Gypsum: IMSBC Code Schedule and Carriage

Gypsum is calcium sulphate dihydrate (CaSO4·2H2O), classified as Group C under the IMSBC Code: non-liquefiable and free of chemical hazards under standard conditions. The practical concerns in carriage are cargo hardening if the hold takes water, dust exposure during handling, and, for very fine moist synthetic forms, the need to verify that the actual cargo characteristics still fall outside the Group A liquefaction threshold.

Gypsum is one of the higher-volume Group C cargoes in global seaborne trade. The IMSBC Code contains a dedicated schedule for gypsum under its Appendix 1, alongside a separate schedule for calcium sulphate (anhydrous and hemihydrate forms). Both are Group C cargoes, meaning the Code imposes no liquefaction testing requirement and no chemical hazard provisions beyond standard good seamanship. That regulatory simplicity can create complacency. Gypsum’s chemistry makes it distinctly unforgiving of water ingress, and the industry occasionally learns this when a receiver at a cement plant or wallboard factory finds a hold full of set plaster instead of pourable granules.

This article covers the mineralogy and distinction between natural and synthetic gypsum, the principal trade routes, the IMSBC schedule particulars, the dust and caking hazards in detail, the Group A fines caveat, hold and hatch preparation, loading and discharge operations, and the draft survey approach for gypsum cargoes. The companion calculators on this site include the IMSBC Gypsum schedule reference and the IMSBC Gypsum Anhydrite reference.

Mineralogy and chemistry of gypsum

Gypsum is the common name for calcium sulphate dihydrate, chemical formula CaSO4·2H2O. In its natural form it is a soft evaporite mineral, Mohs hardness 2, that forms by the evaporation of saline water bodies or by the hydration of anhydrite (CaSO4, the anhydrous form) in the presence of groundwater. The colour ranges from white through grey to pinkish-red depending on trace iron content and impurities. The crystal habit ranges from coarse selenite crystals (transparent, cleavable plates) through compact alabaster to the earthy massive form known as rock gypsum, which is the form mined and shipped in bulk.

When gypsum is heated to between 107 and 170 degrees Celsius, it loses approximately three-quarters of its water of crystallisation, producing calcium sulphate hemihydrate, also written CaSO4·0.5H2O. This is plaster of Paris. At temperatures above 200 degrees Celsius, further dehydration produces soluble anhydrite, and above 400 degrees Celsius, insoluble anhydrite (dead-burnt gypsum). The hemihydrate reaction is important to bulk carriage because it runs in reverse: hemihydrate, once exposed to water, reabsorbs water and recrystallises as dihydrate, releasing heat and expanding volumetrically. This is the setting reaction that makes plaster of Paris a building material. It is also, in a cargo hold, the mechanism that transforms a load of damp gypsum into a solid mass bonded to the hold internals.

The reverse reaction (setting) requires the material to first be calcined (dehydrated to hemihydrate). Natural mined gypsum does not need to be calcined before shipment, and bulk gypsum in a hold will not spontaneously set merely from being wet. What actually happens is different: wet gypsum dissolves fractionally in water, then the solution migrates and, if the water later evaporates, leaves behind recrystallised gypsum bridging the inter-particle spaces. Over a long voyage with repeated wetting and drying, this recrystallisation can produce a product that behaves, at the discharge port, like a partially set plasterboard layer. The effect is less dramatic than true plaster setting but is enough to cause grab-handling difficulty at discharge.

Anhydrite and hemihydrate: related IMSBC entries

The IMSBC Code’s Appendix 1 lists calcium sulphate separately from gypsum. Calcium sulphate covers principally anhydrite, which is the anhydrous natural mineral mined as a co-product or sole product from sulphate rock deposits in Germany, Austria, and the United Kingdom, and used as a cement set-retarder or soil conditioner. Hemihydrate (plaster of Paris and calcined gypsum) is a processed material with somewhat different handling characteristics. All three entries, gypsum, calcium sulphate, and the anhydrite sub-form, are Group C. The IMSBC Gypsum Anhydrite calculator covers the anhydrite schedule particulars.

Natural versus synthetic (FGD) gypsum

Natural mined gypsum

Natural gypsum occurs as evaporite deposits interbedded with limestone, dolomite, and halite. Major producing countries are the United States (Tennessee, Iowa, Texas, Oklahoma, California, Michigan, Nevada), Mexico (Baja California Sur, Sonora), Oman (Dhofar Governorate), Iran (multiple deposits), Spain (Andalucia, Aragon), Türkiye (Sivas, Malatya), Thailand, and Australia. Global natural gypsum mine production has historically been in the range of 120 to 150 million tonnes per year.

Mined rock gypsum goes through crushing and screening to produce a saleable product. Most export-grade rock gypsum is crushed to a maximum particle size of around 50 to 100 millimetres, with the fines fraction screened out for domestic use. The product loaded onto bulk carriers is a coarse, granular material that flows freely, handles easily by grab and conveyor, and has a bulk density in the range of 1.2 to 1.4 tonnes per cubic metre. Moisture content at loading is generally low, typically 2 to 8 percent by mass, because the material has been exposed to arid mine conditions in countries such as Oman and Iran. This relatively low moisture content, combined with the coarse particle size, places natural mined gypsum firmly in the Group C category with little ambiguity.

The dominant natural gypsum trade routes are from Oman and Iran to India and Southeast Asia, from Mexico to the US East Coast and the Mediterranean, and from Spain to North Africa. Oman’s Dhofar region contains large high-purity deposits that have supplied Indian and Bangladeshi cement plants for decades. A single shipment from Salalah or Sohar to an Indian cement plant at Mundra, Vizag, or Chennai typically involves a Handymax or Supramax bulker carrying 30,000 to 55,000 tonnes.

Synthetic FGD gypsum

Flue-gas desulphurisation (FGD) gypsum is produced at coal-fired power stations equipped with wet limestone scrubbers. Sulphur dioxide in the flue gas reacts with a limestone slurry to produce calcium sulphite, which is then oxidised in an air-sparged absorber to produce calcium sulphate dihydrate: the same compound as natural gypsum. The reaction is:

FGD gypsum purity is high, often 93 to 97 percent CaSO4·2H2O, and it is directly substitutable for natural gypsum in both wallboard and cement applications. Power stations in Germany, Japan, South Korea, and the United States produce large quantities of FGD gypsum. In Germany, FGD gypsum has largely displaced natural gypsum in wallboard production since the 1980s; the German wallboard industry relies almost entirely on FGD supply from domestic coal stations.

The key difference from a shipping perspective is particle size and moisture content. FGD gypsum emerges from the filter press or vacuum belt filter as a fine, moist cake with particles typically below 200 micrometres (0.2 mm) and a free moisture content of 10 to 20 percent by mass. This fine, moist character has two practical consequences for shipping. First, dust generation during loading and discharge is significantly higher per tonne handled compared with coarse natural gypsum. Second, the combination of fine particle size and elevated moisture content may, if the cargo is particularly fine and wet, approach the conditions under which Group A liquefaction assessment becomes appropriate.

FGD gypsum: the Group A caveat

The IMSBC Code classifies FGD gypsum as Group C in the standard schedule, matching natural gypsum. This classification is correct for typical commercially supplied FGD gypsum that has been dewatered to a free moisture content below about 15 percent and that has a particle size distribution with a substantial coarse fraction. But IMSBC Code Section 1.3 states explicitly that where a cargo’s actual properties differ from those listed in the schedule, the actual properties must be declared and the appropriate precautions applied.

Group A cargoes are defined as those that may liquefy if moisture content exceeds the Transportable Moisture Limit. The Group A threshold is determined by the physical properties of the cargo, not by its chemical identity. A cargo of very fine FGD gypsum with a high free moisture content can, under repeated ship motion, develop elevated pore-water pressure and shift in the hold. Masters receiving cargo declarations for FGD gypsum with very high moisture content or with particle size data indicating a predominantly sub-0.5-mm fraction should ask the shipper whether a TML test has been performed. If the Flow Moisture Point has been measured and the actual moisture content is shown to be well below 0.9 x FMP, the Group C classification stands. If the cargo is close to or above that threshold, Group A precautions, including a formal TML certificate and cargo declaration per IMSBC Code Section 4.6, apply.

This is not a frequent operational issue for standard FGD gypsum shipped as a commercial product; it is a known edge case that competent masters and cargo superintendents keep in mind when a new supplier or a non-standard specification is presented.

IMSBC Code schedule particulars

The IMSBC Code’s Appendix 1 GYPSUM entry records the following schedule properties. The table below summarises the key parameters from the current consolidated edition, which incorporates amendments through Amendment 07-23 (MSC.539(107), mandatory from 1 January 2025).

The schedule notes that the cargo may harden if wetted and that this hardening can cause serious difficulties at discharge. It also notes that dust may be generated during loading and trimming and that adequate precautions to limit dust exposure are required.

The separate CALCIUM SULPHATE entry in Appendix 1 covers anhydrous calcium sulphate (anhydrite, CaSO4) and related synthetic forms. Anhydrite has a notably higher bulk density, approximately 2,200 to 2,500 kg/m³, reflecting the absence of the two water molecules. Stowage factor is correspondingly lower at around 0.40 to 0.45 m³/t.

Classification under SOLAS Chapter VI

SOLAS Chapter VI Regulation 2 requires that solid bulk cargoes be declared to the master using the IMSBC Code classification. For gypsum shipped as a Group C cargo, the shipper’s cargo declaration must confirm Group C status and state the actual bulk density, moisture content, and particle size, together with the full chemical description. Where the cargo is FGD gypsum with unusual moisture or size characteristics, the shipper must confirm that the Group C classification applies to the actual material.

SOLAS Chapter XII, which covers additional safety measures for bulk carriers, sets structural requirements but does not change the IMSBC Code group classification of gypsum. Class surveys under the Enhanced Survey Programme for bulk carriers, as administered by classification societies, are not modified by the Group C status of a specific cargo.

Principal trade flows

Oman, Iran, and the Indian subcontinent

The largest seaborne gypsum trade is from Oman and Iran to India, Bangladesh, and Sri Lanka. Oman’s Dhofar Governorate holds extensive high-purity gypsum deposits. The main export terminals are Salalah, Sohar, and Mina al-Fahal. Iran exports from Bandar Abbas and Bushehr. Receiving ports in India include Mundra, Kandla, Tuticorin, Vizag, and Haldia; cement plants in these regions depend on imported gypsum as a set-retarder additive because domestic Indian production does not fully meet demand.

Typical vessel size on this trade is 30,000 to 55,000 DWT, matching Handymax and Supramax tonnage. Journey time from Oman to Mundra is approximately seven to ten days; from Oman to South India or Bangladesh, ten to fourteen days. The cargo is routinely presented as dry, coarse rock gypsum with moisture content in the 3 to 7 percent range, well within the Group C classification parameters.

Mexico and the US and Mediterranean markets

Mexico is a large natural gypsum exporter, principally from the Santa Rosalia district of Baja California Sur, which has been producing gypsum for export since the early twentieth century. Exports go to US East and Gulf Coast wallboard plants, to US cement plants, and to customers in Spain, Portugal, and North Africa. Shipments from Santa Rosalia use vessels in the 20,000 to 35,000 DWT range.

The United States itself imports gypsum from Mexico, Canada, and Spain despite its own domestic production, because plant locations do not always align with mine locations. US wallboard plants on the East Coast have historically used imported Canadian and European gypsum as well as FGD gypsum from domestic power stations.

European and North African trades

Spain ships gypsum from Almeria and other Andalucian ports to North Africa, the UK, Ireland, and the Netherlands. Türkiye exports to western European and Black Sea markets. The UK imports natural gypsum from Spain and anhydrite from Germany to supplement declining domestic production from the Nottinghamshire and Cumbrian quarries. These are shorter-haul trades, typically 2,000 to 5,000 DWT coasters for the Mediterranean routes and 10,000 to 20,000 DWT vessels for the Atlantic routes.

FGD gypsum coastal trades

FGD gypsum from German, Japanese, and South Korean coal-fired power stations moves primarily by truck, barge, and coastal vessel to nearby wallboard plants. The material is rarely shipped across ocean basins because its high moisture content increases freight cost relative to its value, and because coal-fired power generation is being phased out in many regions, reducing the supply. Where FGD gypsum is shipped in bulk, voyage times are short, reducing the exposure time to potential water ingress.

End uses and quality requirements

Plasterboard and wallboard

Plasterboard (also called drywall or wallboard) is the dominant end use for gypsum globally. The manufacturing process calcines gypsum at around 150 degrees Celsius to produce alpha or beta hemihydrate, which is then mixed with water to form a slurry, sandwiched between paper facings, formed into boards, and dried in a kiln. The final product is calcium sulphate dihydrate again, recrystallised into an interlocking needle-like microstructure that gives plasterboard its mechanical rigidity.

Wallboard-grade gypsum must meet strict purity requirements, typically 80 to 90 percent CaSO4·2H2O minimum for natural gypsum, with low levels of chloride (below 0.05 percent), low organic content, and controlled heavy metal concentrations. Contamination during shipping, either from residues of a previous cargo or from sea water, can render an entire parcel commercially non-conforming. This creates a strong commercial incentive for careful hold preparation and weathertight hatch covers.

Cement set-retarder

Portland cement clinker, made by burning limestone and clay at 1,400 to 1,500 degrees Celsius in a rotary kiln, contains approximately 50 to 70 percent tricalcium silicate and 5 to 10 percent tricalcium aluminate (C3A). C3A reacts very rapidly with water: without a retarder, the cement would flash-set within minutes of mixing, making it unworkable. Gypsum added during ball milling at around 4 to 6 percent by weight reacts with C3A to form ettringite (calcium sulphoaluminate hydrate), which coats the C3A particles and slows the initial set, allowing normal working time.

Cement-grade gypsum tolerates somewhat lower purity than wallboard-grade, and natural rock gypsum with 70 to 85 percent CaSO4·2H2O and moderate clay content is commonly used. The cement application is more tolerant of the coarser, less-pure rock gypsum that is typical of Omani and Iranian exports. This is one reason why the Oman-India trade in cement-grade gypsum is large and economically viable despite the relatively modest FOB price of the commodity.

Agricultural use

Agricultural gypsum is applied as a soil conditioner, primarily to clay soils to improve permeability (via calcium-sodium exchange on clay particles, which deflocculates soil aggregates and improves drainage) and to supply sulphate-sulphur to sulphur-deficient soils without altering soil pH. Application rates of 1 to 5 tonnes per hectare are typical. Gypsum for agricultural use has the lowest purity requirements of the three main markets and can use the finer or lower-grade material screened out during the preparation of wallboard or cement-grade product.

Agricultural gypsum is often supplied domestically from local mines or FGD gypsum streams rather than via ocean shipment. However, in regions where local production is limited, such as parts of South Asia and Southeast Asia, imports do occur.

Dust: health risk and control

Gypsum dust is a nuisance dust rather than a specifically toxic substance. Unlike silica dust (which causes silicosis), asbestos, or cadmium-containing mineral dusts, gypsum dust does not have an established specific disease pathway. The health concern is for the respiratory system under prolonged exposure to respirable-fraction dust, the sub-4-micrometre particles that penetrate to the alveoli. Occupational hygiene standards for calcium sulphate dust in most jurisdictions fall under general nuisance dust limits, typically 10 milligrams per cubic metre total inhalable dust and 4 milligrams per cubic metre respirable dust (these are representative values from European and US standards; the applicable limits at any port are those of the relevant national authority and any flag-state requirements).

During loading of fine or crushed gypsum, dust clouds can be generated at the load point, at the hatch opening, and in the hold during trimming. The IMSBC Code schedule notes the dust concern. Practical controls include:

• Water misting at the load spout to suppress airborne dust at source.
• Hatch covers partially closed during loading where the configuration allows trimming without full open hatches.
• Dust masks (P2/FFP2 or P3/FFP3 rated depending on dust concentration) for all personnel on the loading deck.
• Ventilation of the hold before entry, with atmospheric testing for oxygen content (minimum 20.5 percent by volume) per IMSBC Code enclosed space entry requirements.

Dust also creates a nuisance on deck and in the superstructure. Gypsum dust is mildly corrosive if it contacts wet steel surfaces because calcium sulphate in solution produces a mildly acidic environment, though the corrosion rate is low compared with, say, fertilizer cargoes. Deck and accommodation washing after cargo operations is standard good practice.

FGD gypsum, being finer than natural rock gypsum, generates significantly more dust per tonne handled. Where FGD gypsum is loaded from enclosed conveyors into a hold, dust suppression by water mist is particularly important.

Caking and hardening: the dominant carriage hazard

The mechanism

Gypsum does not set spontaneously on contact with water in the way that plaster of Paris does, because plaster of Paris is hemihydrate (CaSO4·0.5H2O), not dihydrate. Bulk gypsum at Group C conditions is already the fully hydrated form (dihydrate) and does not have a thermodynamic driving force to absorb more water and crystallise.

What actually happens when gypsum gets wet in a hold is a process of dissolution and recrystallisation. Gypsum has a moderate solubility in water: approximately 2.0 to 2.4 grams per litre at 20 degrees Celsius, which is low compared with table salt (360 g/L) but significant over time with repeated wetting. When sea water or rain enters the hold and contacts the gypsum cargo, fine particles at the surface and inter-particle contact points dissolve partially. As the water evaporates or drains away, the dissolved calcium sulphate recrystallises, often in a coarser needle-like habit that bridges adjacent particles and cement them together.

Over a voyage of two to three weeks with repeated small water exposures, this recrystallisation can produce a hard crust on the cargo surface and a consolidated layer beneath. If the hold takes a significant ingress event, for example a hatch cover seal failure in heavy weather, the deeper layers may also harden. On arrival at the discharge port, the grab operator finds that the cargo surface resists penetration, grabs cannot take full bites, and cleaning the hold becomes very difficult. Manual breaking out with pneumatic chisels may be required, at high cost and slow rate.

Seawater versus fresh water

Sea water accelerates caking compared with fresh water because the dissolved chloride and other salts lower the solubility of calcium sulphate slightly (common-ion effect for sulphate from sea water’s sulphate content) and because salt crystallisation in the inter-particle spaces adds a second binding mechanism. Cargo contaminated with sea water is also commercially non-conforming for the wallboard market because chloride content specifications are strict.

Rain water causes less rapid caking than sea water and does not introduce chloride contamination, but repeated rain wetting still drives recrystallisation cycles. The difference matters at the discharge port: cargo wetted only by rain may be acceptable to the cement plant but not to the wallboard plant.

Consequences at discharge

Hardened gypsum at a cement plant is a serious operational problem. Cement plants receive gypsum for direct feeding to the ball mill; they expect a free-flowing granular material that can be discharged by grab into a hopper, conveyed, and fed by screw or belt to the mill inlet. A hard-caked parcel does not flow. Discharge rates drop from 1,500 to 3,000 tonnes per day (typical for free-flowing gypsum) to a few hundred tonnes per day when manual breaking out is needed. A 50,000-tonne shipment that should discharge in two to three days may take eight to twelve days if seriously hardened, incurring substantial demurrage.

The commercial consequences fall primarily on the shipowner under the bill of lading if a surveyor determines that the hardening resulted from sea water ingress, which implies a breach of the obligation to maintain weathertight hatch covers. If the hardening is traced to condition at loading (cargo too moist, pre-existing caking from stockpile exposure), liability shifts toward the shipper or charterer.

Preventing caking

Practical prevention measures include:

• Loading dry cargo only: cargo with moisture content above about 10 to 12 percent should be queried, particularly for natural gypsum.
• Inspect and test hatch cover seals before loading, using hose-testing or ultrasonic testing per Class requirements.
• Ventilate the hold during the voyage with caution: ventilation that draws moist sea air through a warm hold and deposits condensation on cool cargo contributes to caking. Ventilation should follow the dew-point rule (ventilate only when outside dew point is lower than cargo temperature).
• Place dunnage between the cargo and any exposed steel areas at the hatch coaming junction where condensation tends to concentrate.

Hold preparation

Gypsum is not a chemically demanding cargo in terms of reactivity with steel or with other cargo residues, but it is demanding in terms of cleanliness and dryness. The IMSBC Code schedule specifies that holds must be clean and dry before loading.

Previous cargo compatibility. Gypsum that will be used in wallboard or cement plants is subject to strict purity specifications. Residues of fertilizers (ammonium nitrate, urea, DAP) contaminate gypsum and are objectionable at both wallboard and cement plants. Residues of sulphur can introduce excess sulphate into cement or wallboard. Residues of organic cargoes (grain, coal, petroleum coke) introduce organic contamination that discolours wallboard and can affect cement chemistry. Holds coming from these previous cargoes require thorough washing and drying before accepting gypsum.

Bilge system. All bilge suctions must be tested operational, strum boxes cleaned and free of debris, and bilge wells pumped dry before loading. During the voyage, bilge monitoring is important: any water accumulation in the bilge under the cargo indicates either ingress or moisture drainage from the cargo, and should be pumped promptly to prevent re-absorption by the cargo.

Hatch covers. Weathertightness is the most critical hold preparation item for gypsum. Hatch cover seals should be inspected visually for cracks and compression gaps. Hose testing (or ultrasonic testing using a Class-approved device) must be conducted and documented before loading. Any leaking seal must be replaced. This requirement is not peculiar to gypsum, but the consequences of hatch failure are particularly severe for this cargo given the caking and commercial non-conformance risks.

Hold washing. After a clean previous cargo, sweeping and blowing down with compressed air is usually sufficient. After a potentially contaminating cargo, hot water washing with detergent, followed by a fresh water rinse and thorough drying (by ventilation or by steam heating if available), is standard. The hold must be completely dry at the time loading commences. A cargo inspector or class surveyor should attend to issue a hold cleanliness certificate, which will be needed by the shipowner for any cargo quality dispute at the discharge port.

Loading operations

Loading rate and equipment

Natural gypsum is typically loaded at the export terminal by a shiploader fed from a conveyor. Loading rates of 1,500 to 4,000 tonnes per hour are achievable at dedicated mineral export terminals in Oman, Mexico, and Spain. At smaller ports or quarry-side berths, loading by front-end loader and conveyor or even by grab from stockpile may proceed at lower rates.

Trimming is required to achieve a safe distribution of the cargo mass across the hold width and to avoid excessive loading on the inner bottom near the hatch coamings. Most gypsum cargoes trim freely by gravity from the load spout, but at the final stages of loading, bulldozers or mechanical trimmers may be needed to push the cargo outboard to achieve level trimming. Gypsum does not present the same trimming resistance as a sticky cargo such as wet coal or as a non-cohesive cargo such as grain, but attention to the tank-top loading limit per hold is required during loading at high rates.

Cargo declaration and documentation

The shipper must provide to the master before loading:

• The cargo declaration identifying the Bulk Cargo Shipping Name (GYPSUM), the Group (C), and the actual physical properties.
• A certificate of analysis stating moisture content, particle size distribution, and chemical purity if required by the receiver.
• A statement of bulk density (now required under Amendment 07-23 for all IMSBC Code cargoes).

For FGD gypsum with elevated moisture or fine particle size, the shipper should additionally confirm either the TML test results (showing that moisture is well below the Group A threshold) or a written explanation from a competent authority that the cargo characteristics have been assessed and found to fall within the Group C classification.

The master should verify that the cargo as loaded appears consistent with the declaration. Very fine, wet, or visibly different material compared with the declared properties should be queried before loading continues.

Draft survey for gypsum cargoes

A cargo draft survey is the standard method of determining the quantity of gypsum loaded or discharged on a bulk carrier. Because gypsum is a fairly dense commodity with a relatively stable bulk density compared with some other bulk cargoes, draft surveys are generally reliable for gypsum provided standard precautions are followed.

Pre-loading survey

Before loading commences, surveyors take six draft readings (fore, mid, and aft on each side), measure ballast quantities in all tanks, measure fuel oil, fresh water, and stores quantities, and weigh the constants (permanent equipment on board not otherwise measured). The initial displacement is computed from the vessel’s hydrostatic tables, and the initial condition is established as the reference.

Post-loading survey

After loading is complete, final drafts are taken, all tank soundings are re-measured, and the cargo quantity is computed as the difference between the final and initial displacements, corrected for changes in ballast, bunkers, and other measured items. The result is the draft survey figure for cargo loaded, which should agree with the shore weight (belt-weigher or stockpile survey) within the agreed tolerance, typically plus or minus 0.5 percent.

Gypsum-specific considerations

Gypsum does not require a moisture correction to the draft survey quantity (unlike wet cargoes such as coal, where the quantity on a dry basis may be commercially important). The cargo is typically sold on an as-loaded mass basis inclusive of its natural moisture content.

Density of sea water must be measured at the time of survey. The standard sea water density of 1.025 t/m³ is rarely exact: ports in the approaches to river deltas (India’s west coast ports, Bangladeshi ports) may have densities of 1.005 to 1.020 t/m³ if fresh water discharge is significant. Using the wrong density introduces systematic error into the displacement calculation. Surveyors take water samples from the sea beside the vessel and measure density with a calibrated hydrometer.

Squat and trim corrections are applied per the vessel’s hydrostatic tables. For gypsum cargoes, the vessel is typically loaded deep (gypsum’s moderate bulk density means the hold fills substantially before reaching the load-line draft), and draft reading accuracy is important. A 1-centimetre error in the mean draft of a 50,000 DWT Supramax represents approximately 80 to 100 tonnes of cargo, which at cement-grade gypsum prices of US$15 to 25 per tonne is a non-trivial commercial discrepancy.

Discharge operations

Standard grab discharge

Gypsum is discharged by shore crane or ship’s crane with a mechanical grab at most receiving terminals. Grab capacity is typically 5 to 12 cubic metres, giving individual lift weights of 6 to 17 tonnes for gypsum at 1.3 t/m³ bulk density. Discharge rates of 1,000 to 3,000 tonnes per day are typical at cement plant berths that may not have high-capacity shipunloaders.

Continuous shipunloaders (pneumatic or mechanical screw/bucket elevator types) are used at larger wallboard manufacturing facilities, where throughput is more important. These provide higher rates, typically 500 to 1,500 tonnes per hour.

The cargo trims towards the aft and forward ends of the hold as the grab removes the central pile, and bulldozers are used in the hold during the final cleaning stages to consolidate remaining cargo toward the grab’s reach. Hold cleaning after discharge is important: gypsum that remains damp in the corners and on the tank top will harden and be difficult to remove before the next cargo.

Hardened cargo: the discharge problem

When hardened gypsum is encountered at discharge, the standard response depends on the degree of hardening. A surface crust of a few centimetres can often be broken by the grab teeth and mixed back into the softer cargo below; this adds time but does not stop discharge. Deeper or more complete hardening requires manual intervention.

Personnel entering the hold to break out hardened gypsum must follow enclosed space entry procedures: atmospheric testing for oxygen deficiency and for toxic gases (CO, H2S) and issuance of a permit to enter before any person descends. Gypsum does not itself generate toxic gases, but the hold may have accumulated carbon dioxide from normal atmospheric processes if it has been closed for a long voyage. The atmospheric checks are mandatory under SOLAS and the IMSBC Code regardless of cargo type.

Pneumatic chisels, electric jackhammers, and water jetting have all been used to break out hardened gypsum. Water jetting redissolves the surface crust but creates a slurry that is difficult to pump from the bilge; it is the last resort. Mechanical chiselling is slower but leaves the cargo in a form that the grab can handle.

The commercial resolution of hardened cargo disputes depends on the survey findings. If sea water ingress from a failed hatch cover seal is documented, the P&I club is usually involved and the shipowner is at risk. If the cargo was delivered in a wet or partially caked condition, the shipper or shipper’s surveyor must establish this from a pre-loading survey report. Good documentation on both sides, including hold condition photographs before loading, cargo condition photographs at loading, and hatch cover test records, is the only protection against an inconclusive and costly dispute.

Structural loading and stability

Gypsum at a bulk density of 1.2 to 1.4 t/m³ is a medium-weight cargo for bulk carrier holds. It is lighter than iron ore (approximately 2.0 to 3.0 t/m³) and heavier than grain (approximately 0.75 t/m³). For a Handymax or Supramax bulk carrier designed for alternate-hold loading with iron ore, gypsum may be carried in all holds without exceeding the inner bottom plate loading limit, because the pressure per unit area (bulk density times cargo depth) is lower than for ore.

The master or operator must verify the inner bottom loading limit specified in the vessel’s loading manual before accepting gypsum cargo. For standard bulk carrier designs, the inner bottom plate loading limit is typically 10 to 15 tonnes per square metre; gypsum at 1.3 t/m³ filling a hold to 10 metres depth produces approximately 13 t/m² at the bottom, close to the limit. This means that a full hold of gypsum to the hatch coaming is not automatically safe for all vessels, and the loading manual must be consulted for the specific ship.

Stability during loading is generally straightforward for gypsum. The cargo does not shift under normal conditions (it is Group C, not liquefiable), and the loading sequence can follow the standard procedure of even loading across all holds or the alternate-hold pattern specified in the ship’s loading manual. No special stability precautions apply beyond those required for any solid bulk cargo under SOLAS Chapter VI and SOLAS Chapter XII.

Ventilation during the voyage

The IMSBC Code schedule for gypsum specifies that the cargo should be kept dry. The ventilation guidance for Group C cargoes generally is that holds may be ventilated by surface ventilation when conditions favour drying and no rain or sea water ingress is possible. For gypsum, the dew-point rule applies: ventilate only when the dew point of the outside air is lower than the temperature of the cargo hold atmosphere. If the outside air is warmer and more humid than the cargo (a common condition in tropical trades), ventilation will deposit condensation on the cargo surface and accelerate caking.

In practice, many bulk carriers carrying gypsum on the Oman-India trade keep hatches closed throughout the voyage. The voyage time is short enough (seven to fourteen days) that hold atmosphere quality does not become a concern, and the risk of condensation or sea water ingress from an open hatch is greater than the benefit of fresh air circulation. On longer voyages, such as Mexico to Europe, careful ventilation management is more important.

Limitations

This article describes the standard commercial grades of natural gypsum and FGD gypsum as currently traded and regulated under the IMSBC Code. The following limitations apply:

The IMSBC Code is periodically amended. The schedule particulars in this article reflect the current consolidated edition incorporating Amendment 07-23 (MSC.539(107), mandatory from 1 January 2025). Shippers, shipowners, and masters must verify against the current edition of the Code as maintained by the IMO.

The Group A caveat for fine moist FGD gypsum is this article’s assessment of how the IMSBC Code Section 1.3 provisions interact with the physical properties of atypical FGD gypsum. It is not a formal IMO determination. Masters or operators with a non-standard FGD gypsum shipment should seek advice from their P&I correspondent or a qualified cargo surveyor before loading.

Cargo caking and hardening behaviour depends on the specific cargo, voyage duration, hatch cover condition, weather encountered, and hold temperature. The description in this article is based on the established chemistry and known shipboard experience with gypsum cargoes; it does not substitute for a pre-loading cargo survey and a professional assessment of the specific shipment.

Commercial prices, trade volumes, loading rates, and port throughput data cited in this article are indicative only. They reflect publicly available ranges as of mid-2026 and are subject to change. They are not suitable for freight or commercial decision-making without verification from current market sources.

Draft survey accuracy is affected by vessel trim, heel, sea state at the time of reading, condition of the draft marks, and the accuracy of the vessel’s hydrostatic tables. The draft survey methodology described follows the standard OCIMF/IICL/FOSFA approach, but each survey must be conducted by a qualified and independent marine surveyor.

See also

• IMSBC Code
• IMSBC Group C Cargoes
• IMSBC Group A Cargoes
• Limestone: IMSBC Schedule
• Cement: IMSBC Schedule
• Sulphur: IMSBC Schedule
• Potash: IMSBC Schedule
• Phosphate Rock: IMSBC Schedule
• Cargo Draught Survey, Bulk Carriers
• SOLAS Chapter XII: Bulk Carrier Safety
• Bulk Carrier

Related calculators:

• IMSBC Gypsum schedule reference
• IMSBC Gypsum Anhydrite schedule reference