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.