Ground vs Precipitated Calcium Carbonate: GCC or PCC, and How to Specify

Ground calcium carbonate (GCC) is natural limestone, marble or chalk that has been milled and classified; precipitated calcium carbonate (PCC) is manufactured by reacting lime with carbon dioxide. GCC is the economical default for plastics masterbatch, paint, construction products and many paper uses. PCC costs more and is chosen where a very fine, uniform particle, high whiteness, or a particular crystal shape improves the end product. In both cases, the buying decision should rest on a written specification covering whiteness, D50, D97 (top cut), moisture and surface treatment, not on the product name.

This guide is aimed at buyers and technical purchasers of calcium carbonate who need to compare offers, brief a supplier and avoid the common mismatches. It covers how GCC and PCC are made, what the key specification lines mean, how coating affects performance and how to match grade to application. Our range is described on the calcium carbonate product page.

The short answer: GCC or PCC?

• Choose GCC when cost matters, particle size in the low single-digit to tens of micrometres is acceptable, and the application is filler-driven: filler masterbatch, PVC and polyolefin compounds, decorative paints, tile adhesives, putty, joint compounds, mortar and asphalt filler.
• Choose PCC when you need a finer, more uniform particle, higher whiteness or opacity, a controlled crystal shape, or specific rheology: premium paints, high-brightness paper, sealants, some plastics and specialty applications.
• Choose by specification, not label. A fine GCC and a coarse PCC can overlap in some properties. What matters is the measured whiteness, particle size distribution, surface area, moisture and coating on the CoA.

How GCC is made

GCC starts with a mineral deposit. The rock is crushed, then ground either dry in mills with air classification, or wet as a slurry, which is then dried if powder is required. Classifiers remove coarse particles, which sets the top cut. The ground product can be sold uncoated or surface treated.

Because it is a natural product, GCC quality depends on the deposit. Marble generally gives the highest whiteness and purity, limestone varies widely, and chalk is soft with its own character. Impurities such as iron, magnesium, silica and clay minerals affect whiteness, abrasiveness and compliance. The supplier's choice and control of raw material is therefore as important as the milling.

How PCC is made

PCC is made by calcining limestone to produce quicklime (calcium oxide), slaking it with water to form milk of lime (calcium hydroxide slurry), and bubbling carbon dioxide through the slurry to precipitate calcium carbonate. By controlling temperature, concentration and process conditions, producers shape the crystal habit and size.

The result is a very fine, uniform particle with a narrow size distribution and high purity. These features cost more to produce, and PCC is often made in dedicated plants, or at paper mills in on-site facilities designed to supply that mill directly.

Key specification lines and what they mean

Whiteness and brightness

Whiteness is usually reported as a brightness value (for example ISO brightness for paper-related grades) or as colour coordinates (L*, a*, b*), and sometimes as a yellowness index. Check three things.

1. Which scale. ISO brightness, Hunter L and CIE L* are not interchangeable.
2. Which sample condition. Powder pressed into a tablet, dry powder or slurry will give different readings.
3. Whether yellowness is controlled. Two grades with the same brightness can look different if one is yellower.

Whiteness matters most in paper, white and pastel paints and in plastics where colour is critical. In dark or heavily pigmented products, it matters less, and paying for a higher whiteness grade may be a waste.

Particle size: D50 and D97

Particle size distribution is expressed with percentile values.

• D50 (median): half the particles by volume are finer than this size. It is the general indicator of fineness.
• D97 (top cut): 97% of the particles are finer than this size. It tells you how large the coarse tail is.

D50 and D97 are not the same thing, and the gap between them describes the width of the distribution. Two products with the same D50 can behave very differently if one has a much larger top cut.

• Top cut matters for thin films, fibres, gloss paints and coatings, where a few coarse particles cause defects, die-lip build-up or poor surface finish.
• D50 matters for opacity, packing, viscosity and mechanical properties.

Always state the measurement method. Laser diffraction and sedimentation (for example the Sedigraph method) give different numbers for the same powder because they measure different things. A supplier quoting D50 by laser diffraction and a buyer expecting sedimentation will not see matching results. Write the method into the contract.

As a rule, plastics filler grades sit in the low single-digit micrometre range for D50, ultra-fine coating and sealant grades are finer, and coarser grades are used in construction products. Confirm the actual figures with the supplier, since ranges differ by application.

Residue on sieve

For coarser grades and some paint applications, a residue on a specified sieve (for example 45 micrometres, roughly 325 mesh) is quoted instead of, or in addition to, D97. It is a simple way to limit oversize particles.

Calcium carbonate content and impurities

State the minimum calcium carbonate assay and limits for acid-insoluble matter, magnesium carbonate, iron oxide and other relevant impurities. If the product is going into food contact, drinking water, pharmaceuticals or toys, state the standard that applies and ask for supporting documentation.

Natural carbonates can contain small amounts of crystalline silica (quartz). Check the SDS for the declared content, as this affects dust-handling requirements in your plant.

Moisture

Moisture should be tightly controlled for plastics. Even a small amount of water causes bubbles, voids, die drool and surface streaks when the compound is melted. Paint and construction are more tolerant, but moisture still affects bagging, flow and storage. State the maximum moisture and the test method.

Oil absorption and surface area

• Oil absorption indicates how much resin, plasticiser or binder the filler demands. High oil absorption raises viscosity and cost in formulations.
• BET surface area reflects fineness and porosity. Higher surface area generally means higher binder demand, higher coating consumption and, in plastics, a greater tendency to pick up moisture.

Other lines

Bulk density (for handling and silo design), pH of slurry, and, for paper, abrasion value (such as the Einlehner test) and slurry solids and viscosity if purchased as slurry.

Coated vs uncoated

Calcium carbonate is naturally hydrophilic: it likes water and does not mix readily with non-polar polymers. Surface treatment solves that.

• Coated grades are treated with a fatty acid, most commonly stearic acid or a stearate, which makes the surface hydrophobic. This improves dispersion in polyolefins and PVC, reduces moisture pickup, lowers melt viscosity and improves mechanical properties and processing.
• Uncoated grades are used where water-based systems and mineral binders dominate: water-based paints, paper, cement-based construction products and many adhesives.

Points to check on coated grades:

1. Coating agent and level. Ask what the agent is and how much is applied, usually expressed as a percentage by weight. Too little leaves untreated surface; too much can cause plate-out, odour or processing problems.
2. Coating uniformity and consistency. Request test data such as hydrophobicity, loss on ignition or moisture pickup behaviour.
3. Food-contact and regulatory status. Not every coating agent is suitable for every end use, so state the requirement.

Decision rule: polymers (polyethylene, polypropylene, PVC) generally call for coated grades; water-based and cementitious systems generally call for uncoated grades. Using the wrong one is a common source of complaints.

Matching grade to application

Plastics and filler masterbatch

Filler masterbatch concentrates calcium carbonate in a polymer carrier, often at loadings that make the filler well over half of the masterbatch by weight, and is then let down into the final plastic. It reduces cost, can improve stiffness and shrinkage control, and is widely used in polyethylene and polypropylene films, bags, injection-moulded products and PVC pipe and profile.

• Grade: coated GCC is the usual choice.
• Priorities: low moisture, effective coating, controlled D97 (especially for films), consistent whiteness and good dispersion.
• Cautions: a coarse top cut causes film breaks and pinholes. High surface area raises moisture pickup and lowers the loading you can achieve.

PCC, usually ultra-fine and coated, is used in some PVC and specialty compounds where impact strength, surface finish or particular rheology matter.

Paint and coatings

Calcium carbonate acts as an extender, supports opacity, controls gloss and sheen, and reduces cost by replacing more expensive pigments.

• Decorative emulsions: uncoated GCC across a range of finenesses, selected to set matt level, scrub resistance and cost.
• Premium or high-opacity paints: finer GCC or PCC, chosen for brightness and for the way fine particles space out titanium dioxide.
• Priorities: whiteness, D50 and D97 (a coarse tail affects smoothness and gloss), oil absorption and a consistent grade from batch to batch.

Paper

In alkaline papermaking, calcium carbonate is used as a filler and as a coating pigment, improving brightness, opacity and smoothness while reducing fibre cost.

• GCC is widely used where high brightness and cost efficiency are required, often supplied as slurry.
• PCC is chosen for bulk, opacity and printing performance, and many mills use PCC produced at on-site satellite plants.
• Priorities: brightness, particle size distribution, abrasion, slurry stability and consistency.

Because paper-grade carbonate is frequently supplied as slurry or from on-site plants, bagged powder trade into paper is more limited than into plastics, paint and construction. Confirm the format a mill actually buys before quoting.

Construction and building products

Calcium carbonate is a bulk filler in tile adhesives, grouts, joint compounds, putty, mortar, render, self-levelling compounds and asphalt filler.

• Grade: mostly uncoated GCC, in coarser and mid-range sizes.
• Priorities: cost, consistent grading, moisture control, and an acceptable colour for visible products.
• Cautions: large variations in particle size distribution change workability and water demand. For products with a colour requirement, specify whiteness too.

Packaging, handling and storage

• Packaging. Commonly 25 kg bags and bulk bags of around one tonne, palletised and wrapped. Bulk loads in silo tankers are common for regular large consumers. Slurry is supplied in tankers.
• Moisture protection. Use lined bags and keep pallets dry. Uncoated powder takes up moisture more readily, and coated powder is more resistant but not immune.
• Contamination. Keep coated and uncoated lots separate, and avoid mixing batches in the plant.
• Dust. Calcium carbonate is generally not classified as hazardous for transport, but dust is a nuisance and should be controlled. Follow SDS guidance on respiratory protection and housekeeping.
• Chemical compatibility. It reacts with acids to release carbon dioxide, which matters in acidic formulations and for storage near acids.

Always check the SDS for the specific grade, including the declared crystalline silica content for natural products.

Common specification mistakes

• Quoting D50 only. Without D97 or a sieve residue, you have no control over the coarse tail that causes film defects and rough paint surfaces.
• Omitting the test method. A D50 figure without its method (laser diffraction or sedimentation) cannot be checked or enforced.
• Buying coated grade for water-based systems. A hydrophobic surface can resist wetting and cause dispersion problems in emulsions and cement mixes.
• Paying for whiteness you do not need. Match brightness to the colour of the final product.
• Ignoring moisture on polymer grades. A powder that looks dry can still cause voids and die drool at extrusion temperatures.
• Approving on a single sample. Request a CoA for each lot and compare it with the approved sample, because natural feedstock varies.

Each of these is cheap to fix at the enquiry stage and expensive to fix after a container has landed. Put the numbers and methods in the contract, and keep a retained sample of every approved lot for comparison if a dispute arises.

How to specify calcium carbonate in an RFQ

1. Type: GCC or PCC, and the raw material if it matters (marble, limestone, chalk).
2. Application: masterbatch, paint, paper, construction or other, and the polymer or system.
3. Particle size: D50 and D97 with the measurement method, or residue on a named sieve.
4. Whiteness: value, scale and test condition.
5. Surface treatment: uncoated or coated, with the coating agent and level.
6. Moisture: maximum, with test method.
7. Purity: minimum calcium carbonate and limits for acid-insolubles, magnesium and iron.
8. Other properties: oil absorption, BET surface area, bulk density if relevant.
9. Packing and quantity: bag size, bulk bags or bulk loads, palletisation and total tonnage.
10. Destination, Incoterms, timing and inspection: including whether you want SGS or Intertek pre-shipment inspection.

Ask for a sample and a CoA before committing to volume, and run your own trial. Test results from a laboratory do not always translate to your process.

Frequently asked questions

Is PCC always better than GCC?

No. PCC offers fineness, uniformity and shape control, but costs more, and many applications do not need those properties. In filler masterbatch and construction products, GCC usually delivers the required performance at a lower cost.

What is the difference between D50 and D97?

D50 is the median particle size: half the particles are finer. D97 is the top cut: 97% are finer. D97 shows the coarse tail, which controls defects in films, coatings and fine surfaces.

Should I buy coated or uncoated calcium carbonate for plastics?

Coated, in almost all polyolefin and PVC applications, because the coating improves dispersion and reduces moisture pickup. Uncoated suits water-based and cement-based systems. Confirm with your formulation.

Why do two suppliers quote different D50 for similar grades?

Often because they use different measurement methods or sample preparation. Laser diffraction and sedimentation give different results. Specify the method, and ideally test samples side by side in your own laboratory.

Does higher whiteness mean higher quality?

Not necessarily. Whiteness is critical for paper, white paint and light-coloured plastics, but a premium for whiteness is wasted in dark or heavily pigmented products. Specify the whiteness you need, not the highest available.

Is calcium carbonate a dangerous good for shipping?

Generally it is not classified as dangerous for transport, but confirm with the SDS for the exact grade. Dust control and the declared crystalline silica content are the usual safety considerations.

Next step: send us your enquiry

Chemineral is based in Sohar, Oman, and exports from Sohar and Salalah. We quote Suez and Cape routing side by side, and our firm offers are valid for 72 hours. To receive a quotation for calcium carbonate, send your application, particle size, whiteness, coating and packing requirements to our trade desk by submitting an RFQ through the contact page.