Cement Clinkers: IMSBC Code Schedule and Carriage

Cement clinker is classified as Group C under the IMSBC Code: not liable to liquefy, no chemical hazard in the Group B sense. The practical hazards are more demanding than that Group C label suggests. Clinker is often loaded hot, directly from kiln coolers at 80 to 150 degrees Celsius, which destroys hold coatings and stresses hatch seals. It is hard, dark-grey calcium silicate nodules of 5 to 25 mm, and it is abrasive enough to accelerate wear on grabs and conveyors. When wet, it reacts to form calcium hydroxide at pH above 12, causing chemical burns and caking that complicates discharge. None of these hazards trigger automatic Group B classification, but none of them disappear because the schedule says Group C.

Cement clinker occupies a specific position in the cement supply chain: it is the kiln output, the intermediate product that travels by sea from clinker-producing regions to grinding terminals, where it is milled with about 5% gypsum to become Portland cement. The IMSBC Code, which has been mandatory under SOLAS Chapter VI since 1 January 2011 under IMO Resolution MSC.268(85), lists CEMENT CLINKERS as a separate schedule entry from CEMENT. The two cargoes share a Group C classification but differ in almost every physical and chemical property that matters for carriage.

Seaborne clinker trade has grown alongside the globalization of the cement industry. Clinker kilns are capital-intensive and energy-intensive, and they benefit from location near cheap energy and quality limestone reserves. Grinding mills are lower-capital, easier to permit in urban areas, and can be located near the final market. That economic logic drives cross-ocean clinker shipments of 50 to 70 million metric tons per year. The ships carrying clinker typically carry other bulk cargoes on return legs, so clinker handling knowledge is part of general bulk-carrier operations, not a specialty-vessel domain.

Cement clinker in the cement industry

What clinker is and how it is made

Portland cement clinker is produced by calcining a blended raw meal of limestone (approximately 75 to 80%), clay or shale (approximately 10 to 15%), and corrective materials such as iron ore and bauxite in a rotary kiln at temperatures of 1,400 to 1,500 degrees Celsius. At those temperatures, calcium oxide (from the limestone) reacts with silica, alumina, and iron oxide to form the four main clinker minerals: alite (tricalcium silicate, C3S), belite (dicalcium silicate, C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The molten nodular material exits the kiln and passes through a clinker cooler, where forced air drops the temperature from approximately 1,200 degrees Celsius at the kiln outlet to 60 to 120 degrees Celsius at the cooler exit. These exit temperatures are important: they are the loading temperatures for vessels receiving clinker directly from the cooler.

The nodules produced by this process are hard (Mohs hardness approximately 6 to 7, similar to feldspar) and dark grey to black. Clinker from a well-operated kiln has a nodule size of 5 to 25 mm, with a small fraction of fines from breakage in the cooler and during conveying. Bulk density of freshly produced clinker is approximately 1,200 to 1,500 kg/m3. The nodules are porous rather than glassy: the rapid cooling in the cooler preserves a microstructure with about 3 to 5% accessible porosity, which influences how clinker absorbs moisture during a long voyage.

Free lime (uncombined CaO) content in well-burned clinker is typically 0.5 to 2.0% by weight. Free lime is the fraction of calcium oxide that did not fully react in the kiln. It is the component most reactive with water: CaO + H2O yields Ca(OH)2 (calcium hydroxide), releasing heat and producing a solution with pH above 12. Clinker with elevated free lime (above 3 to 4%) is considered “under-burned” or defective from a cement-quality standpoint, and it carries elevated chemical hazard during marine carriage because of the greater free lime available to react with water.

Clinker versus cement: the key distinctions

The single most common error in clinker carriage documentation is confusing the hazard profile of clinker with that of finished cement. The two cargoes share a Group C classification under the IMSBC Code and are often handled at the same terminals, but their properties differ in ways that change the operational response.

The moisture sensitivity contrast is practically the most important. Cement that contacts water during the voyage sets into a concrete-hard mass that may destroy the shipment and require mechanical chipping to clear the hold. Cement clinker that contacts water does not set in the same way: the calcium silicates in clinker hydrate much more slowly than the fine particles in ground cement, and the reaction without a grinding activator produces calcium hydroxide and calcium silicate hydrate gels that cause caking rather than full setting. The holds can be cleaned with water and mechanical assistance. But the alkaline leachate from wet clinker is a serious chemical hazard, and partially caked clinker in a hold corner presents a grab-discharge problem: the grab cannot break up a 200-tonne alkaline concrete block.

Clinker as a global commodity

Seaborne clinker trade is estimated at 50 to 70 million metric tons per year. Unlike limestone or iron ore, clinker is an industrial intermediate with no direct end-use until it is ground, so the trade is always between a clinker producer and a grinding terminal. The principal export regions are China, Vietnam, Japan, South Korea, Turkey, the UAE, Saudi Arabia, and Egypt. The principal import regions are West Africa, East Africa, the Philippines, Bangladesh, Sri Lanka, and Latin America.

China and Vietnam together account for a large share of clinker exports during periods of domestic overcapacity. Chinese clinker exports vary year by year with domestic demand and government production quotas, ranging from around 10 million to over 30 million metric tons in recent years. Vietnamese exports have grown as new kiln capacity has outpaced domestic demand. Both countries export principally to Southeast Asian neighbours, the Philippines, and Bangladesh.

Turkey is the primary Mediterranean clinker exporter, supplying North Africa and West Africa via the Mediterranean and Atlantic. Türkiye’s cement sector operates at high capacity utilization, and excess clinker is exported during periods of domestic demand softening. UAE and Saudi Arabian kilns, located near high-grade limestone in arid zones with cheap energy, supply East Africa, Sri Lanka, and South Asian markets.

Grinding terminals receiving clinker are typically located at coastal industrial zones near major urban markets. The terminal stores clinker in covered sheds or silos, mills it with gypsum in ball mills or vertical roller mills, and packs or bulk-loads the finished cement for distribution. Vessel sizes used in the clinker trade range from 10,000 DWT coasters in regional trades to 35,000 to 55,000 DWT Handymax and Supramax vessels on deep-sea routes.

The IMSBC Code schedule for cement clinkers

Group classification and regulatory basis

The IMSBC Code Appendix 1 lists CEMENT CLINKERS as a Group C cargo under a dedicated schedule entry, separate from the CEMENT schedule. Group C means the cargo is not liable to liquefy (no Transportable Moisture Limit is required) and does not possess chemical hazards that require Group B treatment (no self-heating, no flammable off-gassing, no toxic gas emission under normal conditions).

IMO Resolution MSC.268(85), which first adopted the IMSBC Code in 2008 (entered into force 1 January 2011), established the CEMENT CLINKERS schedule. Amendment 06-21, adopted by Resolution MSC.500(105), and Amendment 07-23, adopted by Resolution MSC.539(107) and mandatory from 1 January 2025, did not change the Group C classification of cement clinkers. The schedule has been stable across recent amendment cycles.

The Group C classification reflects the genuine absence of liquefaction risk (clinker nodules at 5 to 25 mm do not develop pore-water pressure under ship motion) and the absence of self-heating, flammable gas generation, or acutely toxic gas emission. It does not mean the cargo is without hazard. The IMSBC Code schedule for CEMENT CLINKERS records specific special properties and special requirements that translate the hot-loading, dust, and water-reactivity hazards into operational obligations.

Schedule particulars

The following table summarizes the physical and chemical properties as recorded in the IMSBC Code schedule for CEMENT CLINKERS, with practical ranges from shipper declarations and port-state experience:

The IMSBC Code schedule requires the shipper to declare the cargo temperature at the time of loading and to notify the master if the temperature exceeds the level at which hold coatings and hatch seals may be affected. The Code does not set a universal maximum loading temperature, but the duty to declare is explicit, and the master’s right to decline cargo at excessive temperature is implied by the Code’s general master’s discretion provisions.

What Group C means for operational requirements

Group C status produces three immediate operational simplifications. No TML laboratory test is required before loading. No hold atmosphere monitoring for toxic or flammable gases is required during the voyage. No emergency spillage response for chemical hazard applies.

The specific properties in the schedule, however, impose obligations that in practice are more demanding than those for an ordinary Group C cargo such as iron ore pellets or limestone. The hot-loading provisions, the dust health hazard, and the water reactivity create a package of pre-load, loading, voyage, and discharge requirements that are covered in detail in the sections below.

Hot loading: the defining operational challenge

Why clinker arrives hot

A rotary cement kiln operates continuously at 1,450 to 1,500 degrees Celsius. The clinker cooler attached to the kiln outlet uses a combination of grate coolers and forced-air systems to drop the clinker temperature to 60 to 120 degrees Celsius within the cooler. Cooler efficiency determines the exit temperature: a well-maintained grate cooler with adequate airflow can reduce clinker to 65 to 80 degrees Celsius at exit. An older or overloaded cooler may deliver clinker at 100 to 150 degrees Celsius.

From the cooler, clinker travels by conveyor to storage sheds or clinker silos. In large export terminals with sufficient storage capacity, clinker is held in storage for days or weeks before shipment, and the temperature drops to near-ambient during storage. In facilities where storage is limited and kiln production flows directly to the ship, clinker may reach the vessel’s holds at close to the cooler exit temperature.

This second scenario is the hot-loading situation. It is common at export terminals in developing-country cement clusters where capital investment in clinker storage is constrained and continuous throughput from kiln to ship is the preferred operating model. Masters calling at such ports must expect hot clinker unless the shipper can certify the storage period and resulting temperature.

Temperature effects on hold coatings and structures

Hold coating damage is the most immediate consequence of hot clinker loading. Epoxy coatings typically used in cargo holds are formulated for service temperatures up to 40 to 60 degrees Celsius. Clinker at 100 degrees Celsius placed directly against an epoxy-coated inner bottom can blister, de-bond, or carbonize the coating within hours of loading. Soft epoxy coatings in hot weather (hold steel temperature above 35 degrees Celsius before loading) are at risk even from clinker at 80 degrees Celsius.

The practical consequence of coating failure is twofold. First, exposed steel corrodes at an accelerated rate because the hot clinker provides an aggressive alkaline-moisture environment once any condensation or water ingress occurs. Second, coating fragments that peel from the hold plating contaminate the clinker cargo, which is a quality problem for the grinding terminal: residue that is ground with the clinker reduces cement quality.

Hatch cover rubber seals are the second vulnerable point. Synthetic rubber seals designed for ambient cargo service begin to degrade at 70 to 80 degrees Celsius. A full hold of clinker at 100 degrees Celsius raises the hold atmosphere temperature to similar levels, and the hatch coaming rubber in contact with that atmosphere softens, compresses permanently, and loses its sealing integrity. Hatch cover seal replacement is expensive and sometimes requires off-hire periods; responsible operators check seal condition after every clinker voyage.

The hold structure itself can withstand much higher temperatures than the coatings or seals: mild steel has no structural concern at 100 to 150 degrees Celsius, and the thermal expansion of the plating at those temperatures is manageable. But the frame web-to-plating bond relies on the coating for corrosion protection, and frame wastage from coating loss accumulates over multiple clinker voyages.

Temperature limits in practice

No single regulatory temperature limit applies universally to cement clinker loading. The IMSBC Code requires declaration and master’s assessment. Some cement majors with ship vetting programs set internal temperature limits: 70 degrees Celsius is a commonly cited threshold in vetting questionnaires, above which additional coating inspection records are required before the vessel is approved for loading. Some P&I clubs and hull underwriters impose conditions on clinker loading that reference coating integrity and temperature history.

Masters who encounter hot clinker above approximately 80 degrees Celsius at a port where this is not declared should request a written temperature declaration from the shipper and document the refusal or compliance in the mate’s receipt and Statement of Facts. The right to decline excessively hot cargo is not removed by the Group C classification; the master’s duty of care for the ship’s structure and coatings is a separate obligation under the standard maritime law of seaworthiness.

Cooling during the voyage

Clinker cools from its loading temperature toward ambient during the voyage. The rate depends on the initial temperature, the hold insulation effect of the cargo mass, and the ambient sea and air temperature. Empirical experience from clinker trades suggests that a full hold of clinker loaded at 100 degrees Celsius will cool to below 60 degrees Celsius within 2 to 4 days and below 40 degrees Celsius within 5 to 7 days in typical tropical trade conditions. The cooling is driven by conduction through the hold plating and by natural convection of moisture-laden vapor through any ventilation paths.

During the cooling period, hold atmosphere humidity is elevated because the clinker’s residual moisture and any absorbed atmospheric moisture evaporate. This humidity combines with CO2 from the atmosphere to form a mild carbonic acid environment that is mildly corrosive to exposed steel. Adequate ventilation during the early days of a clinker voyage both cools the cargo faster and dilutes the humid hold atmosphere.

Dust: caustic and abrasive

How clinker dust is generated

Cement clinker is a hard, brittle material. Mechanical impact during loading, vessel vibration during the voyage, and grab discharge all generate a fraction of fine particles. The fines from clinker are not calcium carbonate (as in limestone dust) but calcium silicate, calcium aluminate, and free calcium oxide: the same mineral compounds present in the nodules but in fine form.

Free lime (CaO) in clinker dust is the primary health hazard. CaO reacts with moisture (including moisture on skin and in the respiratory tract) to form Ca(OH)2, which at pH above 12 is strongly caustic. Contact with dry clinker dust that subsequently absorbs moisture from skin or mucous membranes causes alkali burns. These burns have a delayed onset (the exothermic reaction takes seconds to minutes) and can penetrate tissue deeply before pain is felt. The IMSBC Code schedule records the dust hazard explicitly and requires appropriate personal protective equipment for personnel working in dust environments during loading and discharge.

Crystalline silica content in clinker dust is lower than in limestone dust: the calcination process converts most quartz (crystalline SiO2) in the raw meal into amorphous calcium silicates. Well-burned clinker contains negligible free quartz. Silicosis risk from clinker dust is therefore lower than from limestone or quartz sand dust, but the free-lime alkali hazard is higher.

Dust exposure standards

The occupational exposure limits for Portland cement dust (which clinker dust closely resembles in composition) vary by jurisdiction. The UK Health and Safety Executive EH40 limit for Portland cement dust is 10 mg/m3 (inhalable fraction) and 4 mg/m3 (respirable fraction) as an 8-hour time-weighted average. The US OSHA permissible exposure limit for cement dust is 15 mg/m3 (total dust) and 5 mg/m3 (respirable fraction). The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value for Portland cement is 1 mg/m3 (respirable fraction), a more conservative limit that reflects concerns about lung irritation from prolonged exposure.

During loading of clinker by belt conveyor and shiploader, dust concentrations at the loading spout and hatch coaming can substantially exceed these limits if no controls are applied. Water spray at the cargo drop point is the standard control and can reduce respirable dust by 60 to 80% in practical operations. Personnel working within the dust zone during loading should wear half-face respirators with P2 (Europe) or N95 (US) filters at minimum. Full-face respirators provide eye protection against the caustic dust, which is recommended given the alkali burn risk.

During discharge by grab crane, the grab break-out and drop from hold to barge or hopper generates concentrated dust clouds. Clinker being abraded by the grab teeth, and fines disturbed by the grab plunging into the cargo mass, become airborne at rates that are higher per grab cycle than in a typical limestone or iron ore grab discharge. Shore-side stevedores at receiving terminals handle this with enclosed grab systems at some advanced facilities, but open-grab discharge with water spraying on the cargo surface between grab cycles is more common.

Abrasion of equipment

Clinker at Mohs hardness 6 to 7 is harder than most dry bulk cargoes. Limestone crushed grades are typically Mohs 3 to 4. Iron ore is Mohs 5 to 6.5 depending on the mineral (hematite is 5.5 to 6.5; magnetite is 5.5 to 6.5). Clinker is at the high end of the hardness range encountered in bulk carrier operations.

This hardness means that grabs, conveyor belts, and shiploader chute linings wear faster on clinker cargoes than on most other bulk commodities. Grab bucket teeth and lip plates typically require replacement after 50 to 80 cycles of clinker discharge compared to 200 to 400 cycles on iron ore pellets. Shore terminals that handle clinker regularly use manganese steel or hard-faced grab liners and maintain higher spare-parts inventories for discharge equipment. The master should record any observable damage to hold plating from loading equipment during clinker operations.

Water reactivity, caking, and the caustic-when-wet hazard

The chemistry of clinker hydration

When cement clinker contacts water, three parallel reactions begin. Free lime (CaO) hydrates rapidly: CaO + H2O yields Ca(OH)2, releasing approximately 1.15 MJ per kilogram of CaO. Tricalcium aluminate (C3A) hydrates rapidly in the presence of water to form calcium aluminate hydrates. Tricalcium silicate (alite, C3S) hydrates more slowly to form calcium silicate hydrate (CSH gel) and additional Ca(OH)2. The belite phase (C2S) hydrates even more slowly, over weeks to months.

In a maritime context, the practical consequences are different from the laboratory chemistry. A clinker cargo that absorbs water slowly from humid hold atmosphere over a long voyage does not set hard: the reaction proceeds without the mechanical disturbance and mixing that promotes strength development in concrete. Instead, the free lime hydrates to Ca(OH)2, which precipitates as a white powder on the clinker surface, and a localized caking occurs where particles are in contact under pressure, cemented by calcium silicate hydrate gel. The result is that affected areas of the cargo mass develop compressive strength of a few kilopascals to tens of kilopascals: soft enough to break up with moderate force but hard enough to resist a grab’s first pass at the cargo surface.

Where the wetting event is more severe (a hatch seal failure, a rainstorm during uncovered loading, or seawater ingress through a failed bilge), the hydration is more complete, and the caked zone can develop significant compressive strength over the days of the voyage. A caked zone of 200 to 500 tonnes of clinker presents a serious discharge problem and may require manual pneumatic hammer work or a hydraulic concrete breaker on the cargo surface before grab discharge can proceed.

Alkaline leachate from wet clinker

The calcium hydroxide produced by clinker hydration dissolves in water to form a solution with pH of approximately 12.4 at saturation. Bilge water from a hold carrying wet clinker will have pH above 12. This alkaline leachate damages bilge pump components (gaskets and impeller seals) that are rated for slightly alkaline bilge water at pH 8 to 9 but not for pH 12. It also flows through any hull penetrations (bilge sea-suction, drain valve seats) and can deposit calcium carbonate scale when it contacts CO2 in seawater. Bilge inspection after a clinker voyage that experienced any water ingress is a maintenance requirement that responsible shipowners include in their post-voyage checklist.

Contact of alkaline leachate with skin requires immediate flushing with large volumes of water. Eyes are particularly at risk: Ca(OH)2 solution causes liquefactive necrosis of eye tissue, which is more severe than acid burns of comparable pH. Crew working in holds with wet clinker must wear chemical splash goggles, not just safety glasses, and must know the location of the nearest emergency eyewash station before entering.

Caking in practice: implications for discharge

A clinker cargo that arrives at the discharge port partially caked is an operational problem that the vessel master and charterer must manage together. The standard charter party for bulk clinker typically includes a clause that the cargo shall be delivered in the same condition as loaded, and caking caused by water ingress attributable to the vessel’s hatch cover failure is a shipowner’s problem. Caking caused by water ingress during loading in an uncovered rainstorm is the shipper’s problem. The dispute is common, and the contemporaneous records in the SOF (Statement of Facts), mate’s receipts, and the pre-loading hatch cover water test certificate are the evidence.

From an operational standpoint, the mate should document the condition of the cargo surface at the opening of each hold at discharge port. A visual description in the log of any caked areas, their approximate extent, and whether they are on the surface or deeper in the cargo (indicating earlier wetting) is important for both claims management and for discharge planning. The chief officer should notify the shore crane operator if caked areas require additional breaking effort, as this affects the discharge rate calculation.

Hold preparation and coating protection

Pre-load cleanliness

The shipper’s cargo declaration for cement clinker states that the cargo is for grinding into Portland cement or similar cementitious products. Grinding terminal quality controllers test incoming clinker for trace contaminants at the parts-per-thousand to parts-per-million level. A hold that previously carried coal introduces combustible carbon and sulphur into the clinker. Coal residue above 0.01% by weight in a 40,000-tonne clinker cargo means 4 tonnes of coal contamination entering the grinding mill, which is detectable at the cement quality stage and is a cargo claim against the shipowner.

Prior cargoes that contaminate clinker for cement use include: coal and petroleum coke (carbon and sulphur), sulphur (sulphate contamination), fertilizers (nitrogen, phosphorus, potassium), salt (chloride), and any cargo with significant heavy-metal content. Prior cargoes that are generally compatible with clinker include limestone, iron ore, sand, and other clinker. The standard pre-load hold preparation for a clinker cargo after any contaminating prior cargo is full hold washing with high-pressure fresh water, drying, and a surveyor’s hold cleanliness inspection with certificate.

Bilge wells require particular attention. Clinker fines are among the most problematic bilge contaminants because they hydrate in the bilge water to form a calcium silicate paste that sets into a hard mass. A bilge that has accumulated clinker paste and is then neglected will require mechanical cleaning with a pneumatic hammer before it can be used again. Bilge strainer covers must be intact before loading begins, and bilge clearing should be scheduled daily during the voyage on any clinker cargo that has experienced moisture exposure.

Hold coating inspection and preparation

The inspection of hold coatings before loading a hot clinker cargo is a commercial and insurance requirement, not just a maintenance recommendation. Many vetting programs for cement-company chartered vessels require a recent hold condition survey report. Hull class survey records are the baseline; a supplementary pre-load inspection by an approved coating surveyor is required by some cargo owners.

Areas of coating failure (blistering, de-bonding, bare steel) in a hold that will receive clinker at elevated temperature should be treated before loading. Touch-up with a heat-resistant coating rated for the expected temperature is the standard repair. Where coating damage is extensive, some operators apply sacrificial zinc silicate coating on the inner bottom and lower hopper areas before loading clinker, accepting that the sacrificial layer will be damaged or removed during discharge but that it protects the underlying steel from the hot-load corrosion environment.

Hold frames and frame web-to-plating connections are the area most susceptible to hidden wastage after repeat clinker cargoes without coating maintenance. Class surveyors conducting hold thickness measurements on vessels with clinker trading history should pay particular attention to frame webs in the lower hopper area, where hot clinker sits for the voyage duration.

Hatch cover maintenance before clinker loading

Hatch cover rubber seals must be inspected before every clinker loading, because both the temperature and the alkaline environment of a clinker cargo degrade seal material faster than most other cargoes. A hatch water test (hosing the hatch perimeter while observing for drips inside the hold) is standard pre-loading procedure and should be recorded in the mate’s receipt documentation.

Seals that show cracking, set compression (permanent deformation that reduces the seal height), or delamination from the compression bar should be replaced before loading. This is not unique to clinker but is more important for clinker than for most other Group C cargoes because of the combined stresses of heat and alkali. A seal failure during a clinker voyage in a rainy trade route can cause a significant caking event in the affected hold, with all the discharge and claims consequences described above.

Loading operations

Receiving the cargo temperature declaration

Before accepting any clinker cargo, the master is entitled under IMSBC Code Section 4 provisions to request a cargo declaration that includes the temperature of the cargo at the time of loading. This declaration is the shipper’s responsibility. At terminals where clinker flows directly from the cooler to the ship, temperature measurements at the conveyor transfer point are the practical data source. At terminals with storage sheds, a representative temperature measurement from the shed using an infrared thermometer or embedded sensor is the basis for the declaration.

Where the shipper cannot or will not provide a temperature declaration, the master should arrange independent measurement at the loading spout before cargo flow begins. Infrared thermometry of the cargo falling from the spout gives a reasonable surface temperature. Core temperature of the cargo pile is typically 10 to 15 degrees Celsius higher than surface temperature in a freshly loaded hold, but this variation matters mainly for hatch seal and coating assessment.

If the measured or declared temperature is above the level at which coating damage is expected (typically above 80 degrees Celsius for standard epoxy coatings, and above 70 degrees Celsius for vessels with marginal coating condition), the master has grounds to refuse loading until the shipper cools the cargo or demonstrates that the coating is rated for the loading temperature. This refusal should be documented and communicated in writing to the terminal and to the operator.

Loading rate and dust control

Clinker loading rates at modern bulk export terminals are typically 1,500 to 3,500 t/h for a single shiploader. The loading rate is constrained by the conveyor capacity and by hatch size (the shiploader boom must access each hatch opening), not by the clinker’s physical properties. Trimming within the hold is needed only at the end of loading each hatch, to push cargo from under the loading point toward the hatch sides so that the hatch can close: this is standard trimming practice for most bulk cargoes and may use a bulldozer on the cargo surface or a telescoping spout that can direct the cargo flow sideways.

Dust control during loading uses water spraying at the conveyor transfer point and at the loading spout. Water application should be controlled: excessive wetting of clinker at the loading point starts the hydration process and produces a surface crust that traps fine material. The goal is dust suppression by binding the fines, not full wetting of the cargo. Water application rates of 0.3 to 0.8 liters per tonne of clinker loaded are typical in practice.

Hatch openings not currently receiving cargo should be kept closed during loading to prevent dust from adjacent loading operations from entering those holds. Fugitive dust from clinker loading can deposit on vessel superstructure, deck equipment, and navigation aids if wind conditions spread it across the deck. A deck wash-down with fresh water after loading is complete should be standard procedure, particularly around deck machinery that can be damaged by clinker fines accumulating in moving parts.

Shipper’s cargo information requirements

SOLAS Chapter VI Regulation 2 and IMSBC Code Section 4 require the shipper to provide a cargo declaration before loading begins. For cement clinker, the declaration should state: the cargo name (CEMENT CLINKERS, as in the IMSBC Code schedule), the quantity (metric tons), the stowage factor or bulk density, the moisture content, the cargo temperature at loading, the particle size distribution (including the fine fraction below 5 mm), the free lime content (if elevated), and any special properties relevant to safe loading.

The cargo temperature declaration is the item most frequently absent or inaccurate in clinker cargo documentation. P&I correspondents handling clinker cargo claims consistently note that temperature declarations either are not provided or are provided as nominal values based on the cooler design specifications rather than actual measurements on the cargo being loaded. The master’s independent temperature check at the loading spout, recorded in the mate’s receipt and SOF, provides the shipowner’s defense if coating damage claims arise later.

Voyage handling and ventilation

Ventilation during the voyage

Cement clinker is not listed among the cargoes for which the IMSBC Code requires specific hold ventilation regimes, unlike some Group B cargoes where toxic or flammable gas generation makes ventilation mandatory. However, the practical case for ventilating clinker holds during the early days of the voyage is strong.

Hot clinker generates water vapor by driving off moisture absorbed by the cargo during cooler transit and conveying. This moisture, combined with the elevated hold atmosphere temperature, creates conditions that accelerate corrosion of any exposed steel. Surface ventilation (natural or forced) during the first 2 to 3 days of the voyage helps remove this hot humid air, cooling the hold and reducing the corrosion environment. Once the clinker has cooled to within 10 degrees Celsius of ambient, the case for continued ventilation is no weaker (and no stronger) than for any other Group C cargo: ventilate if the dew point of the outside air is below the dew point of the hold atmosphere, to avoid condensation on the cargo or the hold structure.

Hold ventilation openings should be checked before the voyage to confirm that they are not blocked by clinker fines deposited during loading. A blocked vent that cannot be opened from inside the hold eliminates the ventilation option. Hold ventilators should be swept clear of fines before closing after loading.

Monitoring cargo condition during the voyage

The cargo should be visually inspected through hatch inspection ports or by brief hatch opening during the voyage, particularly if weather conditions have included significant wave action or if any rain has fallen on the hatch covers. The signs to look for are: a white powder deposit on the cargo surface (indicating free lime hydration), localized dark-wet patches (indicating water ingress), or surface hardening (indicating early caking).

Hold atmosphere temperature can be measured through the vent pipe using a thermometer on a lanyard to track the cooling of hot clinker. Measurements on day 1, 2, and 3 of a hot clinker voyage give the master data to confirm or question the shipper’s loading temperature declaration. If the measured hold atmosphere temperature on day 1 is 70 degrees Celsius, the cargo was almost certainly loaded above 80 degrees Celsius. This measurement, recorded in the log, is useful evidence.

Discharge operations

Receiving terminal preparation

Clinker grinding terminals receive the cargo into covered sheds or open stockpiles adjacent to the milling plant. Shed-based terminals can begin unloading directly into storage at loading rates limited by the terminal conveyor capacity. Open stockpile terminals, more common in warmer climates, may be constrained by weather: rain on freshly discharged clinker starts the hydration clock, so storage time before milling is a quality concern for the terminal.

The vessel master should establish the terminal’s discharge rate expectation and compare it to the vessel’s allowable crane load per unit area on the cargo hold inner bottom. Some clinker discharge operations use heavy grab cranes with individual grab weights of 10 to 25 tonnes; the dynamic load from a fully loaded grab dropped into the hold can exceed the inner-bottom design loading if the crane operator is not careful. This is a shipowner’s concern, and the mate should specify maximum allowable grab weight before discharge begins.

Grab discharge mechanics and residue

Grab discharge of clinker is operationally similar to iron ore grab discharge, with two differences that reflect the abrasive hardness of the cargo. Grab bucket teeth and lip plates wear faster than on most cargoes. The grab plunging into the clinker mass disturbs the surface and generates fine dust; adequate water spray on the cargo surface between grab cycles reduces this.

Residue after grab discharge is typically 100 to 300 tonnes per hold in a 30,000 to 40,000 DWT vessel, remaining in the corners and under the hatch landing area where the grab cannot reach cleanly. Clinker residue is removed by bulldozer on the cargo surface followed by hand-shoveling of the remaining corners. If any caking has occurred, the bulldozer cannot break the caked mass, and a hydraulic breaker or pneumatic hammer is required first. The residue clearing time is a factor in port time and laytime calculation, and the charter party should specify whether caked cargo resulting from water ingress is at the vessel’s risk (hatch failure) or the cargo’s risk (loading moisture).

Discharge rates for grab operations on clinker range from 500 to 2,000 t/h per crane, depending on grab size, crane speed, and cargo condition. Uncaked clinker in good condition is at the upper end; partially caked or hot cargo requiring extra care at the hatch opening stage is at the lower end.

Post-discharge hold cleaning

After clinker discharge, the hold should be swept and washed before loading the next cargo. The principal concern is clinker fines in the bilge well, which will have partially set if any moisture was present during the voyage. A fresh-water high-pressure wash of the bilge well and sump before they dry out further is the most effective approach. Once clinker paste in a bilge well has dried and set, it requires mechanical breaking before it can be pumped out.

Clinker residue on the hold frames and upper wing tanks (from splashing during loading and discharge) should also be washed. These residues are alkaline and will continue to react slowly with atmospheric moisture, producing calcium hydroxide deposits that can block drainage channels and coat steelwork.

Draft survey for cement clinker

The cargo draught survey is the standard quantity-determination method for clinker shipments where the discharge terminal does not have calibrated weighbridge or belt-scale facilities. Many clinker grinding terminals in developing-country markets lack shore scales with capacity for bulk shipments, making the draft survey the primary quantity measurement.

Clinker’s bulk density of 1,200 to 1,500 kg/m3 and the fact that it is relatively non-compressible (the hard nodules do not compact as much as fine powders during the voyage) mean that the loaded and discharged tonnage should agree within 0.3 to 0.5% on a well-conducted draft survey. Discrepancies above 0.5% are worth investigating for: uncorrected trim or list at the time of one survey, density of dock water not directly measured, or error in the vessel’s displacement tables.

The freshwater allowance is important at river or estuarine clinker loading ports. Many export terminals in Vietnam, Bangladesh, and West Africa are located in rivers or bays where dock water density can be 1.000 to 1.015 t/m3, far below the standard 1.025 t/m3. Failure to apply the freshwater allowance at a loading port where the dock water is 1.010 t/m3 introduces an error of approximately 0.74% of displacement into the draft survey, which is a significant shortfall on a 40,000-tonne cargo.

Wedge corrections for a clinker cargo are straightforward when the cargo fills all holds to a uniform depth. Complications arise when some holds are loaded to a lower level than others (uneven loading sequence), or when the cargo has caked into an uneven surface profile that does not match a flat-surface assumption. Surveyors should observe the actual cargo surface profile through the hatch before the hold is closed for the voyage, and record any irregularities that may affect the wedge calculation.

Comparable cargoes and distinctions

Cement clinker occupies a position in the Group C schedule that makes it comparable to several other calcium-bearing or mineral-aggregate cargoes, but it is distinct from each in at least one important practical dimension.

Limestone is the feedstock for clinker production. It is coarser, softer (Mohs 3 to 4), not hot when loaded, non-caustic when wet (calcium carbonate is chemically neutral rather than alkaline), and does not cake. Hold coatings are not at elevated risk from limestone. Limestone is in many respects a simpler cargo to carry than the clinker made from it.

Gypsum is added to clinker during grinding to produce cement. Gypsum is also Group C, also calcium-bearing, and also not liable to liquefy. It is much softer than clinker (Mohs 2), not abrasive to equipment, and not hot when loaded. Gypsum is moderately soluble in water (unlike clinker, which reacts chemically with water rather than dissolving), and its moisture-related carriage concern is dissolution of calcium sulfate at the cargo surface rather than hydration caking.

Cement is the product made from clinker. Its carriage profile is in many ways more severe: cement reacts with water orders of magnitude faster than clinker (hours versus days), and a wetting event during the voyage can destroy the cargo completely. Cement dust is more caustic than clinker dust on a per-unit-mass basis because the fine grinding has enormously increased the reactive surface area. But cement is not loaded hot and does not abrade hold coatings. Most cement carriers carry cement without the hot-loading and coating concerns that define clinker carriage.

Iron ore pellets are a rough comparison from a loading perspective: both are roughly spherical nodules at similar bulk densities, both Group C, and both discharge by grab. But iron ore pellets are not hot when loaded, not caustic when wet, and not reactive with moisture.

Limitations

This article describes the IMSBC Code schedule for CEMENT CLINKERS as defined through Amendment 07-23 (Resolution MSC.539(107), mandatory from 1 January 2025). The Code is amended every two years by the IMO MSC; the current schedule text should be verified against the edition published by the IMO directly, not against commercial reproductions. The IMO IMSBC Code is available from the IMO Publications catalogue.

Bulk density, stowage factor, and angle of repose values given here are from industry practice and engineering data across the range of Portland cement clinkers traded globally. Actual values vary with kiln type, raw material composition, cooler design, and the proportion of fines from mechanical breakage during conveying. The shipper’s cargo declaration is the contractual statement of properties for any specific shipment, and it supersedes the typical ranges in this article.

The hot-loading temperature thresholds given here (70 to 80 degrees Celsius) are from industry vetting practice and P&I guidance, not from an explicit IMSBC Code numerical limit. The Code imposes a declaration duty and a master’s discretion; the numbers that trigger specific actions in practice vary by coating specification, vessel age, and cargo owner requirements.

The water reactivity and caking discussion describes the physical and chemical mechanisms based on established cement chemistry. It does not provide engineering calculations for the rate of strength development in a caked clinker mass, which depends on free lime content, water quantity, temperature, and time: all variables that differ by shipment and voyage. Where a caking event is suspected or confirmed, a geotechnical or cement chemistry specialist should be consulted for the discharge operation planning.

Seaborne trade volume estimates (50 to 70 million metric tons per year) are from industry reporting and are approximate. Clinker trade data are not systematically reported with the precision of iron ore or coal volumes. Masters and operators should consult port authority requirements for dust emissions at specific discharge ports, which are not covered in this article.

See also

• IMSBC Code
• IMSBC Code Group C Cargoes
• Cement: IMSBC Code Schedule and Carriage
• Limestone: IMSBC Code Schedule and Carriage
• Gypsum: IMSBC Code Schedule and Carriage
• Cargo Hold Preparation Standards
• Cargo Draught Survey for Bulk Carriers
• Marine Cargo Hold Ventilation
• SOLAS Chapter XII: Additional Safety Measures for Bulk Carriers
• IMSBC Group A/B/C Classification
• IMSBC Cement Clinker Calculator
• IMSBC Cement Calculator
• Cement Carrier CII