Using Gravimetric Sorption Instrumentation to Investigate the Vapor Sorption Properties of Building Materials

Water vapor interactions with solid materials greatly influence a number of processes in industries and materials such as polymers, fuel cell membranes, foods, pharmaceuticals, and building materials. With regards to building materials, moisture sorption is a critical parameter requiring stringent assessment, as damage caused by moisture shortens the lifespan of buildings. In addition, moisture infusion through the outer structure of buildings can have negative effects on air-conditioning and indoor air quality.

In many industries, measuring vapor sorption properties of solid materials using automated gravimetric vapor sorption instruments has become common practice. This article covers the applicability of Dynamic Vapor Sorption (DVS) instruments to analyze various building materials.

Experimental Procedure

Figure 1 shows a schematic of the DVS-Advantage instrument, which gravimetrically measures vapor uptake and loss using the SMS UltraBalance. The SMS UltraBalance comes with two configurations; a capacity of 1g and mass resolution of at least 0.1µg, and a capacity of 4g and mass resolution of at least 1.0µg. Generatation of the partial vapor pressure around the sample is achieved using electronic mass flow controllers to mix the saturated and dry carrier gas streams.

Figure 1. Schematic overview of the SMS DVS-Advantage instrument

The concentration of water and a wide variety of organic vapors can be actively measured and controlled using the DVS-Advantage instrument, thanks to its proprietary optical sensor that is specifically tuned for a variety of solvents.

With this technology, organic vapor concentrations can be measured and controlled in real time using the instrument. Although balance configurations are similar (1 gram or 4 gram capacity), the DVS-Intrinsic instrument is designed for water vapor only.

Experimental Results

Cements

It is crucial to study moisture transport behavior of porous building materials, such as cement, to improve performance. Temperature and vapor concentration are considered to be the key driving principals to understand moisture transfer in these materials.

For concrete and reinforced concrete structures, degradation pathways, such as; alkali-silica reactions, freezing and thawing cycles, chloride or sulphate ingress, and carbonation, are all moisture dependent.

For example, a critical relative humidity range exists where carbonation is favored, and this is important for accelerated carbonation tests. In addition to this, the mechanical durability of reinforced concrete structures can be impaired by water. Also, hardened cement paste is known to be a dynamic system with regards to moisture content.

Moreover, the hardening processes of cement can be effectively studied with water sorption isotherms, and water sorption-desorption isotherms are important parameters in durability evaluation and prediction. As a result, water sorption analyses on cements or cement components by means of gravimetric sorption instruments have been increasingly performed.

The dehydration kinetics (a.) and resulting desorption isotherm (b.) for a cement paste sample at 25°C are illustrated in Figure 2. Figure 2b shows an isotherm with evidence of mass loss. The rapid mass loss beyond 90% RH is most probably because of the loss of loosely bound water.

Between 90% and 40% RH, mass loss is gradual, but a sharper mass loss is observed at 30% RH, possibly due to loss of internal or hydrated water. At 0% RH, another sharp mass loss is observed when the sample is completely dried out.

Figure 2. Drying kinetics (a. [top]) and desorption isotherm (b. [bottom]) for a cement paste sample at 25°C

Figure 3 shows the moisture sorption kinetics for two different dry cement powder samples at 95% RH and 40°C. The mass loss trend over time suggests that the lifetime of Cement 2 is much longer than Cement 1. These two simple analyses demonstrate the applicability of gravimetric moisture sorption instruments to investigate cement-based materials.

Figure 3. Moisture sorption kinetics for two dry cement powders at 40°C

Wood and Wood Composites

For wood and wood composites, measuring and controlling moisture is essential due to its role in the fungal degradation and weathering of wood-plastic composites. Cupping of boards can be caused by moisture content change and moisture distribution.

Oriented strandboard, which is hydroscopic, loses its aesthetic quality and mechanical properties as it becomes dimensionally unstable under exposure to high humidity conditions.

The rate of photochemical degradation of wood is also affected by water sorption on its surface. Moisture sorption behavior and diffusion processes can be of interest for the evaluation of drying kinetics.

Figure 4 presents representative water sorption isotherms for sawdust (a.) and a solid piece of wood (b.), showing the capability of the DVS to study different forms of wood, including slabs, fibers, films, chunks, and sawdust. Moisture sorption kinetics and diffusion coefficients as well as equilibrium isotherm values can be determined using these geometries.

Figure 4. Water sorption (red) and desorption (blue) isotherms at 25°C measured on sawdust (a. [top]) and a solid wood sample (b.[bottom])

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Insulation, Fibers, and Textiles

Moisture sorption measurements are critical for interior and exterior insulation, and interior foams, fibers, and fabrics. Many factors such as fiber type, spin and twist of the yarn, weaving technique, and fabric weight, affect the moisture properties of textile fabrics. For these materials, managing water vapor is crucial due to impact on microbial growth, moisture buffering, long-term material performance, and indoor air quality.

The DVS instrument is a proven technique to measure water vapor sorption characteristics of these materials. Water sorption properties for two carbon cloth samples can be observed in Figure 5a, where the plain sample (red line) exhibits a very low uptake due to its hydrophobicity with a maximum uptake of below 0.01wt% at 95% RH. In spite of the low uptake, the SMS UltraBalance helps record accurate isotherm measurements due to its sensitivity and outstanding baseline stability.

The presence of highly porous carbon powder with a much higher water sorption capacity allows the bilayer sample to show much higher uptake and a hysteresis typical for mesoporus materials.

The use of the DVS helps with the clear differentiation between these samples. The water sorption properties of glass fiber are illustrated in Figure 5b, where the water uptake of the sample during the sorption phase is instantaneous and increases by almost 1 wt % at 95% RH.

Figure 5. Water sorption isotherms for
two carbon cloth samples (a. [top]) and glass fibers (b. [bottom])

However, the water removal is very slow during the desorption phase due to the fiber surface’s strong affinity for water. As a result, the sample retains almost 0.4 wt% water at the end of the desorption isotherm. This water retention tendency over a longer period could result in microbial growth or reduce fiber performance in insulation applications.

Asphalts

Measuring water sorption/diffusion properties is crucial for improving the performance of roofing components. It is important to investigate how water permeation occurs in roofing materials, the drying rate of a wet roof, and how internal moisture escapes from a roofing assembly.

The degree of moisture damage relies on the type, quality, and internal structure of the materials used in the asphalt mixture. Debonding of asphalt from aggregates can be observed in roadway materials because of displacement by water.

Furthermore, the physicochemical properties of asphalt filler are affected by the hydrophilicity of filler materials. Therefore, the surface chemistry of roadway materials must be studied, as well as their moisture sorption properties. The DVS-Advantage instrument can measure both organic and water vapor sorption isotherms, making it suitable to investigate surface energetics of minerals.

The DVS measurements of the dispersive and specific surface energy components for various aggregate samples are illustrated in Figure 6. For these samples, dispersive interactions dominated the total surface energy. Based on the total surface energy, the samples are sequentially ranked: Basalt > Augite > Calcite > Quartz > Feldspars~Granite.

This ranking is in agreement with the empirical observation that these aggregates exhibit a similar order of affinity to the most common asphalt binders. The Quartz sample may vary from expectation because of variations in geographic location or impurity levels, which will have an impact on the surface energy.

Figure 6. Dispersive and specific surface energies of the different aggregate samples

Other Building Material Applications

Using dynamic vapor sorption, moisture sorption properties of other building materials, such as plasters and gypsum board, can be measured. These measurements are gaining significance in recent years due to needing to prevent microbial growth and the capability of these materials to serve as hydroscopic buffers. Since differences in moisture deforms the substrate and coating, it is essential to study the water sorption behavior of paints.

The DVS-Advantage can measure water as well as other vapors, making it suitable to measure the rainfastness and retention behavior of pesticides and herbicides, to hard surfaces such as asphalt and concrete. It can also explore the ability of various interior building materials to serve as sinks for VOCs emitted by other materials.

Furthermore, in-situ video microscopy and spectroscopy (Raman and/or Near-IR) can be coupled with gravimetric sorption measurements using the DVS-Advantage. Vapor-induced phase change or color change can be analyzed with video microscopy and more subtle structural changes in the material such as hydrogen bonding, polymorph identification, and hydrate formation can be elucidated with in-situ spectroscopic measurements (Figure 7).

Figure 7. DVS water sorption results (a. [top]) and in-situ
Raman spectra (b. [bottom]) for MCC at 25°C

Conclusion

The results clearly demonstrate the applicability of the DVS instrument to measure the vapor sorption characteristics of materials such as cement, wood, asphalt, textiles, paint, and insulation in order to handle building-related problems.

Gaining insights into the water sorption behavior of these materials is helpful to understand and control exterior weathering, inter-material adhesion, material stability, and mould growth. Additionally, organic vapor sorption analyses provide information on herbicide/insecticide retention behavior, VOC sorption capacity, and material surface energetics.

This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.

For more information on this source, please visit Surface Measurement Systems Ltd.