Dynamic Vapor Sorption (DVS) provides fundamental information about cement and concrete reaction to relative humidity changes and how this may affect, for example, its durability, shelf life, and response to addition of additives. The information provided by the DVS can help then to improve these and other characteristics of cement and concrete.

Pre-hydration

The sorption of water vapor by cement exposed to humidity is known as pre-hydration. Pre-hydration of cement is a subject of interest for manufacturers and applicators. It may lead to undesired consequences, like increased setting time, decreased compressive strength, altered rheological properties and poor response to superplasticiser addition. Pre-hydration involves surface interaction with gaseous water molecules and capillary condensation between particles.

At room conditions, pre-hydration takes place when the ambient relative humidity exceeds the threshold value at which moisture uptake begins (onset point). This implies, for example, that storing cements below this critical relative humidity value would prevent pre-hydration and extend shelf-life.

DVS instruments are excellently suited to measure the water uptake of cement, establish at which relative humidity level pre-hydration starts to take place and at what rate.

Storage

For the impact of storage time, or shelf-life of cement, the amount of sorbed water is an important factor to take into consideration. This will be dependent on factors such as temperature, exposure time, relative humidity and specific properties of the material. It is also possible that cement may already be pre-hydrated before it is delivered to the customer. This can have the effect of altering the properties of cement considerably, depending on its history of manufacture and storage.

With the aid of the DVS, the shelf-life of cement can be estimated at different humidity levels and different temperatures, simulating real-life conditions.

Application Note 09: Measuring the Moisture Sorption Kinetics of Cement using DVS | Request a copy

Pore size

Pore size correlates directly with the durability and sustainability of concrete. It influences the amount of water sorbed via particle size and hence the specific surface area of the pores available for capillary condensation. The transport of these water vapor molecules in porous cement is influenced by condensation and vaporisation –an exchange process of water molecules taking place between pore surfaces and the air inside the pores.

The pore size distribution can be associated with the permeability of cement –the presence of larger pores leading to greater permeability. An accurate characterization of the distribution of the pore size can help in the development of improved and more durable concrete.

Using the analysis software provided by Surface Measurement Systems, the information obtained from the water sorbtion and desorption processes can be used to characterise the pore size distribution.

Diffusion

The diffusion of water vapor depends on the porosity, the geometrical arrangements of the pores in concrete, and on the liquid water content in the pores. For concrete to be durable it must be able to resist damage due to several factors, among them freezing and thawing cycles, corrosion and sulphate attack. The mechanisms of these factors are partially controlled by the concrete’s resistance to ingress of humidity.

With the DVS, the diffusion constants of water vapor into cement and concrete materials can be obtained.

Sorption / Desorption Isotherms

Sorption and desorption isotherms can yield important information for cement and concrete. Besides the most common uses of water vapor sorption isotherms, which are the evaluation of the pore size distribution and the specific surface area, they also yield other valuable information derived from the hysteresis of the isotherm. Its interpretation has been attributed, for instance, to the presence of interconnected small pores which limit the accessibility of larger interior pores to the outside water vapour molecules. With the DVS instruments, obtaining the sorption and desorption isotherms is not a matter of weeks or months of error-prone, painstaking work, but just a few days of automatic and accurate measurements.

Surface Properties

Adhesion, cohesion, dispersive and acid-base (specific) interactions of cementitious compounds and fillers can be studied with the Inverse Gas Chromatography Surface Energy Analyser (iGC SEA) at different temperatures. These properties are of fundamental importance for the understanding and prediction of the surface thermodynamic properties of mineral compounds of cement pastes. This information can help in preventing failure of the resulting assemblies during the whole service life of a structure.

Interaction with other materials

The interaction of construction and building materials, like cement, with other materials of interest, like fuels and explosives, can be studied via the measurement of the adsorption enthalpy of the latter on the former. For this purpose, the Inverse Gas Chromatography Surface Energy Analyser (iGC SEA) is a very useful instrument. The information that this method yields can be used for predicting the useful life of cement when in contact with those materials, or in forensic analysis.

Composite materials are made of a matrix and a reinforcement, having both different physical and chemical properties. The adhesion between them takes place at their interface, yielding then the desired mechanical properties of a composite.

There are many types of composites used as building materials. Examples are wood-plastic composites, glass fiber reinforced and mineral filled thermoplastics. The first ones are made of wood and a combination of a thermoplastic or thermoset, known as wood-plastic composites. Source of wood are generally residuals (sawdust, wood shavings), then mixed with a thermoplastic or thermoset polymer.

Water Sorption Characterization

When subject to environmental attack, as fluctuating changes in the relative humidity, water molecules will diffuse into the matrix of a composite. This can have adverse consequences to the matrix-reinforcement bond. In general for composites, long term exposure to moisture can yield irreversible damage to it due to hydrolysis or microcracking of the polymer matrix. Higher temperatures can accelerate this process. Also, moisture diffusion into the matrix can lead to a reduction in glass transition temperature and softening, resulting in loss of strength. Hence, moisture content and its effects on composite materials are very important for structure designers. DVS instruments can be used to obtain data on the water vapour sorption of composites at different temperatures. This can help in the characterisation of this type of materials and help predict its properties when used in the field.

In the case of wood-plastic composites, moisture sorption in composites directly affects the dimensions and durability of the material due to the swelling of the wood present in the composite. It can also increase its susceptibility to decay by fungi and other micro-organisms. The amount of moisture sorption can be related to the amount of wood, the polymer, and any treatment given to the wood prior to the manufacturing of the composite.

Matrix / reinforcement compatibility

The compatibility of the matrix and the reinforcement in a composite building material can be predicted by measuring the surface energy of both materials independently. The surface energy can be measured using the Inverse Gas Chromatography Surface Energy Analyser (iGC SEA). This is a powerful instrument that yields information about to the surface characteristics of materials (energy, basicity and acidity, specific and dispersive surface energy components) used to study and foretell which materials are best suited to work together as a composite.

Wood is a hygroscopic material because its cell walls contain substantial water sorption sites, composed mainly from polar hydroxyl groups. Many of the wood properties regarding its use as a building material, such as dimensional instability, durability, and fungal decay, are closely related to a wood’s water sorption behavior. This behaviour is complex due to the main components of wood, (cellule and lignocellulose), which build intricate internal cell wall geometries and continuously change structurally, resulting in cell wall dimensional variation.

Sorption Isotherms

Sorption isotherms yield information that has been used, for instance, to estimate the durability of wood, prevent fungal degradation, evaluate water vapor uptake, measure moisture content (hygroscopicity), help to differentiate wood types and age of wood, and study the sorption hysteresis. Water vapor sorption isotherms can be easily and accurately obtained with the DVS instruments in a matter of days due to its ease of operation and automatic data collection.

In restoration, compatibility of old and new materials is highly desired. Hence, investigating the moisture ingress properties of used building materials in an old structure is particularly important. Moisture infusion through a building’s outer structure can have a significant effect on thermal behavior of materials, indoor air quality and air conditioning load. When compatible materials with near identical moisture holding capacity, hygroscopic resistance to increasing/decreasing relative humidity conditions, and thermal expansion properties are used, it is likely that the combination will act well against hydration and humidification. These water vapor sorption properties can be tested with the DVS instruments.

Application Note 61: Characterizing the Restoration Materials for Historic Buildings Using Dynamic Vapor Sorption Technique | Request a copy