POP (Plaster of Paris) — Complete Guide
Plaster of Paris (POP) is a fine white powder manufactured by heating gypsum to drive off most of its water of crystallisation. When mixed with water, it forms a smooth paste that sets rapidly into a hard, white, dense and easily-carvable solid — perfect for ceiling work, decorative cornices, false ceilings, wall punning, sculpture and mouldings.
POP gets its name from the Montmartre quarry near Paris, France, which historically supplied gypsum of exceptional quality. In India, POP is governed by IS 2547 (Parts 1 & 2) — Specification for Gypsum Building Plaster. Major Indian brands include Saint-Gobain Gyproc, Trimix Premium POP, Brar POP, Rajdhani POP and JK Lakshmi PowerMax.
What is Plaster of Paris?
Chemically, POP is calcium sulphate hemihydrate (CaSO₄ · ½H₂O). It is made by heating gypsum (CaSO₄ · 2H₂O) to 150–180°C, which removes three-quarters of its bound water. The dehydrated calcined material is then ground to fine powder.
CaSO₄ · 2H₂O → CaSO₄ · ½H₂O + 1½ H₂O↑
When POP powder is mixed with water again, the reaction reverses: it absorbs water, recrystallises and hardens into gypsum — setting in 5–15 minutes! This rapid setting time is both its greatest strength and limitation.
Types of POP
| Type | Description | Use |
|---|
| Class A — Casting POP | Fine, high purity, for moulding | Decorative cornices, statues |
| Class B — Building POP | Coarse, slower setting | Ceiling punning, false ceiling |
| Class C — Dental POP | Ultra-fine, fast setting | Dental moulds (medical use) |
| Alpha Hemihydrate POP | High strength, dense | Industrial moulds |
| Beta Hemihydrate POP | Standard building POP | General construction work |
| Modified / Polymer POP | POP + retarder + polymer | Spray ceiling plaster (Saint-Gobain Gyproc) |
| Coloured POP | POP with pigment | Decorative coloured ceilings |
| Fibre-Reinforced POP | POP + glass / sisal fibre | Strong panels, FRG products |
Physical Properties (IS 2547)
| Property | Typical Value |
|---|
| Colour | Pure white |
| Bulk Density (Powder) | 0.6 – 0.9 g/cm³ |
| Specific Gravity (Hard set) | 2.3 – 2.4 |
| Initial Setting Time | 3 – 5 min (Class A); 5 – 10 min (Class B) |
| Final Setting Time | 15 – 30 min |
| Water-Powder Ratio | 0.50 – 0.70 (by weight) |
| Compressive Strength (24 hr) | 4 – 8 MPa |
| Compressive Strength (7 days) | 6 – 12 MPa |
| Tensile Strength | 2 – 3 MPa |
| Density (Hardened) | 800 – 1,000 kg/m³ |
| Thermal Conductivity | 0.20 – 0.40 W/m.K (good insulator) |
| Fire Resistance | 2 – 3 hours (contains crystal water) |
| Expansion on Setting | 0.2 – 0.5% (slight expansion) |
| Solubility in Water | 0.21 g/100 ml (partially soluble!) |
| pH | 7.0 – 8.0 (neutral) |
| Fineness (residue on 90 micron) | < 1% |
Manufacturing Process
- Gypsum Mining: Mined from Rajasthan, Gujarat, Bihar deposits
- Crushing: Boulders crushed to < 50 mm pieces
- Drying: Surface moisture removed at 60–80°C
- Calcination: Heated in rotary kiln to 150–180°C, removes 75% bound water
- Grinding: Calcined material ground to fineness passing 90 micron sieve
- Quality Check: Setting time, fineness, strength testing
- Packing: 25 kg / 40 kg HDPE woven bags (moisture-proof)
- Storage: Dry warehouse on pallets, 6-month shelf life
Applications
Construction & Interior Finishing
- False ceiling (gypsum board + POP suspended)
- POP punning over RCC slab ceiling (smooth finish)
- Decorative wall corners and edge moulding
- Cornices at wall-ceiling junction
- Decorative arches, columns, brackets
- Centre roses / medallions (ceiling decor)
- Wall niches and shelves
- POP cove moulding for indirect lighting
- Wall paneling and feature walls
- Wall murals / 3D wall art
- Bulkhead and beam encasement
- Sculpting decorative columns
Other Applications
- Sculpture and statues (Ganesh / Durga idols)
- Mould-making (casting moulds for concrete, metal, ceramic)
- Dental moulds and casts
- Medical splints / orthopedic casts
- Theatre stage scenery / film set decor
- Pottery and ceramics (slip-casting moulds)
- Restoration of heritage buildings
- Patching cracks in plaster walls (small repairs)
- Educational/school models
POP False Ceiling Installation
- Marking: Mark ceiling level at desired height (typically 200–300 mm below slab)
- Channel Framework: Fix wall angle, then GI / aluminium ceiling channels at 600 mm c/c
- Suspension Rods: Hang 6 mm GI rods from RCC slab at 900 mm c/c
- Gypsum Board: Screw 12.5 mm gypsum board on framework (or POP-cast board)
- Jointing: Apply paper tape + jointing compound at panel joints
- POP Mix: Mix POP with water (0.6 ratio) + binder. Apply 8–15 mm thick coats.
- Cornices & Mouldings: Cast or fix factory-made cornices around perimeter
- Smoothing: Use plain trowel + sponge to smoothen wet POP
- Drying: Wait 24–72 hours for full set
- Sanding: Light sanding with 320 grit
- Putty & Paint: Apply wall putty, primer, paint
POP Punning (Direct on Slab/Wall)
- Roughen RCC slab surface with chisel / chemical primer for grip
- Apply 6–8 mm thick scratch coat
- Apply finishing coat 4–6 mm thick
- Smoothen with metal trowel
- Allow 24 hours setting
- Apply wall putty if extra smoothness needed
- Primer + paint after 3–7 days
Mixing Procedure
- Use clean, fresh, cool water (warm water accelerates setting!)
- Pour water into clean mixing tray (never into bag)
- Sprinkle POP powder uniformly into water
- Let it “slake” for 2–3 minutes (no stirring)
- Then mix gently with hand / spatula to creamy paste
- Use within 5–10 minutes before setting begins
- Never re-mix once setting starts — discard and start fresh
Always mix small batches (1–2 kg). Setting cannot be reversed by adding more water. Once it sets even partially, it loses 50%+ strength.
Setting Time Control
| Method | Effect |
|---|
| Cold water (10°C) | Slows setting (12–15 min) |
| Warm water (35°C) | Speeds setting (3–5 min) |
| Add salt (NaCl) 0.5% | Speeds setting |
| Add gypsum (set POP) dust | Speeds setting (seeding) |
| Add citric acid 0.1% | Slows setting |
| Add borax (Na₂B₄O₇) | Slows setting significantly |
| Add glue / arabic gum | Slows setting (used in casting) |
| Vibrate mix | Speeds setting (denser) |
| Parameter | POP | Gypsum Plaster | Wall Putty |
|---|
| Material | Calcined gypsum | Gypsum + retarders | White cement / acrylic |
| Setting Time | 5–15 min (fast!) | 15–30 min (controlled) | 2–4 hr (paste) |
| Thickness | 6–15 mm (punning) | 10–15 mm | 1–2 mm |
| Strength (28 d) | 6–12 MPa | 4–6 MPa | 3–6 MPa |
| Water Resistance | Poor | Poor | Excellent (exterior grade) |
| Fire Resistance | 2 hr | 2 hr | 1 hr |
| Use Location | Interior dry only | Interior dry only | Interior + Exterior |
| Carving Ability | Excellent | Good | Poor |
| Cost | Rs 12 – 22 / kg | Rs 10 – 18 / kg | Rs 25 – 45 / kg |
| Best For | Cornices, false ceiling | Smooth wall plaster | Surface finish for paint |
Advantages
- Sets fast (5–15 min) — quick construction
- Excellent smooth finish — mirror-like surface
- Easy to mould, carve, sculpt
- Fire-resistant (2–3 hr rating, releases crystal water)
- Lightweight (800–1000 kg/m³)
- Good thermal insulation (low conductivity)
- Sound absorbent (acoustic dampening)
- Does not shrink (slight expansion on setting)
- No curing needed
- Eco-friendly (non-toxic, recyclable)
- No chemical hazards for workers
- Excellent base for paint (after putty)
- Easy to repair (just patch with fresh POP)
- Lower labour cost than cement plaster (no curing)
- Beautiful for decorative work
- Allows complex 3D shapes, cornices, arches
- Can be cast into pre-fabricated panels
- Wide variety of finishes
Disadvantages
- Not water-resistant! Dissolves slowly in water
- Cannot use in bathrooms, kitchens, exteriors
- Damaged by humidity / dampness
- Lower strength than cement plaster
- Cannot withstand impacts well (brittle)
- Cannot be applied on damp / wet surfaces
- Setting is irreversible once started — wastage if not used promptly
- Sometimes develops cracks if applied too thick
- Cannot be screwed / drilled like wood
- Heavy false ceiling needs strong framework (POP boards are heavy)
- Joints in POP false ceiling may crack over time
- Difficult to remove once set (requires complete demolition)
- POP idols banned in some states due to non-biodegradability in water bodies
- Discolours / yellows when exposed to sunlight
Tests on POP (IS 2547)
- Fineness Test — 90 micron sieve, < 1% residue
- Setting Time Test (Vicat needle) — Initial & Final
- Compressive Strength Test (24 hr, 7 d, 28 d) — IS 2542
- Tensile Strength Test (briquettes)
- Transverse Strength Test
- Bulk Density Test
- Water-Powder Ratio
- Setting Expansion / Shrinkage
- Soluble Salts Test
- SO₃ Content (anhydrous calcium sulphate)
- Free Water (loss on ignition)
- Insoluble Residue Test
- Crystal Water Content
Quantity Estimation
| Application | Coverage |
|---|
| POP Punning on slab (8 mm thick) | ~10 kg / m² |
| POP Punning on slab (12 mm thick) | ~14 kg / m² |
| POP False Ceiling (cast) | ~14 kg / m² |
| Cornice (small) | ~3 kg / m run |
| Cornice (medium) | ~5 kg / m run |
| Cornice (large designer) | ~10 kg / m run |
| Wastage allowance | 10 – 15% |
POP Quantity (kg) = Area (m²) × Thickness (mm) × 1.4 (density factor)
Indian Manufacturers
- Saint-Gobain Gyproc (false ceiling system)
- Trimix Premium POP
- JK Lakshmi PowerMax
- Rajdhani POP
- Brar POP
- Modi POP
- Vaishak POP
- Yashoda Plaster
- Birla POP (rare)
- Dolphin POP
- Universal POP
- Sharp POP
Cost (Approximate, Indian Market)
| Item | Rate |
|---|
| POP Powder (25 kg bag) | Rs 300 – 550 |
| POP Powder (40 kg bag) | Rs 500 – 900 |
| Premium / Branded POP per kg | Rs 15 – 25 |
| POP punning (material + labour) | Rs 25 – 45 / sqft |
| POP false ceiling (basic) | Rs 65 – 95 / sqft |
| POP false ceiling (designer with cove lights) | Rs 90 – 150 / sqft |
| POP cornice (small) | Rs 80 – 150 / m run |
| POP cornice (large designer) | Rs 200 – 500 / m run |
| Pre-fabricated POP panel | Rs 40 – 80 / sqft |
Best Practices
- Buy fresh POP — check manufacturing date (use within 6 months)
- Store in dry place, off the floor, sealed
- Use clean, fresh, cool water for mixing
- Mix small batches (1–2 kg) for use within 10 min
- Apply max 8 mm per coat — thicker layers crack
- Roughen substrate for grip on smooth RCC
- Apply scratch coat first if total thickness > 12 mm
- Maintain temperature 20–30°C during application
- Allow 24 hr setting before sanding
- Allow 3–7 days before painting
- Use only in dry interior locations
- Never apply in bathrooms, kitchens, exterior walls
- Use mechanical fasteners (not just adhesion) for cornices > 200 mm
- Apply paint with sealer to prevent moisture absorption
- Use gypsum board base for false ceiling (not POP cast board) for crack-free joints
- Provide expansion gaps in long POP runs (every 6 m)
Applicable Standards
| Standard | Description |
|---|
| IS 2547 (Part 1) | Gypsum Building Plaster — Casting Plaster |
| IS 2547 (Part 2) | Gypsum Building Plaster — Building Plaster |
| IS 2542 (Parts 1 & 2) | Methods of Test for Gypsum Plaster, Concrete & Products |
| IS 12860 | Glass Fibre Reinforced Gypsum Plaster Boards — Specification |
| IS 2095 (Parts 1–3) | Gypsum Plaster Boards — Specification |
| IS 1290 | Mineral Gypsum — Specification |
| BS 1191 (Parts 1&2) | Specification for Gypsum building plasters |
| ASTM C28 / C28M | Standard Specification for Gypsum Plasters |
| EN 13279 | Gypsum binders and gypsum plasters |
Conclusion
Plaster of Paris remains an indispensable material in Indian interior construction. From the elegant cornices of luxury homes and the smooth ceilings of every modern apartment, to the intricate Ganesh idols of Maharashtra and the dental moulds of dentists — POP’s rapid setting, fine finish, fire resistance and easy carvability give it unique advantages no other material matches.
Remember: POP is for interior dry locations only. For exterior walls and bathrooms, use water-resistant wall putty; for wall plastering, use gypsum plaster; for primer and paint, see primer, interior emulsion and exterior weatherproof paint.