against established methods like gravimetry.

The drying kinetics of limestone under low humidity show two distinct phases. Phase I is a constant drying rate period followed by phase II with a falling rate. The diffusivity during phase II was measured at 3.0 × 10^-9 ± 1 × 10^-10 m² s^-1. CRDS-derived mass loss values align closely with gravimetric measurements. Isotopic analysis of HDO fluxes was successfully performed using dual-wavelength CRDS. The HDO diffusivity was found to be 3.2 × 10^-9 ± 4 × 10^-10 m² s^-1. At higher humidity, the drying curves deviate from the two-phase model. These findings confirm the sensitivity and selectivity of CRDS for isotopic tracking.

The authors propose that CRDS is a viable method for monitoring water transport in porous materials. The technique provides both mass and isotopic resolution during drying. The measured diffusivity values are consistent with established models under low humidity. Deviations at higher humidity suggest limitations in current drying models. The ability to track isotopically labeled water is a key advantage of CRDS. The study demonstrates the potential of CRDS for refining transport models. The results support the use of spectroscopy for real-time monitoring of drying processes. The authors suggest that these findings may improve understanding of water movement in heritage materials.