The softening coefficient is the key indicator of the water resistance of gypsum plaster (rendering plaster), defined as the ratio of the compressive strength of water-saturated specimens to that of dry specimens. This article explains the determination principle, specimen preparation, test procedure, calculation method, governing standards such as GB/T 28627 and EN 13279, and typical applications of softening coefficient testing for gypsum-based renders.
Technical Overview
Gypsum plaster, also referred to as rendering plaster or plastering gypsum, is a factory-premixed building material based on calcium sulfate binders such as hemihydrate, anhydrite II, or blends of the two. After mixing with water and application, the binder hydrates into a matrix of interlocking calcium sulfate dihydrate crystals that provide high early strength, good workability, and excellent fire performance. However, gypsum-based materials are inherently sensitive to moisture: calcium sulfate dihydrate has an appreciable solubility in water of roughly 2 g/L at 20 °C, and capillary water uptake partially dissolves the crystal contacts and weakens the intercrystalline bonds. As a result, the strength of gypsum plaster decreases significantly when it becomes saturated.
The softening coefficient is the quantitative measure of this water sensitivity. It is defined as the ratio of the compressive strength of water-saturated specimens to the compressive strength of companion dry specimens prepared and cured under identical conditions, expressed as K = fc,s / fc,d. The value ranges from 0 to 1.0 in practical terms; ordinary gypsum binders typically show softening coefficients of approximately 0.2 to 0.4, while water-resistant formulations containing hydrophobic or polymer modifications can achieve values of 0.6 or higher. Determination of the softening coefficient is therefore an essential part of the technical evaluation of gypsum plaster for humid service environments and for conformity assessment against product standards such as GB/T 28627 and the EN 13279 series.
- Core parameter: ratio of saturated compressive strength to dry compressive strength
- Purpose: characterization of water resistance and wet strength retention
- Specimen basis: standard 40 mm × 40 mm × 160 mm prisms
- Typical result range: 0.2 to 0.4 for plain gypsum plasters; 0.6 or above for water-resistant grades
Testing Samples
Laboratory-Prepared Standard Prisms
The preferred sample form is the standard triple-gang prism of 40 mm × 40 mm × 160 mm cross-section, cast from the gypsum plaster under test. The plaster powder is sampled from a delivery lot in accordance with the sampling rules of the applicable product standard, homogenized, and mixed with the amount of water corresponding to the standard consistency or the manufacturer's recommended water demand. Mixing is performed with a standard laboratory mixer to ensure reproducible dispersion and to avoid air entrapment beyond normal levels. The fresh paste is cast into the molds, compacted or vibrated gently, and struck off level.
Production and Field Samples
In addition to laboratory-cast prisms, softening coefficient determination may be performed on samples taken from production batches of hemihydrate, anhydrite II, or blended gypsum renders, on machine-applied plaster from trial panels, and on drilled core specimens recovered from finished render layers during engineering investigations. In such cases, the specimen geometry, drying history, and saturation state must be documented so that the dry and wet strength series remain strictly comparable.
Specimen Grouping
For each determination, a minimum of two statistically comparable series are prepared from the same batch: a dry series and an immersion series. Each series should contain at least three prisms, providing six compressive test values per series when the prisms are broken first in flexure and then in compression on their halved sections. Specimens showing visible defects, improper compaction, or dimensional deviations outside tolerance must be discarded before testing.
Testing Items
The determination program typically covers the following measured and derived quantities:
- Dry compressive strength of the reference specimens, determined after drying to constant mass
- Saturated compressive strength of the specimens after the specified water immersion period
- Softening coefficient K, calculated as the ratio of the two strength values
- Supporting properties such as dry and wet flexural strength, water absorption by mass, and bulk density, which assist in interpreting the softening behavior
Testing Methods
Specimen Preparation and Conditioning
After the initial set, specimens are demolded without damage and conditioned in air at a controlled temperature of approximately (23 ± 2) °C and relative humidity of (50 ± 5) % until the specified age. For the dry series, the prisms are subsequently dried at (40 ± 4) °C to constant mass, defined as less than the permitted mass change between successive weighings. Drying temperatures must remain moderate, because temperatures well above this range can partially dehydrate the dihydrate phase and alter the strength of the gypsum matrix itself. After drying, specimens are cooled to test temperature in a desiccator or dry container before loading.
Determination of Dry Compressive Strength
The dry prisms are first broken in flexure where required by the test scheme, and the resulting halves are loaded in compression between bearing platens or auxiliary plates of 40 mm × 40 mm contact area. Loading is applied at the constant rate specified by the governing standard until failure, and the maximum load is recorded. The dry compressive strength fc,d is calculated by dividing the maximum load by the loaded area for each half-specimen, and the series mean is determined after application of the prescribed validity criteria.
Determination of Saturated Compressive Strength
The immersion series is submerged in clean water at (23 ± 2) °C for the period specified by the product standard, commonly 24 hours, with the water level maintained at least 10 to 20 mm above the top surfaces of the prisms. At the end of the immersion period, each specimen is removed, and surface water is promptly removed with a damp cloth. The specimens are tested in compression in the same manner and on the same loading arrangement as the dry series, within the short time window allowed after removal so that the saturated state is preserved. The saturated compressive strength fc,s is calculated for each valid half-specimen and averaged.
Calculation, Expression, and Reporting
The softening coefficient is calculated as K = fc,s / fc,d, using either the ratio of the series means or the mean of the paired individual ratios, according to the rule defined by the applicable standard. The result is reported to two decimal places or as a percentage. The test report should identify the plaster type and batch, water demand, specimen preparation and curing history, drying and immersion conditions, individual and mean strength values, the calculated softening coefficient, and references to the test standards applied. Where certification or independent verification is required, testing is performed by accredited third-party laboratories under recognized quality systems.
Influencing Factors and Precautions
- Water-to-plaster ratio: higher mixing water increases porosity and lowers both dry and wet strength
- Drying regime: excessive drying temperature can dehydrate the matrix and distort the dry strength reference
- Immersion duration and water temperature: these define the saturation degree and must be held constant between series
- Testing speed and platen alignment: misalignment or uncontrolled loading causes premature failure and underestimates strength
- Specimen age: gypsum hydration and carbonation progress with time, so both series must be tested at the same age
Applications
Softening coefficient determination supports a wide range of technical and commercial decisions in the gypsum plaster industry:
- Factory quality control and batch release testing of rendering plasters, including hemihydrate, anhydrite II, and blended binders
- Conformity assessment of products against national and international requirements, including the GB/T 28627 product standard and the EN 13279 series for gypsum binders and gypsum plasters
- Mix design and optimization of water-resistant plasters containing hydrophobic admixtures, redispersible polymer powders, or finely divided mineral additions
- Material selection for renders exposed to elevated humidity or occasional wetting, such as kitchens, bathrooms, basements, and stairwells
- Research and development of anhydrite-based and blended gypsum systems, where wet strength retention is a central performance criterion
- Forensic engineering analysis of failed or degraded render layers, in which a low softening coefficient can confirm moisture-induced strength loss
FAQ
What is the softening coefficient of gypsum plaster?
The softening coefficient is the ratio of the compressive strength of water-saturated gypsum plaster specimens to the compressive strength of identical dry specimens. It expresses, on a scale from 0 to 1.0, how much strength the material retains after saturation and is the standard index of water resistance for gypsum-based renders.
Why is gypsum plaster sensitive to water?
The hardened gypsum matrix consists of interlocking calcium sulfate dihydrate crystals. Because dihydrate is moderately soluble in water, immersion partially dissolves and weakens the intercrystalline contacts, and absorbed water also acts as a lubricant between crystal surfaces. Both effects reduce strength, which is why untreated gypsum plasters commonly show softening coefficients of only 0.2 to 0.4.
Which standards govern the determination?
The determination is performed in accordance with the test methods referenced by the applicable product standards, including GB/T 28627 and the supporting gypsum test method standards, together with the EN 13279 series for gypsum binders and plasters and ASTM C472 for physical testing of gypsum plaster products. The specific conditioning, immersion, and calculation rules always follow the standard invoked by the product specification.
How long are specimens immersed in water?
A 24-hour immersion at (23 ± 2) °C is the most common requirement for full saturation of 40 mm × 40 mm × 160 mm prisms, although some product standards specify shorter or longer periods. The key requirement is that the immersion conditions match those defined by the governing standard and are identical for all specimens in the wet series.
What softening coefficient value is considered acceptable?
Plain gypsum plasters typically fall between 0.2 and 0.4. For applications with moisture exposure, water-resistant grades are generally specified with a softening coefficient of at least 0.60, and high-performance modified formulations can reach 0.7 or above. The acceptance threshold must always be taken from the relevant project specification or product standard.
Can the softening coefficient exceed 1.0?
In principle it should not, since saturation only weakens a gypsum matrix. A result above 1.0 almost always indicates a testing artifact, such as insufficient drying of the reference series, testing the two series at different ages, or systematic differences in specimen preparation, and the determination should be repeated with corrected procedures.