HT_1D: Pore structure descriptors and measurements of moisture storage

This is the first of three test protocols developed in year 2003.

1. Pore structure descriptors

The following descriptors of the pore structure are required:

· Solid phase density and bulk density

· Total and total open porosity

· Capillary moisture content (see protocol 1E)

· Water absorption coefficient (link to HT-1H)

1.1     Solid phase density

The solid phase (or matrix density), r_s, refers to the density of the solid phase and is expressed as the ratio of the total mass of the solid material to the volume of the material (note that it excludes pore spaces). It is expressed in kg/m³. Typically, the solid phase density is measured with a commercial helium-pycnometer. The helium is selected because the molecules of this inert gas are smallest of all existing molecules and can enter practically all pores. The tested material is crushed into a powder grade and dried. It is then placed in the measurement chamber of a pycnometer.

A gas pycnometer contains two calibrated volume chambers. Initially, the gas is contained in the reservoir chamber with volume Vr, and pressure Pr, and in the sample chamber with volume Vs, and pressure Ps. The pressures inside the two chambers are different.  The test is initiated by opening the valve separating the two chambers and the resultant pressure (P) upon attaining equilibrium is measured. This process is defined by equation (1)

VsPs + VrPr = (Vs + Vr) P                                                                                    (1)

Where: the subscripts s and r denotes the sample- and the reservoir-chambers respectively. P is the common pressure after the equalization of both chambers. Initially, the measurement is performed with an empty sample chamber to check the precision of the volume measurement for the empty chamber (Vso). Next, the measurement is conducted with the test specimen placed inside the sample chamber. The actual free volume of the sample chamber becomes

Vs = Vso – Vf                                                                                       (2)

Where: Vf is the volume of the test specimen and the resulting pressure Pr relates to the actual free volume of the chamber.

Calculating this volume from equation (3)

Vs = (Pr –P) Vr / (P- Ps)                                                                              (3)

Knowing the volume of the empty sample chamber Vso, the volume Vf for the solid material placed inside the sample chamber can be determined.

1.2.    Material (bulk) density

The density (sometimes also called the bulk density), r, kg/m³; is the ratio of the mass of the dry solids to the volume of the material with inclusion of the pore space. The weight of a specimen dried to a constant mass is measured. Typical drying temperatures include: 105°C for ceramic bricks, 80°C for calcium silicate, 60°C plasters and 55 ^oC for organic foams. After the specimen’s geometrical dimensions are measured. The volume and the resulting density of the material are calculated.

1.3. Total porosity

Knowing solid phase density (matrix density) and material density (bulk density) the total porosity, f_t [-], for the material can be calculated. The ratio r/r_s is the fraction of the total volume occupied by the solids. Total porosity is defined by the remaining part of the volume:

f_t = (1 -   r/r_s )                                                                                        (4)

1.4. Total open porosity (or vacuum saturation)

The open porosity, f_o [-] is defined as the ratio of the open-pore volume to the total volume of the sample, V. This concept defines the amount of pore space that can be occupied by water. Typically, the total porosity is measured by means of a vacuum saturation. A test container is connected to a vacuum pump on one side and de-aired, distilled water on the other side. The measurement begins with a placement of the specimen into the container and evacuating the air from the container to a level of 10 to 20% of its atmospheric pressure. Then by a partial opening of the valve connected to the water tank, the container is slowly filled with de-aired water until the specimen is fully immersed. To avoid breakage of the material structure, the water level is increased at a slow rate. Typically 1 to 2 hours is required to have the specimen fully immersed in water.

Sometimes a solution containing salts/sugars extracted from the specimen is used to determine the vacuum saturation. The specimen is first immersed in a suitable solution and saturated without applying the vacuum. The specimen immersed in the solution is then subjected to a vacuum with the rate being increased slowly and steadily over the period of one to two hours.

The open porosity is calculated using the following equation:

f_o = (m_w – m_d) /(V r_w)                                                                             (5)

Where r_w is the density of water [kg/m³], m_w and m_d are the mass of the water-saturated specimen and the mass of the dry specimen, respectively.

1.5.       Capillary moisture content

As discussed later the second stage of free water intake (imbibition) starts when the specimen reaches capillary moisture content. Since there is no adequate method to determine this property, a provisional test procedure is presented in the protocol 1.

2.         measurements of Moisture storage under desoprption (drying)

Moisture storage results are expressed as the relationship between the volumetric water content and a descriptor of the moisture potential. In hygroscopic region, such a descriptor is the relative humidity of the pore-air, in above-hygroscopic region it is the capillary suction (the difference between pore-water pressure and air pressure acting on the water meniscus in the equilibrium state). The relation between moisture content and capillary suction is called the moisture retention curve (MRC).

The moisture (water) content, i.e. a difference between wet and dry mass can be expressed either in volumetric (m³), or in mass (kg) basis of water, in relation to the volume (m³) of the material. It is denoted with the symbol q when using the volume basis [m³/m³], and with w when using mass per volume relation (kg/m³). Equations (6) and (7) present these concepts

q = (m – m_d) /(r_w V)                                                                                (6)

w = (m – m_d) /V                                                                                     (7)

Figure 1 shows two series of measurements repeated on the same set of specimens following drainage and a subsequent water re-saturation. The relation between moisture content and the capillary suction (expressed as water overpressure) in the repeated test was different from one established in the original test series. While the difference in overall may appear small, it is important when one wants to determine the pressure conditions where the drying run would start. (The air pressure needed to pass an air bubble through the water saturated material is called the bubbling point).

Traditionally, the extreme drying curve (desorption MRC curve) would be started at the total open porosity (vacuum saturation). Yet, such a measurement includes the time dependent part of moisture hysteresis (see Figure 2). Therefore, a preferred option includes starting the desorption measurements just above the capillary moisture content (the latter property is defined in the protocol 2 on measurements of water absorption coefficient (A-coefficient).

Figure 1. The equilibrium moisture content of Calcium silicate HAMSTAD measured at TUD with a pressure plate apparatus on the same specimens in a repeated test.

Figure 2. Schematic representation of wetting (adsorption) and drying (desorption) branches of the moisture retention curves (MRC) indicating different types of moisture hystereses.

Starting the desorption loop from a level slightly above the capillary moisture content eliminates the time dependent effects from the hysteresis and, as shown in Figure 2, permits measurement of the primary MRC desorption loop.

2.1        Desorption MRC measured with pressure plates

Figure 3 shows several equilibrium moisture content points measured with a pressure plate apparatus. Values measured at 1 Pa and 1400 kPa are the averages calculated from either 30 or 23 tests. Superimposed on this graph are values measured on one set of specimens at four different pressures. The agreement between these two sets of data is good.

Figure 3 allows discussing a minimum number of the measuring points. It also highlights the importance of the information on the ends of the measured range. The first measuring point must be below, but not too far off, from the expected bubbling point (i.e., minimum pressure at which the drainage process can be started). In the discussed case, the test at 80 kPa appears quite adequate.

The lower end of the over-hygroscopic moisture content is determined by the upper range of the hygroscopic measurements. If the test at 96.9% RH is performed with a sufficient degree of temperature control i.e., at capillary suction of p = 4265 kPa [(1000 x 462 x 293 x ln(0.969)] so measuring the next equilibrium moisture content at 1400 kPa is quite appropriate.

Next is the question on a number of required points between the upper and lower bounds. The answer depends on the type of a model used to evaluate the consistency of input data. In the engineering model for material characterisation, one point will be used for the curve fitting and the second point is needed for the curve optimisation, thus two points are considered as a minimum. For the enhanced precision, one requires three measurement points.

It is evident that (see Figures 1 and 3) that an MRC desorption should be measured on the same set of specimens starting from a capillary saturation. If one knew the pore-size distribution curves, the first measuring point should be a large-pore peak and the second point at the lower end of the measuring range (here 1400 kPa). As these measurements must be performed on the same specimen, the limits of time preclude more extensive testing. The third test is recommended only for testing with enhanced precision.

Figure 3. Equilibrium moisture content measured with a pressure plate apparatus at TUD. Moisture content values measured at 1 Pa and 1400 kPa are the averages from either 30 or 23 tests. Superimposed on this graph are values measured on one set of specimens at four different pressures.

To determine the water retention characteristics a pressure plate apparatus is used (ISO 11274).

Dependent upon the range of capillary potential, a mid-range and high-range systems are used. They permit measurements at air over-pressures ranging from 1Pa to 1500 kPa. The pressure plates used by TUD are shown in Figure 4.

Figure 4 A pressure plate apparatus used in determining moisture equilibrium in over hygroscopic range of the moisture retention characteristics.

In principle, each apparatus consist of a pressure chamber, a ceramic plate with a specified bubbling pressure (i.e., pressure at which an air bubble may pass the specimen) and a gas pressure supply regulating system. Typically, the ceramic plates have the bubbling points of 50, 100, 300, 500 and 1500 kPa and allow the MRC to be determined within the range of a few Pa up to 1500 kPa.

The test starts with a saturated specimen whose moisture content is slightly above that of a capillary saturation.  The specimen is placed inside a pressure plate chamber on a filter paper or silt/kaolin mixture to ensure a good contact between the specimen and the ceramic plate. The chamber is closed and a desired air over-pressure is applied, causing the water to be drained from the specimen. The outflow continues until the equilibrium is reached between the capillary suction and the applied external air pressure. The moisture content is determined from the difference between the total and dry mass of the specimen. The readings of the water outflow are taken every 3 to 5 days. A criterion for establishing the equilibrium moisture content requires that the difference in mass between two non-consecutive weightings be less than 0.1% over a minimum period of 10 days. 

NOTE: If the specimen was previously tested in the pressure plate apparatus the excess of water in the specimen may be insufficient to ensure a good contact between the specimen and the pressure plate. Additional water is therefore added in the beginning of the test.

2.2        Desorption MRC measured with salts solutions (sorption isotherm)

This method involves moisture exchange between the specimen and the atmosphere with constant and known relative humidity.

Figure 5.  A system of desiccators at TUD; low energy ventilators inside the chamber reduce the time to achieving the defined boundary conditions around the tested specimen.

Moisture content at equilibrium is determined and associated with the thermodynamic water potential calculated from the relative humidity. The specimens are placed in a desiccators containing a pre-selected saturated salt solution (Figure 5).

The pre-selected saturated salt solutions have the corresponding relative humidity levels at a room temperature: 97.3% by K₂SO₄; 96.0% by KH₂PO₄; 84.7% by KCl; 75.4% by NaCl; 58.2% by NaBr; 32.9% by MgCl; 22.5% by CH₃COOK and 11.3% by LiCl. A criterion for establishing the equilibrium moisture content requires that the difference in mass between two non-consecutive weightings be less than 0.1% over a minimum period of 10 days.  

After completing the required series of measurement within the hygroscopic range of the water retention characteristics, the specimens are re-dried to a constant mass. A specimen is assumed to be oven dry, when the weight loss is less than 0.1% between three non-consecutive measurements performed within 24 hours.