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A gypsum plaster retarder delays setting by interfering with the growth of gypsum crystals. Producers mostly use citric or tartaric acid, their salts, or protein hydrolysates. Published doses are small: about 0.04–0.06% in one machine-plaster formulation. Too much cuts strength, so the dose is set by trial on your own stucco, against a measured setting time.
Unretarded plaster sets too fast to apply. Commercial bagged plasters work for an hour and a half to more than three hours, and that gap is what the retarder closes. This article covers what producers need to decide: which retarder, how much, what it costs in strength, why set drifts from week to week, and how to measure it. For the limits themselves and the rest of the standard, see our EN 13279 guide.
Plaster of Paris Setting Time Before Any Retarder
Mixed with water, hemihydrate reverts to gypsum, whose crystals grow until they interlock. There is an induction period at the start, during which very little seems to happen (NZIC). Retarders act mainly on that stage and on crystal growth after it.
How fast does plain plaster go? Published figures sit in minutes:
- One study of a commercial construction plaster gives 7–15 minutes in its materials section, 16 minutes initial and 19 minutes final set in its test results, and 30 minutes in its abstract (Voinitchi et al., 2022). The spread is a useful warning: always ask which plaster, which water ratio and which method.
- Calorimetry on a pure hemihydrate paste put the main hydration peak at about 10 minutes (Fobbe et al., 2023). That peak is not a setting time, but it shows how quickly the reaction runs.
- ASTM C28 sets 20–40 minutes for unretarded gauging plaster (ASTM C28/C28M-10 preview; check the current edition).
Gypsum Plaster Setting Time: The Window You Are Dosing To
Set the target before choosing the chemical. The floor comes from the standard; the working figure comes from the applicator.
| Product or requirement | Setting or working figure | Source |
|---|---|---|
| EN 13279 plasters, as shown in a draft revision | Initial set above 20 min; a lower value is permitted for some manual applications. Machine plasters may be declared higher (see below) | prEN 13279-1:2017 draft; check the current edition |
| ASTM C28 mill-mixed base coat | 1½–8 h (Vicat) | ASTM C28/C28M-10; check the current edition |
| ASTM C28 neat plaster | 2–16 h (with 3 parts Ottawa sand) | ASTM C28/C28M-10; check the current edition |
| Finish plaster, EN type B1/20/2 | 1.5 h setting time | Thistle Board Finish data sheet |
| Lightweight base coat, types B4/20/2 and C3/20 | 1.5–2 h setting time | Thistle Hardwall data sheet |
| Lightweight machine plaster, B4/20/5 | About 180 min processing time | Knauf MP 75 G/F-Leicht |
| Lightweight machine plaster, B4/50/2 | About 3–3.5 h from mixing to finishing | Knauf MP 75 (Belgium) |
The practical reading: 20 minutes is the lowest general figure we could check, and only in a draft, so confirm the limit for your plaster type in the current edition. It will not satisfy a plastering crew anyway. Note the designations too: machine plasters are often printed with /50/ where hand plasters carry /20/ (VG Orth range), which suggests a higher declared figure, although one Knauf machine plaster above prints /20/ and these sheets do not define the number. The commercial sheets above run several times longer, and machine plasters sit at the long end because the plaster has to survive the pump, spreading, leveling and finishing. “Setting time” and “processing time” are also different measurements, so compare them with care. A producer entering a market should buy the local leading bag and measure it in its own lab, with its own method. That number is the real target.
Types of Gypsum Plaster Retarder
| Family | Examples | What the literature reports | Watch for |
|---|---|---|---|
| Fruit acids and salts | Citric acid, sodium citrate, tartaric acid, malic acid | Citrate adsorbs on gypsum and affects nucleation and crystal growth (Koslowski and Ludwig, ASTM STP) | Strength falls as the dose rises; sodium citrate is described as causing a “marked decrease” (US 4,218,261) |
| Protein hydrolysates | Collagen, keratin, wheat protein | Carboxylic groups bind Ca²⁺ ions (Voinitchi et al.); a wheat protein study reports good retardation with lower strength loss (Ding et al., 2015) | Batch variation, odor, storage stability and mildew are listed as drawbacks in US 4,293,344, a patent promoting a rival chemistry |
| Modified proteins | Carboxyalkylated gelatin, keratin, casein, soy | Patented to overcome the poor cold-water solubility and spoilage of plain animal proteins (US 4,166,751) | Supplier-specific; verify with your own trials |
| Synthetic polymers | Low-molecular-weight polyacrylates | At 0.1 part per hundred, one patent example extended the start of setting from 26 to about 1,020 minutes (US 4,293,344) | A very strong effect makes small dosing errors costly |
| Phosphates | Sodium hexametaphosphate, phosphoric acid | Listed as known retarders (US 4,293,344) | The same patent links this group, and citric acid, to higher water need and later efflorescence and blistering |
Treat the drawbacks stated in patents as claims: a patent background exists partly to make the invention look necessary. Still, the pattern across sources is consistent. None of these sources shows a retarder that is free of a strength penalty at working doses; they differ in how much delay you get for a given penalty, how stable they are in the bag, and how reliable the supply is.
Among simple acids, an isothermal calorimetry paper hosted by TA Instruments ranked retarding effect at equal molar dose, weakest to strongest, as tartaric, succinic, citric, malic. It also found that all four worked by lengthening the induction period. Take the ranking as a starting point for trials; compare on molar dose, purity and cost in your own lab.
One trap for anyone coming from cement: calcium formate, a familiar cement accelerator, retards hemihydrate. In the study by Fobbe et al., 5 wt.% moved the main hydration peak from about 10 to 45 minutes. Do not transfer additive habits from one binder to the other.
Retarder Dosage and Strength Loss
Published doses
The doses are tiny, which is what makes this section of a plaster plant difficult.
| Source | Retarder | Dose range or example | Result reported |
|---|---|---|---|
| Dow, US 10,494,303 | Retarder in a machine plaster; example with tartaric acid | About 0.04–0.06%; example 0.05% | Formulation data only |
| Lanzón and García-Ruiz, 2012 | Citric acid | 125–3,000 ppm (0.0125–0.3%) | Efficiency peaked at 1,500 ppm in that study |
| Voinitchi et al., 2022 | Collagen and keratin hydrolysates | 0.05–0.20% of plaster mass | 1–1.5 h delay at 0.20% |
| US 4,166,751 | Carboxyalkylated protein | Preferred 0.01–1 part per 100 parts gypsum | Above 5 parts, setting becomes impractically long |
To see the scale, take 0.05% as an illustrative figure: that is 0.5 kg per tonne of plaster, or 12.5 g in a 25 kg bag. That cannot be added accurately by hand into a large mixer, so it needs a dedicated small-scale feeder or a pre-blend with a carrier powder. Dosing and mixing are covered in our article on gypsum plaster manufacturing after calcination.
The response curve need not be a straight line. Voinitchi et al. found delay rising roughly linearly with dose across their range. USG’s site guidance describes a different picture for its retarder: one dose adds 30 minutes, two add 90, three add several hours (USG technical guide). Run your own curve. If you operate on a steep part of it, a small dosing error becomes a large change in set.
What the dose costs in strength
Slowing the reaction also changes the crystals that form. The evidence:
- Koslowski and Ludwig report that strength drops significantly as citric acid content increases, particularly at higher humidity.
- Lanzón and García-Ruiz found that citric acid partly inhibits conversion of hemihydrate to dihydrate, and that microstructural interlocking was greatly reduced even at very low doses.
- Voinitchi et al. measured bending strength about 10–40% below the control at 0.20% hydrolysate, while compressive strength was unaffected at low doses and fell at higher ones.
- USG warns that excess retarder “will drastically reduce strength” and can cause “dry-out”, where the plaster loses its water before it has set.
The last point matters most on a hot site with a porous background. Water leaves the coat while the set is still pending, so the retarder and the water-retention additive have to be designed together. For a bagged plaster, the strength that counts is the requirement declared on the bag, so test strength at the dose you will actually use. Do not rely on figures from the undosed stucco.
Why Set Drifts: The Causes Retarder Cannot Fix
Many setting complaints are blamed on the retarder but start upstream. Adding more retarder hides the cause and spends strength.
| Cause | Effect on set | What to check |
|---|---|---|
| Under-calcined stucco leaving raw gypsum | Faster: ground gypsum gives sites for crystals to grow (NZIC) | Combined water every shift, about 6.2% for pure hemihydrate |
| Soluble salts | Faster: potassium sulfate is a known accelerator (NZIC) | Raw gypsum analysis, especially by-product gypsum |
| Fresh, unaged stucco | Unstable: water demand is higher and its properties are still settling (US 9,751,805) | Aging silo residence time |
| Fineness, water amount, temperature, contaminants | All change setting time (Warsaw University of Technology manual) | Sieve results, water ratio, water temperature, clean lab tools |
| Lime in the formulation | Changes retarder response: USG sells a separate retarder for lime-containing products (CGC data sheet) | Re-run the dose curve when lime content or source changes |
| Set residue in mixers | Faster: USG advises clean equipment between batches to prevent quick-setting | Mixer and hopper cleaning routine |
| Damp bags | Faster, with lumps and possibly weaker plaster (Thistle Hardwall) | Packaging, pallet storage, stock rotation |
The stable answer is a stable calciner. On the steam-heated rotary kilns we supply, gypsum calcines slowly at a low 150–200 °C, which gives a stable phase composition. Whichever calciner you use, the point stands: temperature control and aging decide how much the retarder has to correct. See gypsum calcination temperature for what happens at each stage, and the plaster of Paris manufacturing process for where aging fits.
Shelf life is part of the same problem. Published bag lives are short: about 3 months for Knauf MP 75 G/F-Leicht, 4 months for Thistle plasters, and about 8 months for bags and 4 months for bulk on the Belgian MP 75 sheet. Those differences are one reason to test set on retained samples through the stated life, not only on the day of packing.
Measuring Plaster Setting Time
EN 13279-2. The 2014 edition gives two methods for setting time: the knife method and the Vicat cone method (STN EN 13279-2:2014 preview). A laboratory manual following the earlier Polish adoption (PN-EN 13279-2:2006) describes the knife method as follows; check the details against the current edition before you equip the lab.
- Determine the water/binder ratio first, then weigh water and binder to make 200 g in total.
- Pour the binder into the water, start the clock and mix thoroughly.
- Pour three pats on a glass plate, each about 100–120 mm across and about 5 mm thick.
- Cut two pats with a clean metal knife at set intervals. Keep the third for a control cut.
- Record the start of setting: the time from pouring binder into water until the cut edges no longer flow back together.
ASTM C472. It includes a standard setting time test and a temperature rise method (ASTM C472-26 listing). ASTM C28 refers to Vicat setting time for plaster types.
Three habits make the numbers comparable. Use one method for release decisions and do not mix results across methods. Fix the water ratio by the standard procedure, because more water lengthens the apparent set. And condition lab materials and water to a set temperature, then repeat the test at the warmest water temperature your market’s crews will use.
Setting a Retarder Dose: A Plant Procedure
- Define the target from the market: declared standard, and working time of the benchmark bag measured in your lab.
- Shortlist two or three retarders from suppliers who can give a batch certificate and stable supply in your country.
- Run a dose curve on aged stucco from normal production. Use five or more doses spanning the target, at the fixed water ratio, including any lime and water-retention additive in the final formula.
- Test strength at each dose. Cast prisms, test flexural and compressive strength as in EN 13279-2 and plot them against set.
- Choose the flatter part of the curve if you have a choice. Allow a margin above the declared strength.
- Repeat for variables such as stucco age and water temperature. Also repeat for every new gypsum source.
- Write the control rule. Set a target and a tolerance for each shift’s setting time, with the action if it drifts: check calciner and combined water first, then adjust dose.
- Check retained bags at intervals through the printed shelf life.
FAQ
What is the best retarder for gypsum plaster?
There is no single best one. Citric and tartaric acids are simple and widely available but carry a strength penalty as the dose rises. Protein hydrolysates can give long delay with a lower penalty in some studies, but quality and storage vary by supplier. Choose by running a dose curve on your own stucco, measuring set and strength together.
How much citric acid is used to retard gypsum plaster?
One study tested 125–3,000 ppm (0.0125–0.3%) at a constant water/binder ratio and reported peak efficiency at 1,500 ppm, with interlocking reduced even at very low doses. Your dose depends on the target set, stucco quality and lime content. Start low and measure strength at every step instead of copying a figure from a paper.
What is the setting time of plaster of Paris?
Unretarded plaster sets in minutes. One study reported 16 minutes initial and 19 minutes final set for a commercial plaster, and ASTM C28 gives 20–40 minutes for unretarded gauging plaster. Results depend on water ratio, fineness, temperature and method. Retarded bagged plasters run from about 1.5 hours to more than 3 hours on published data sheets.
Does a retarder reduce plaster strength?
Yes, at practical doses most do, and more so as the dose rises. Published work shows strength falling with citric acid, and bending strength 10–40% below the control with 0.20% protein hydrolysate. Excess retarder can also cause dry-out, where water leaves before the plaster sets. Compare retarders by the strength left at your target setting time.
How is setting time tested under EN 13279?
EN 13279-2:2014 gives the knife method and the Vicat cone method. In the knife method, as described in one university manual following an earlier edition, thin pats of plaster are cut at intervals and the start of setting is when the cut edges no longer flow back together. Check the procedure against the current edition before equipping a lab.
Why does my plaster set too fast even with retarder?
Look upstream first. Raw gypsum left in under-calcined stucco, soluble salts such as potassium sulfate, damp bags, warm water and set residue in the mixer all shorten setting time. Check combined water, the raw gypsum analysis and the cleaning routine before you raise the dose, because extra retarder costs strength and does not remove the cause.
Planning a Plaster Plant?
Stable set starts with the calciner, the aging silos and accurate additive dosing, so they should be specified together. See our gypsum powder production line for how the sections fit, and the gypsum powder plant setup guide for the full planning sequence.
Sources
- New Zealand Institute of Chemistry, Plaster and Plaster Board Production: https://nzic.org.nz/unsecure_files/book/9D.pdf
- C. Voinitchi et al., “Collagen and Keratin Hydrolysates to Delay the Setting of Gypsum Plaster,” Materials 15(24), 8817 (2022): https://pmc.ncbi.nlm.nih.gov/articles/PMC9785065
- N. Fobbe, F. Goetz-Neunhoeffer, D. Jansen, “The influence of calcium formate on the hydration of calcium sulfate hemihydrate,” Materials and Structures 56, 92 (2023): https://link.springer.com/article/10.1617/s11527-023-02181-1
- M. Lanzón, P. A. García-Ruiz, “Effect of citric acid on setting inhibition and mechanical properties of gypsum building plasters,” Construction and Building Materials 28(1), 506–511 (2012), catalog record: https://bvbvf1.bib-bvb.de/vufind/PrimoRecord/cdi_gale_infotracmisc_A283591651
- T. Koslowski, U. Ludwig, “Retardation of Gypsum Plasters with Citric Acid: Mechanism and Properties,” The Chemistry and Technology of Gypsum, ASTM STP (1984): https://store.astm.org/stp30274s.html
- Y. Ding et al., “Study on the Retarding Mechanism and Strength Loss of Gypsum from Hydrolyzed Wheat Protein Retarder,” Journal of the Korean Ceramic Society 52(1), 28–32 (2015): https://www.koreascience.or.kr/article/JAKO201506849871912.page
- Effect of carboxylic acids on the hydration of calcium sulfate hemihydrate (calorimetry paper, TA Instruments literature no. 314-07): https://www.tainstruments.com/pdf/literature/314-07%20Effect%20of%20carboxylic%20acids%20on%20the%20hydration%20of%20cal.pdf
- US 4,218,261, Retarder for gypsum plaster and a process for retarding the setting of gypsum plaster: https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/4218261
- US 4,293,344, Setting retardatives for compositions with a gypsum plaster base or anhydrite: https://patents.google.com/patent/US4293344
- US 4,166,751, Method for setting gypsum utilizing retarder: https://patents.google.com/patent/US4166751
- Dow Global Technologies, US 10,494,303 B2, Gypsum compositions containing crosslinked cellulose ethers for mortars with reduced stickiness: https://patents.google.com/patent/US10494303
- US 9,751,805 B2, Method for stabilizing beta-hemihydrate plaster: https://patents.google.com/patent/US9751805
- USG, Plastering technical guide: job use of retarder in plaster: https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plastering-technical-guide-job-use-of-retarder-in-plaster-en-PM7.pdf
- CGC/USG, Gypsum plaster retarders data sheet: https://www.usg.com/content/dam/USG_Marketing_Communications/canada/product_promotional_materials/finished_assets/cgc-gypsum-plaster-retarders-data-sheet-en-can-P783.pdf
- iTeh Standards, oSIST prEN 13279-1:2017 draft preview: https://iteh.es/catalog/standards/sist/6116aa3a-eb37-4889-aac9-6dee96b091a6/osist-pren-13279-1-2017
- Slovak Office of Standards, STN EN 13279-2:2014 preview: https://normy.normoff.gov.sk/norma/119434/nahlad
- ASTM C28/C28M-10, Standard Specification for Gypsum Plasters (iTeh preview): https://cdn.standards.iteh.ai/samples/75132/b4161b6922034e149cc85c30a5db831d/ASTM-C28-C28M-10.pdf
- ASTM C472-26, Standard Test Methods for Physical Testing of Gypsum, Gypsum Plasters, and Gypsum Concrete (catalog listing): https://www.en-standard.eu/astm-c0472-26-standard-test-methods-for-physical-testing-of-gypsum-gypsum-plasters-and-gypsum-concrete/
- Warsaw University of Technology, Non-hydraulic building binders: Gypsum binders (laboratory task): https://www.zimb.il.pw.edu.pl/wp-content/uploads/EN-2B-Gypsum-binders.pdf
- British Gypsum, Thistle Board Finish data sheet: https://www.insulationshop.co/image/catalog/pdf/British_Gypsum/Thsitle/Thistle-Board-Finish.pdf
- British Gypsum, Thistle Hardwall data sheet: https://cdn.shopify.com/s/files/1/0222/7290/1216/files/PDS-Thistle-HardWall.pdf
- Knauf, MP 75 G/F-Leicht data sheet (German): https://baustoffshop.de/media/pdf/knaufwdvs/mp_75_g_f-leicht_p118_de_0813_ger_screen.pdf
- VG Orth, gypsum plaster product range: https://www.baulinks.de/bkd_file/vg-orth/VG_ORTH_gipsputz_07062022.pdf
- Knauf, MP 75 data sheet (Belgium, Dutch): https://sleurs.marcando.be/mediastorage/sleurs/FSDocument/A0/61/tf_mp75.pdf
What to Send for a Quotation
Send the same list to every supplier you talk to: proposals built on the same inputs are the only ones you can compare.
- Target annual output (t a year), or the board output the plant will feed
- End use: board line feed, bagged plaster, blocks or other products, and the standard they must meet
- Gypsum source (natural, FGD or phosphogypsum), with a recent analysis
- Heat source available: natural gas, steam, thermal oil or other fuel
- Power supply (voltage, frequency, available capacity)
- Site or building dimensions
- Destination country and target timeline
See the full scope of our gypsum powder production line, or browse every guide in the Gypsum Powder Plant Knowledge Center. With your output target, destination country and timeline we can start, and you receive a written sourcing report within 48 hours of a complete requirement brief. We respond within one business day.
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