Radioactivity Testing of Plastering Gypsum: Standards, Exposure Indices, and Test Methods

Radioactivity testing of plastering gypsum determines the activity concentrations of naturally occurring radionuclides, primarily radium-226 (Ra-226), thorium-232 (Th-232), and potassium-40 (K-40), in gypsum-based interior plastering materials, verifying compliance with building material safety standards such as GB 6566-2010. This technical article explains the applicable standards, internal and external exposure indices, gamma-ray spectrometry procedures, sampling requirements, and typical applications, serving as a practical reference for manufacturers, contractors, specifiers, and accredited third-party laboratories.

Technical Overview

Plastering gypsum is a factory-blended dry-mix building material based on hemihydrate calcium sulfate (CaSO4·1/2H2O), combined with graded aggregates, lightweight fillers, retarders, water-retention agents, and other admixtures. It is applied by hand or machine spraying to interior walls and ceilings as a base coat or finishing render, where it has largely replaced conventional cement-lime mortars due to its fire resistance, smooth finish, and rapid setting behavior.

The radiological significance of plastering gypsum arises from its raw material sources. Natural gypsum is generally low in radioactivity, but industrial by-product gypsums, particularly flue gas desulfurization (FGD) gypsum and especially phosphogypsum (a by-product of phosphoric acid production), may contain elevated levels of naturally occurring radioactive materials (NORM) inherited from phosphate rock. Because plastering gypsum covers large interior surface areas with prolonged occupant exposure, the Chinese product standard GB/T 28627-2012 for plastering gypsum explicitly requires that radioactivity comply with GB 6566-2010, the mandatory standard for radionuclide limits in building materials.

GB 6566-2010 evaluates building materials through two dimensionless indices: the internal exposure index (IRa), which controls the radon inhalation pathway governed by radium-226, and the external exposure index (Iγ), which controls direct gamma exposure from Ra-226, Th-232, and K-40. Testing is performed by low-background gamma-ray spectrometry in accredited third-party laboratories or qualified in-house facilities, following principles consistent with ISO 18589 and IEC 61452.

Testing Samples

Applicable Sample Types

• Hand-applied plastering gypsum, including base coat, single-coat, and finishing (skim coat) grades
• Machine-applied (spray-applied) plastering gypsum for mechanized rendering
• Lightweight or thermal-insulating plastering gypsum containing perlite, vitrified bubbles, or other low-density fillers
• Building (calcined) gypsum and gypsum-based leveling compounds used as raw materials or binders
• Hardened plaster specimens cored or chiseled from finished works for supervision and acceptance testing

Sampling and Sample Preparation Requirements

Representative sampling is critical because radionuclide distribution can vary between raw material sources and production batches. A composite sample, typically 1 to 3 kg, should be collected from multiple increments taken at different positions within the same batch, silo, or set of bags, then homogenized and reduced by quartering. The sample must be sealed in airtight packaging and clearly labeled with product name, batch number, production date, and sampling location.

• Dry the sample to constant mass where moisture content is significant
• Crush and grind the material to pass a 0.16 mm sieve to ensure homogeneity
• Fill a container matching the calibrated detector geometry (commonly a cylindrical vessel of approximately 75 mm diameter or a Marinelli beaker, holding 300 to 600 g), record the net mass
• Seal the container with tape or wax to prevent radon-222 leakage
• Store the sealed sample for no fewer than 15 days, with 20 to 28 days being common practice, to establish secular equilibrium between Ra-226 and its short-lived radon progeny

Testing Items

The core testing items under GB 6566-2010 for plastering gypsum are:

• Specific activity concentration of Ra-226, expressed in Bq/kg
• Specific activity concentration of Th-232, expressed in Bq/kg
• Specific activity concentration of K-40, expressed in Bq/kg
• Internal exposure index: IRa = CRa / 200
• External exposure index: Iγ = CRa / 370 + CTh / 260 + CK / 4200

In the formulas above, C denotes the measured activity concentration in Bq/kg, and the denominators represent the activity concentration that, acting alone, would reach the corresponding limit. For main building materials, which include plastering gypsum, the acceptance criteria are IRa less than or equal to 1.0 and Iγ less than or equal to 1.0. For hollow building materials with a hollow ratio greater than 25 percent, the external exposure index limit is relaxed to 1.3 while the internal exposure index limit remains 1.0.

Optional or supplementary items may include the radon-222 exhalation rate, the radium equivalent activity, and activity concentrations of additional natural radionuclides such as U-238, which are sometimes requested for research purposes, export documentation, or specific national requirements in destination markets.

Testing Methods

Instrumentation

The standard technique is low-background gamma-ray spectrometry. A high-purity germanium (HPGe) coaxial detector, typically with a relative efficiency of 20 to 60 percent, provides the best energy resolution and lowest detection limits, while a thallium-activated sodium iodide NaI(Tl) scintillation detector offers a lower-cost alternative suitable for screening purposes. The detector is housed in a lead shield with copper, cadmium, or acrylic lining to suppress environmental background radiation, and connected to a multichannel analyzer with spectral analysis software.

Calibration

• Energy calibration using certified point sources such as Am-241, Co-57, Cs-137, and Co-60 to establish the channel-energy relationship
• Efficiency calibration using certified matrix reference materials with known activities of Ra-226, Th-232, and K-40, packed in the same geometry and density as the test samples, in accordance with ISO 18589-3 and IEC 61452, with general detector calibration practices consistent with ASTM E181
• Background measurement on an empty or blank container over an extended counting period, typically 24 to 48 hours, to determine baseline and minimum detectable activity

Measurement Procedure

After the sealed equilibrium period, the sample is counted on the detector, typically for 12 to 24 hours or until counting statistics are acceptable. Ra-226 is quantified through the gamma emissions of its progeny Pb-214 at 295.2 and 351.9 keV and Bi-214 at 609.3, 1120.3, and 1764.5 keV. Th-232 is quantified through Ac-228 at 338.3, 911.2, and 969.0 keV, Pb-212 at 238.6 keV, and Tl-208 at 583.2 and 2614.7 keV. K-40 is measured directly through its 1460.8 keV emission. Net peak counts are corrected for background, and where multiple lines are available, a weighted average activity is calculated.

Calculation, Evaluation, and Reporting

The measured activity concentrations are substituted into the index formulas, and the results are compared against the GB 6566-2010 limits. The test report should state the activity concentrations with their expanded uncertainties (coverage factor of approximately 2), the minimum detectable activity, the calculated IRa and Iγ values, the sealing and counting durations, and a clear conformity conclusion. For well-configured HPGe systems, minimum detectable activities are typically well below 10 Bq/kg for Ra-226 and Th-232, comfortably below the levels relevant to the limit calculation.

Quality Assurance

Reliable results depend on laboratory quality management aligned with ISO/IEC 17025, including regular detector calibration verification, participation in interlaboratory comparisons, duplicate and blank testing, control charts on reference materials, and documented measurement uncertainty budgets.

Applications

Radioactivity testing of plastering gypsum supports quality control, regulatory compliance, and market access across the entire supply chain. Typical applications include:

• Type testing and product conformity assessment against GB/T 28627 for market access in China
• Factory production control whenever raw material sources change, for example when switching from natural gypsum to FGD gypsum or phosphogypsum
• Supervision, arbitration, and acceptance testing of plastering works on construction sites
• Green building material certification programs and green building evaluation credits
• Demonstration of compliance for comprehensive phosphogypsum resource utilization policies
• Indoor environmental quality management aligned with GB 50325 for controlling indoor radon levels
• Export documentation where buyers or destination markets require radioactivity or NORM declarations

By integrating radioactivity testing into routine quality plans, manufacturers can document the radiological safety of their products, protect occupants, and avoid costly rejections or remediation during project acceptance.

FAQ

Why does plastering gypsum need radioactivity testing at all?

Gypsum raw materials, especially phosphogypsum derived from phosphate rock, may carry elevated naturally occurring radionuclides. Because plastering gypsum covers large interior surfaces with long-term occupant exposure, GB/T 28627 mandates that radioactivity comply with GB 6566, making testing an essential conformity item alongside setting time, strength, and bond strength.

What are the pass criteria for the test?

For plastering gypsum treated as a main building material, the internal exposure index IRa must not exceed 1.0, which corresponds to a Ra-226 activity concentration of 200 Bq/kg, and the external exposure index Iγ must not exceed 1.0 based on the combined contributions of Ra-226, Th-232, and K-40.

How much sample is required and how should it be taken?

A composite sample of approximately 1 to 3 kg is generally sufficient. It should be assembled from several increments taken at different positions within the same batch, thoroughly mixed, and reduced by quartering, then sealed and labeled with full traceability information before dispatch to the laboratory.

Why must the sample be sealed and stored before measurement?

Ra-226 is quantified through the gamma rays of its short-lived radon progeny. Sealing traps radon-222 inside the container, and an equilibration period of at least 15 days, preferably 20 to 28 days, allows the progeny activities to reach equilibrium with the parent radium so that Ra-226 can be measured accurately.

How long does the complete test take?

The equilibration period dominates the timeline: approximately 15 to 28 days of sealed storage, followed by 12 to 24 hours of counting and reporting. In total, clients should typically allow three to five weeks and plan project schedules accordingly.

Are plasters made from FGD gypsum or phosphogypsum automatically non-compliant?

No. FGD gypsum is usually radiologically similar to natural gypsum and commonly passes the limits. Phosphogypsum varies widely depending on the ore source and purification processes, so each material source and, in practice, each production change should be verified through testing by accredited third-party laboratories before commercial release.