Phase composition determination is the core analytical procedure for evaluating plastering gypsum (rendering plaster), quantifying the hemihydrate, dihydrate, and anhydrite phases that control setting time, strength development, and workability. This technical article explains the sampling, testing items, analytical methods such as thermogravimetric analysis and X-ray diffraction, and the applicable standards including GB/T 9776, EN 13279, ASTM C471M, and ISO 3052. The information is intended for manufacturers, contractors, specifiers, and accredited third-party laboratories involved in gypsum product testing and certification.
Technical Overview
Plastering gypsum is a factory-premixed dry-mix mortar based on calcium sulfate binders, typically beta-hemihydrate (basanite) produced by calcination of natural or industrial by-product gypsum, blended with fillers, lightweight aggregates, and chemical admixtures such as retarders, accelerators, water-retention agents, and thickeners. The engineering performance of the finished plaster, including setting time, water demand, adhesion to substrates, flexural and compressive strength, and shrinkage behavior, is governed primarily by the relative proportions of the calcium sulfate phases present in the binder.
Phase composition determination is therefore a fundamental quality-control and conformity-assessment test. It quantifies the individual crystalline and semi-crystalline phases that coexist in calcined gypsum, each of which behaves differently when mixed with water:
- Dihydrate (gypsum, CaSO4·2H2O): the fully hydrated and final reaction product; in fresh plaster powder it indicates residual un-calcined core or partial pre-hydration and acts as a setting accelerator (nucleation seed).
- Hemihydrate (basanite, CaSO4·0.5H2O): the principal reactive phase, existing as alpha (autoclaved, low water demand) or beta (kettle/kiln calcined) modifications; it is the main contributor to early strength.
- Soluble anhydrite (anhydrite III, CaSO4): highly hygroscopic and rapidly hydrating; excessive content can cause flash setting and poor storage stability.
- Insoluble anhydrite (anhydrite II, dead-burned CaSO4): slow-reacting phase that contributes to late strength under humid curing conditions.
- Free water: surface moisture that promotes pre-hydration of the binder during storage and reduces shelf life.
Theoretical reference values used in calculations are well established: pure hemihydrate contains 6.2% combined (crystallization) water, while pure dihydrate contains 20.9%. Product standards such as GB/T 9776 in China and EN 13279-1 in Europe define classification and composition-related requirements for gypsum binders and gypsum plasters, making reliable phase quantification essential for market access and certification.
Testing Samples
Finished Plastering Gypsum Products
Bagged factory-premixed plastering gypsum, including base coat (undercoat), finishing coat (skim), and lightweight or thermal-insulation grade products, is the most common sample type. Sampling is performed on representative composite samples drawn from multiple bags of the same batch, which are then reduced by splitting or cone-and-quartering to obtain a laboratory portion, typically 500 g to 1 kg, sealed immediately in airtight containers.
Raw Materials and Binder Precursors
Phase analysis is also applied to upstream materials: natural gypsum ore, flue-gas desulfurization (FGD) gypsum, phosphogypsum and other chemical gypsums, and calcined stucco sampled directly from kettles, rotary kilns, or roller mills. This supports calcination process control and incoming-material acceptance testing.
Hardened Plaster and Site Samples
Drilled or crushed powder samples of hardened plaster from construction sites are tested to determine the degree of hydration, residual hemihydrate, or unreacted anhydrite content. This is widely used in failure diagnostics, dispute resolution, and verification of on-site mixing and application quality.
Sample Handling and Preparation
Calcined gypsum phases are metastable and sensitive to atmospheric moisture, so strict handling protocols are required: airtight packaging immediately after sampling, exclusion of humid environments, drying of hardened samples at low temperature, grinding and homogenization to a fine powder (typically passing a 0.075 mm to 0.2 mm sieve), and complete documentation of sampling date, batch number, and storage history. Improper handling can shift the measured dihydrate and hemihydrate contents by several percentage points.
Testing Items
A complete phase composition determination for plastering gypsum typically reports the following parameters, calculated on a dry-mass or as-received basis as agreed with the client:
- Free water (moisture) content, %
- Hemihydrate (CaSO4·0.5H2O) content, %
- Dihydrate (CaSO4·2H2O) content, %
- Soluble anhydrite III content, %
- Insoluble anhydrite II content, %
- Total combined (crystallization) water content, %
- Calcination degree and ratio of reactive to inert calcium sulfate phases
- Supporting items where required: acid-insoluble residue, calcium carbonate content, organic matter or LOI, chloride content, and pH
Reporting conventions and acceptance limits are aligned with the applicable product specification, for example GB/T 9776 for plastering gypsum in the Chinese market or the composition-related classifications of EN 13279-1 for gypsum binders and plasters under CE marking.
Testing Methods
Thermogravimetric Analysis (TGA/DTG)
Thermogravimetric analysis is the most widely used instrumental method for routine phase quantification. A small, representative specimen (typically 20 mg to 100 mg) is heated at a controlled rate, commonly 10 °C/min under a nitrogen or dry air purge in an open crucible, while mass loss is recorded continuously. Stepwise mass losses are assigned to characteristic dehydration reactions:
- Room temperature to approximately 60 °C: loss of free (surface) water.
- Approximately 60 °C to 170 °C: dehydration of dihydrate to hemihydrate, DTG peak near 130–150 °C, loss of 1.5 mol H2O per mol CaSO4·2H2O.
- Approximately 170 °C to 300 °C: dehydration of hemihydrate (and any anhydrite III) to anhydrite, loss of the remaining 0.5 mol H2O per mol CaSO4·0.5H2O.
- Above approximately 1000 °C: decomposition of calcium sulfate; mass losses between 550 °C and 850 °C are used diagnostically for carbonate impurities such as calcite or dolomite.
Phase contents are calculated from the stepwise mass losses using stoichiometric conversion factors: dihydrate = mass loss (1.5 H2O step) × 6.37; hemihydrate = mass loss (0.5 H2O step) × 16.11; dihydrate = mass loss (total water of dihydrate) × 4.78. Because the dehydration steps partially overlap, deconvolution of the DTG curve, consistent heating-rate protocols, and validation against reference materials are essential for accurate results.
Gravimetric Drying and Ignition Method
The classical approach, standardized in ISO 3052 (determination of water of crystallization content of gypsum plasters), ASTM C471M (chemical analysis of gypsum and gypsum products), and the test-method framework of EN 13279-2, determines free water by drying at low temperature (typically 40 °C to 45 °C) to constant mass, followed by ignition at a standardized elevated temperature to constant mass to obtain combined water. Hemihydrate and dihydrate contents are then derived stoichiometrically, assuming negligible anhydrite III. The method requires only laboratory ovens and balances, which makes it robust and cost-effective for high-frequency production control, although it is slower and less able to separate overlapping reactions than TGA.
X-Ray Diffraction with Rietveld Refinement
Quantitative X-ray diffraction (QXRD) directly identifies and quantifies all crystalline phases, including insoluble anhydrite II, soluble anhydrite III, calcite, quartz, and mica impurities that thermal methods cannot distinguish individually. Rietveld refinement provides absolute phase fractions with typical absolute uncertainties of 1–3 wt.% for major phases. Addition of a known internal standard (for example, corundum) allows estimation of amorphous content and organic admixtures, which is important for ready-mixed plasters containing significant polymer or filler fractions. XRD is also the reference technique for detecting phase transformation during storage studies.
Differential Scanning Calorimetry (DSC)
DSC measures the enthalpy of the dehydration endotherms of dihydrate and hemihydrate and serves as a complementary or confirmatory technique to TGA, particularly for low dihydrate contents where peak integration of heat flow can be more sensitive than small mass-loss steps.
Wet-Chemical and Selective Dissolution Procedures
Classical wet-chemical methods, incorporated in standards such as ASTM C471M and GB/T 5484 (chemical analysis of gypsum), include selective dissolution for soluble anhydrite, determination of sulfate and calcium, acid-insoluble residue, and carbonate. They are applied as supporting techniques where instrumental results require confirmation or where specific impurities must be quantified for raw-material qualification.
Combined Protocols and Reporting
Best practice in accredited third-party laboratories combines TGA for water-related phases, XRD for full crystalline phase identification, and gravimetric reference methods for verification. Test reports state the methods, standards, calculation basis (as-received or dry), and measurement uncertainty, and are suitable for certification, arbitration, and regulatory submission purposes.
Applications
Phase composition determination supports the entire value chain of the gypsum plaster industry:
- Calcination process control: monitoring stucco from kettles, rotary kilns, and roller mills to detect over-calcined anhydrite II or under-calcined dihydrate cores and to tune burning temperature and residence time.
- Raw material acceptance: verifying purity, free water, and dihydrate content of natural gypsum, FGD gypsum, and phosphogypsum supplies.
- Product conformity and certification: type testing and factory production control supporting compliance with GB/T 9776, EN 13279, and related specifications, including CE marking and national product certification schemes.
- Shelf-life and storage stability studies: tracking gradual pre-hydration of hemihydrate to dihydrate in bagged premixed plasters.
- Failure analysis: diagnosing delayed or flash setting, low strength, cracking, and poor adhesion in hardened plaster by quantifying residual unhydrated phases.
- Research and development: formulation of alpha-hemihydrate binders, anhydrite-based self-leveling screeds, and blended calcium sulfate cements.
- Trade and dispute resolution: providing objective third-party evidence for incoming-inspection disputes between producers, distributors, and contractors.
FAQ
Why is excess dihydrate content in plastering gypsum powder a problem?
Excess dihydrate indicates incomplete calcination or pre-hydration during storage. It accelerates setting by acting as a nucleation seed, reduces the effective reactive binder content, lowers final strength, and shortens the shelf life of the bagged product, which is why product specifications place limits on it.
Can TGA alone determine the complete phase composition?
TGA accurately quantifies free water, dihydrate, and hemihydrate through their characteristic water losses, but it cannot reliably distinguish anhydrite III from anhydrite II, since neither releases water in the measurement range, and it cannot identify silicate or carbonate impurities. A combined TGA plus XRD protocol is recommended for a complete phase assessment.
Can the analysis distinguish alpha from beta hemihydrate?
Both are chemically CaSO4·0.5H2O, so TGA cannot separate them. XRD shows subtle differences in peak positions and crystallinity, and scanning electron microscopy clearly reveals the dense prismatic alpha crystals versus the porous beta morphology, so complementary techniques are used when the distinction matters.
How much sample is required and how should it be shipped?
A representative sample of approximately 500 g to 1 kg per batch is generally sufficient for TGA, XRD, and gravimetric testing. Samples must be sealed in airtight containers immediately after sampling and shipped without exposure to moisture, because hemihydrate and soluble anhydrite absorb atmospheric water and distort results.
Which standards govern the testing?
The key references are GB/T 9776 (plastering gypsum product standard), GB/T 5484 (chemical analysis of gypsum), EN 13279-1 and EN 13279-2 (gypsum binders and gypsum plasters, specifications and test methods), ISO 3052 (water of crystallization content of gypsum plasters), and ASTM C471M (chemical analysis of gypsum and gypsum products).
What is the typical turnaround time?
At accredited third-party laboratories, routine phase composition testing by TGA and gravimetric methods is typically completed within three to seven working days after sample receipt; protocols including XRD with Rietveld refinement or arbitration-level combined testing may require additional time for validation and reporting.