Porosity Determination of Gypsum Plaster: Testing Methods, Standards, and Applications

Porosity determination of gypsum plaster is a critical microstructural test that reveals how pore volume and pore size distribution control the strength, thermal insulation, moisture regulation, and durability of plastered surfaces. This article explains the sampling requirements, measurable parameters, standard test methods, and practical applications of porosity analysis for gypsum-based rendering and plastering materials, referencing recognized standards organizations such as GB, EN, ASTM, and ISO. Whether you are a manufacturer conducting quality control or a researcher developing lightweight plaster formulations, this guide provides a complete technical overview of the porosity determination process.

Technical Overview

Gypsum plaster is a hemihydrate-based (CaSO4·0.5H2O) binding material that hydrates and hardens into a matrix of interlocking calcium sulfate dihydrate (CaSO4·2H2O) crystals. Because gypsum plasters are mixed with substantially more water than is chemically required for hydration, the excess mixing water evaporates during hardening and leaves behind an interconnected network of capillary pores. As a result, hardened gypsum plaster typically exhibits a total porosity of approximately 30% to 60% by volume, depending on the water-to-plaster ratio, admixture dosage, aggregate content, and application technique.

Porosity is one of the most influential microstructural parameters of gypsum plaster, directly governing the following performance characteristics:

• Mechanical strength: higher porosity generally reduces flexural and compressive strength.
• Thermal insulation: entrained air within pores lowers thermal conductivity, improving energy efficiency.
• Moisture buffering and vapor permeability: the open capillary network allows gypsum plaster to absorb and release humidity, regulating indoor climate.
• Sound absorption: interconnected pores dissipate acoustic energy.
• Density and dead load: porosity controls bulk density, which affects structural loading on walls and ceilings.
• Durability: pore structure influences water absorption, freeze-thaw resistance, and long-term integrity of the render.

Porosity determination for gypsum plaster is typically performed in accredited third-party laboratories or in-house research facilities using principles defined in standards such as GB/T 28627 (Gypsum plaster), EN 13279-1 and EN 13279-2 (gypsum binders and gypsum plasters), and ASTM C472 (physical testing of gypsum and gypsum plasters), often combined with generic porosity measurement standards including ISO 5017, ASTM C20, ISO 15901-1, and ISO 12154.

Testing Samples

Standard-Cured Laboratory Prisms

Laboratory samples are prepared by mixing the plaster with water at the manufacturer's recommended or standard consistency, casting the fresh paste into 40 mm × 40 mm × 160 mm steel prisms (or comparable cube molds), and curing at 23 ± 2 °C and 50% to 60% relative humidity. Specimens are then dried to constant mass at a low temperature of 40 ± 5 °C. Low-temperature drying is essential: gypsum dihydrate begins losing crystal water at elevated temperatures, which would artificially alter the pore structure and produce inaccurate results.

Field Core and Cut Samples

For construction quality verification, cylindrical cores (commonly 50 mm in diameter) or rectangular sections are extracted from plastered walls and ceilings using a rotary core drill or saw. The sampling location, plaster thickness, and age of the render are documented, and surface layers contaminated with substrate material are removed by grinding before testing.

Granular and Powder Samples

For true (skeleton) density measurement by gas pycnometry, dried plaster fragments are crushed and sieved to a fine granular fraction. Powder samples eliminate sealed macro-voids and allow the displacement gas to access the full skeleton volume, which is required for accurate total porosity calculation.

Testing Items

A comprehensive porosity characterization of gypsum plaster typically includes the following measurable parameters:

• Total porosity (volume percentage of all open and closed pores).
• Open (apparent) porosity: pores connected to the surface and accessible to saturating fluids.
• Closed (sealed) porosity: isolated pores, calculated as total porosity minus open porosity.
• Pore size distribution across capillary and macropore ranges.
• Median and average pore diameter (most frequent pore throat size).
• Total pore volume per unit mass (cm³/g) or per bulk volume (cm³/cm³).
• Bulk density (g/cm³) and true (skeleton) density (g/cm³).
• Water absorption and saturation coefficient, when required for durability assessment.

Testing Methods

Vacuum Saturation with Hydrostatic (Archimedes) Weighing

This is the most widely used method for determining the open porosity of hardened gypsum plaster. A dried specimen of known dry mass (m1) is placed in a vacuum desiccator, evacuated to remove air from the pore network, and then saturated with degassed water under vacuum. The saturated specimen is weighed while suspended in water (m2) and again in air in the surface-dry condition (m3). Open porosity is calculated as Pa = (m3 − m1) / (m3 − m2) × 100%. The approach follows the general principles of ASTM C20 and ISO 5017, adapted for the water-sensitive, temperature-sensitive nature of gypsum materials.

Gas Pycnometry Combined with Bulk Density Measurement

Total porosity is calculated from the ratio of bulk density to true density using the relationship P = (1 − ρbulk / ρtrue) × 100%. The true (skeleton) density of the powdered specimen is measured by helium pycnometry in accordance with ISO 12154, while geometric bulk density is determined from the mass and dimensional volume of the dried specimen, or by hydrostatic weighing. Subtracting the open porosity from the total porosity yields the closed porosity.

Mercury Intrusion Porosimetry (MIP)

MIP provides both pore volume and pore size distribution, typically covering pore throat diameters from approximately 3.6 nm up to several hundred micrometers. Mercury is forced into an evacuated specimen under incrementally increasing pressure, and intrusion data are interpreted through the Washburn equation using a mercury contact angle of about 130° and surface tension of about 0.485 N/m. Testing is conducted in accordance with ISO 15901-1 and ASTM D4404. MIP is particularly valuable for characterizing the fine capillary pore structure that dominates gypsum plaster behavior, although the high pressures involved may slightly distort very weak, highly porous specimens.

Low-Field Nuclear Magnetic Resonance (LF-NMR)

Water-saturated specimens are analyzed by measuring the transverse relaxation time (T2) of hydrogen protons, which correlates with pore size. LF-NMR is non-destructive, does not require mercury, and is well suited to monitoring water distribution, drying behavior, and pore structure evolution in gypsum plaster over time.

Scanning Electron Microscopy (SEM) and Image Analysis

SEM on fractured or polished cross-sections, optionally supplemented with automated image analysis, provides qualitative and semi-quantitative visualization of crystal intergrowth, pore geometry, air voids, and the effects of lightweight aggregates or admixtures on the microstructure.

X-Ray Computed Tomography (CT)

Micro-CT enables three-dimensional, non-destructive reconstruction of the pore network in small specimens, allowing direct visualization of macropores, air voids, and pore connectivity without specimen preparation artifacts.

Applications

Porosity determination of gypsum plaster supports a wide range of industrial, regulatory, and research objectives:

• Factory quality control and conformity assessment of plaster products against GB/T 28627, EN 13279-1, and related national specifications.
• Formulation development, including optimization of retarders, air-entraining agents, cellulose ethers, and lightweight aggregates to balance workability, strength, and insulation.
• Thermal insulation plaster research, where porosity data feed thermal conductivity models and energy-performance declarations.
• Hygrothermal building simulation, using measured open porosity and moisture storage curves as input parameters for moisture buffering and vapor transport modeling.
• Construction dispute analysis and failure diagnosis, identifying over-watered mixes, poor curing, or frost damage in field-rendered plaster.
• Durability and freeze-thaw studies, where pore size distribution and saturation coefficient indicate susceptibility to moisture-related degradation.
• Acoustic panel and sound-absorbing plaster development, correlating pore connectivity with absorption coefficients.

FAQ

What is the typical porosity of hardened gypsum plaster?

Depending on the water-to-plaster ratio and formulation, hardened gypsum plaster generally shows total porosity in the range of roughly 30% to 60% by volume. Machine-applied base coats and lightweight insulating plasters tend toward the upper end of this range, while dense finishing plasters occupy the lower end.

What is the difference between open and closed porosity?

Open porosity consists of pores connected to the specimen surface and accessible to water, mercury, or gas, while closed porosity consists of isolated, sealed voids. Open porosity governs water absorption, vapor transport, and moisture buffering, whereas closed porosity mainly contributes to thermal insulation and density reduction.

Why must gypsum plaster specimens be dried at low temperature?

Calcium sulfate dihydrate begins to release crystal water at moderately elevated temperatures, converting back toward hemihydrate and permanently altering the pore structure. Drying to constant mass at 40 ± 5 °C, rather than at 105 °C as used for cementitious materials, preserves the authentic microstructure of the hardened plaster.

Which test method is best for pore size distribution?

Mercury intrusion porosimetry is the established method for quantitative pore size distribution in the nanometer-to-micrometer range, while low-field NMR and micro-CT offer non-destructive alternatives for larger pores and moisture-related studies. Combining MIP with pycnometry gives a complete picture of both total and distributed porosity.

How does the water-to-plaster ratio affect porosity?

Excess mixing water above the stoichiometric hydration requirement evaporates and leaves capillary pores, so porosity increases almost linearly with the water-to-plaster ratio. Each increase in porosity typically corresponds to a measurable reduction in flexural and compressive strength, which is why consistency and porosity must be balanced in mix design.

How long does porosity testing take?

Standard vacuum saturation and density-based tests are usually completed within one to two weeks, including low-temperature drying to constant mass, which may require several days for dense specimens. MIP, NMR, and micro-CT analyses add only a few days once conditioned specimens are available.