Limestone: IMSBC Code Schedule and Carriage

Limestone is classified as a Group C cargo under the IMSBC Code, meaning it is not liable to liquefy and presents no chemical hazard in standard lump or crushed form. The practical hazards are dust generation during loading and discharge, and the risk that finely ground limestone shipped at elevated moisture may behave as a Group A cargo. It is one of the highest-volume dry bulk commodities globally, serving the cement, steel, agriculture, and power industries.

Limestone is calcium carbonate (CaCO3), a sedimentary rock formed from the accumulation of marine shells, coral, and calcareous mud over geological time. It is quarried on every continent, and its chemical uniformity, high availability, and reactivity with acids make it indispensable across heavy industry. The IMSBC Code, first entering force on 1 January 2011 under IMO Resolution MSC.268(85) and mandatory under SOLAS Chapter VI, governs its carriage in bulk. The Code records limestone under a Group C schedule, and separately records CALCIUM CARBONATE as a closely related entry covering finely divided carbonate products.

The cargo does not appear in most maritime incident databases as a cause of vessel loss. That is both its chief practical virtue and a reason that detailed carriage guidance is thin in the literature: limestone generates no post-incident research pressure. What incidents do occur involve dust inhalation injuries, hatch sealing failures that allow cargo wetting, and the occasional liquefaction event in finely ground material that was incorrectly declared as Group C. Understanding those edge conditions is the job of the responsible master and shipper.

Limestone as a global commodity

Production and quarrying

Limestone is the most widely quarried non-fuel mineral on Earth. The United States Geological Survey’s annual Mineral Commodity Summary records US production at approximately 800 million metric tons per year across all limestone and related carbonate rock categories, including dolomite. China quarries an estimated 1.4 to 1.6 billion metric tons per year, reflecting the scale of its cement industry. Global cement production alone consumed approximately 4.1 billion metric tons of limestone feedstock in 2022, based on the clinker-to-limestone conversion ratio of roughly 1.5 to 1.6 metric tons of limestone per ton of clinker.

Most limestone consumption occurs within 100 to 200 kilometers of the quarry, because the rock is heavy (bulk density approximately 1,400 to 1,600 kg/m3 in crushed form, approximately 2,600 kg/m3 solid) and low-value relative to its weight. Long-distance seaborne trade arises where limestone of adequate chemical quality is absent near major industrial consumers, or where quarry-adjacent ports can produce at a cost below the delivered price from local sources.

Major seaborne trade routes

Global seaborne limestone trade is estimated at 25 to 40 million metric tons per year, a fraction of total production but a consistent source of cargo for Handysize and Handymax bulk carriers. The main identified trade routes are:

Caribbean to US Gulf and East Coast. The Bahamas (principally Freeport and South Riding Point), Mexico (Yucatan coast), and Belize ship crushed limestone to cement plants and aggregate terminals along the US Gulf and East Coast. The Bahamas became a significant exporter partly because its oolitic limestone quarries are close to sheltered deep-water anchorages, and partly because US domestic quarrying is constrained near coastal population centres. Typical vessel sizes are 20,000 to 55,000 DWT Handysize and Supramax.

Mediterranean short-sea and coastal trade. Turkey, Greece, Italy, and North Africa all quarry and export limestone in coastal trade. Turkish exports feed cement plants in Egypt and Israel. Greek and Italian coastal trade is largely intra-country or intra-EU. Vessel sizes in this trade are typically 5,000 to 20,000 DWT.

Norway and Northern Europe. Norwegian carbonate quarries in Nordland and Troms supply limestone to UK, Dutch, and German steel mills and chemical plants. Omya and Omya-related producers historically ran small dedicated carriers on these routes. Cargo volumes are modest by global standards but the high chemical purity (CaCO3 content above 97%) supports premium pricing.

Southeast Asia. Vietnam has become a significant regional exporter of limestone to China and elsewhere in Southeast Asia. Philippine, Indonesian, and Thai producers supply regional cement and aggregate markets. Vessel sizes vary widely, from small coasters to Supramax.

Africa and Middle East. Egypt, Tunisia, and Morocco supply regional cement and construction markets. Gulf cement plants import limestone from Oman’s quarries near Muscat and from the Ras al-Khaimah coastal quarries in the UAE.

End-use markets and their carriage implications

The end-use determines the required purity, particle size, and acceptable moisture content, all of which affect cargo declaration and handling:

Cement kiln feedstock. Cement manufacture requires limestone with CaCO3 content typically above 80%, and most modern plants prefer above 90%. The raw mill accepts a wide particle-size range (25 to 200 mm) and the kiln calcines it to calcium oxide (quicklime) at 900 to 1,000 degrees Celsius. This trade uses crushed limestone in the 10 to 80 mm size range. Dust generation at loading is moderate. Moisture sensitivity is low because the kiln evaporates water efficiently.

Steel-making flux. Basic oxygen furnaces and electric arc furnaces use limestone (or calcined limestone, i.e., quicklime) as a flux to remove silica, phosphorus, and sulphur from the iron melt. The flux reacts with those impurities to form slag. Steel mills want limestone with CaCO3 above 95%, with closely controlled MgO and SiO2 contents, in a particle size of roughly 10 to 40 mm for blast furnace use and 5 to 25 mm for basic oxygen furnace use. Fines below 6 mm are often rejected by blast furnaces because they impair the permeability of the coke-ore-limestone burden. Steel mill cargoes therefore tend to be coarser than cement-plant cargoes, with less dust but more irregular lump shapes that resist self-trimming.

Agricultural lime. Ground limestone spread on farmland raises soil pH, corrects acidity from fertilizer application, and supplies calcium for crop nutrition. Agricultural lime is finely ground, with 60 to 80% of the product passing a 0.15 mm (100-mesh) sieve, and sometimes finer. This is the particle size range most associated with the Group A fines risk discussed in the section below. Agricultural lime cargoes require more rigorous moisture assessment and closer attention to dust exposure for stevedores.

Flue-gas desulfurization (FGD). Coal-fired power stations inject slurried or finely ground limestone into flue gas to absorb sulphur dioxide, forming calcium sulphate (gypsum) as a byproduct. FGD-grade limestone is typically ground to 90% passing 0.044 mm (325-mesh). The product is almost always slurried with water at the plant rather than shipped as dry powder, but dry FGD limestone is shipped in bulk from producing regions to power-plant clusters. It has the finest particle size of any commercial limestone grade and carries the highest fines-related carriage risk.

Construction aggregate. Crushed limestone is used as road base, concrete aggregate, and railway ballast. These cargoes are typically the coarsest, 20 to 100 mm broken stone, and the lowest in CaCO3 purity requirements. Aggregate cargoes may include chert, flint, dolomite, and other carbonates, particularly for non-critical applications. Dust generation during loading and discharge is moderate.

Chemical and industrial uses. Glass manufacture uses high-purity limestone for CaO content. Paper mills use precipitated calcium carbonate (PCC) produced from limestone as a filler and coating. Water treatment plants use crushed limestone for pH adjustment. These specialty uses account for a small fraction of total seaborne trade.

The IMSBC Code schedule for limestone

Group assignment and regulatory basis

The IMSBC Code Appendix 1 lists LIMESTONE as a Group C cargo. Group C, under the Code’s definitions, covers solid bulk cargoes that are neither liable to liquefy (Group A) nor possess chemical hazards sufficient to require Group B treatment. Group C cargoes carry no Transportable Moisture Limit requirement and no chemical hazard notice in the standard schedule entry.

The Code also lists CALCIUM CARBONATE as a separate entry, likewise Group C. Calcium carbonate in bulk commerce refers either to ground limestone of high purity or to precipitated calcium carbonate (PCC), a refined synthetic product. Both entries share the same Group C status and the same practical carriage rules.

The IMSBC Code Amendment 07-23, adopted by IMO Resolution MSC.539(107) on 8 June 2023 and mandatory from 1 January 2025, did not alter the Group C status of limestone. Amendment 06-21 (Resolution MSC.500(105)) similarly left the limestone schedule unchanged. The schedule has been stable across recent amendment cycles, reflecting the low incident rate associated with standard crushed and lump limestone.

Schedule particulars table

The following table summarizes the physical properties as recorded or implied by the IMSBC Code schedule for LIMESTONE and CALCIUM CARBONATE, with practical ranges derived from shipper declarations and port-state control experience:

Bulk density values in the table are at-rest values for the cargo as loaded. Compaction under ship motion and cargo weight increases the in-hold density slightly, typically 3 to 6% above the at-rest value for a Handysize vessel on a 5-day coastal voyage and up to 10% on longer deep-sea voyages. This compaction effect is small compared to liquefaction-prone Group A cargoes but can affect tonnage reconciliation in draft surveys.

The Group C classification: what it means in practice

Group C status means three things for the commercial and operational team. First, no Transportable Moisture Limit test is required before loading standard limestone. Second, no chemical hazard provisions apply: no hold atmosphere testing, no fire watch, no emergency response for spill. Third, the cargo can be loaded continuously through rain without a mandatory loading suspension, because wetting does not trigger a liquefaction risk in a coarse crushed product.

That said, Group C is a condition of the product, not a blanket exemption for all products named “limestone.” The IMSBC Code requires shippers to declare the cargo truthfully, including its actual particle-size distribution. A declaration of LIMESTONE on a cargo that is actually a finely ground product with particle size below 1 mm and moisture content of 15% misrepresents the cargo. The Code’s Section 4 cargo information requirements apply regardless of group: the shipper must provide a cargo declaration, with actual physical and chemical properties, before the vessel accepts the cargo.

The fines and moisture risk: when limestone approaches Group A

Particle size and pore-pressure physics

The physical mechanism behind Group A liquefaction applies to any cohesive fine material, not just ores and concentrates. Calcium carbonate particles below approximately 1 mm diameter at moisture contents above approximately 10 to 15% (by weight) can generate pore-water pressures under ship motion that temporarily reduce effective inter-particle friction to near zero. The cargo does not need to be classified as Group A for this physics to operate; the physics operates on the material properties, not the label.

The IMSBC Code’s own guidance in Appendix 3, Section 1 (the “non-cohesive” cargo guidance) recognizes that some Group C materials can behave unexpectedly if particle size and moisture conditions fall into ranges normally associated with Group A. The Code’s answer is that shippers and masters must assess the actual material, not just the cargo name.

Ground limestone presents this risk at finer particle sizes. Agricultural lime at 60 to 80% passing 0.15 mm, and FGD-grade limestone at 90% passing 0.044 mm, both have particle-size distributions well within the range where liquefaction can occur under sufficient moisture. No formal amendment has moved these products to Group A, but the practical recommendation from P&I clubs and some flag states is that ground limestone shipments should be treated as Group A for TML purposes when the product is finer than approximately 1 mm and the moisture content at loading exceeds 10%.

Industry incidents and claims

Documented liquefaction events involving limestone are rare compared to those involving nickel ore or iron ore fines, partly because limestone is generally shipped from well-regulated terminals in established trade routes, and partly because the bulk of limestone cargo is coarse crushed material not susceptible to liquefaction. However, P&I club loss-prevention publications reference several cases in which finely ground limestone or calcium carbonate was loaded with elevated moisture and subsequently exhibited shifting behavior, causing list and concern about cargo stability. None of these events resulted in vessel loss, but several required port-of-refuge calls for cargo redistribution.

The typical scenario in these cases is a cement-plant cargo loaded during the wet season in a tropical port, where the plant’s raw-material stockpile has been exposed to rain and the surface moisture is elevated. The shipper declares the cargo as LIMESTONE (Group C) because that is the correct name, without measuring the actual particle size or moisture content of the finer fraction of the crushing product. On a 3 to 5 day voyage with sea conditions above Force 4, the fine fraction at the lower boundary of the particle-size distribution exhibits localized flow at the cargo surface in affected holds.

P&I correspondents handling limestone casualties consistently note the same root cause: the shipper’s cargo declaration described a nominally Group C material, but the actual product delivered to the vessel had a significant fine fraction that behaved as a Group A material at the loading moisture level.

What a prudent master should do

When presented with a limestone cargo that appears unusually fine, shows surface moisture, or is described as “ground limestone,” “agricultural lime,” “limestone powder,” or “limestone fines,” the master should:

1. Request the particle-size distribution from the shipper or independent surveyor.
2. If a significant fraction (say, more than 20% by weight) is below 1 mm and the moisture content exceeds 10%, treat the cargo as a potential Group A material and ask for a TML test under IMSBC Code Appendix 2 procedures.
3. Apply the can test (the rapid shipboard screening procedure in IMSBC Code Appendix 2, Section 8.4) to check for free moisture: fill a 0.5-liter container, drop it 25 times from 0.2 meters onto a hard surface, and check for surface moisture. A wet result means loading should stop pending a certified laboratory assessment.
4. Notify the operator and P&I correspondent if the shipper is unwilling to provide particle-size and moisture documentation, or if the can test is wet. Consult the IMSBC Code Section 4 cargo information provisions and SOLAS Chapter VI Regulation 2.

These steps do not apply to standard crushed or lump limestone where the product is coarser than 5 mm. They apply specifically when the material is fine enough to raise a reasonable question.

Dust: the principal hazard of limestone carriage

Why limestone generates dust

Limestone is a brittle crystalline rock. Crushing generates a particle-size distribution that includes a fine fraction below 0.1 mm, which becomes airborne under conveyor transfer impacts, loader fall height, and the ventilation effects of cargo pile collapse into the hold. The finer the product size specification, the higher the airborne dust fraction. A coarse blast-furnace limestone (10 to 40 mm) generates less dust per tonne than a crushed cement-plant grade (5 to 25 mm), which in turn generates less dust per tonne than an agricultural lime product.

Dust from limestone is calcium carbonate particulate plus small amounts of silica (quartz) and clay minerals from the quarry matrix. The silica fraction is the primary health concern. Pure CaCO3 dust at typical occupational exposures is classified as a “nuisance dust” by occupational health authorities; the UK Health and Safety Executive’s EH40 occupational exposure limit for calcium carbonate dust is 10 mg/m3 (inhalable) and 4 mg/m3 (respirable). The OSHA permissible exposure limit (PEL) for calcium carbonate (nuisance dust) is 15 mg/m3 (total) and 5 mg/m3 (respirable fraction, 8-hour TWA).

Crystalline silica contamination

The health risk escalates when the limestone contains a significant fraction of crystalline silica. All limestones contain trace silica from chert bands, flint nodules, and quartz-bearing shale partings. High-silica limestone from impure quarries may contain 5 to 15% SiO2. Respirable crystalline silica (RCS) at concentrations above 0.1 mg/m3 is a known human carcinogen (IARC Group 1), causing silicosis and lung cancer under repeated long-term exposure.

Ship’s crew working on deck during grab-discharge of limestone should use half-face respirators with P2/N95 filters when bulk density measurements, visual inspection, or the cargo declaration indicate a high-silica limestone. Port-state control inspections have cited vessels for failure to provide adequate respiratory protection during limestone discharge.

IMSBC Code dust provisions

The IMSBC Code schedule for LIMESTONE includes a dust hazard notation. The Code’s standard text for Group C cargoes with a dust notation reads, broadly, that appropriate precautions should be taken to minimize the risk of dust to personnel. In practice this translates to the following requirements, which responsible operators apply as standard:

• Loading: reduce conveyor fall height at the loading spout to the minimum practical to suppress dust generation. Use water spraying at the point of cargo drop and at the ship’s hatch coaming. Close hatch covers partially on holds not currently loading if wind is spreading dust across the deck.
• Voyage: no special action required for standard limestone. The cargo is not prone to spontaneous dust generation during the voyage.
• Discharge: grab crane discharge generates coarse dust plumes that can blanket the vessel’s deck. Deck crew working during grab discharge should wear appropriate respiratory protection. Some terminals use enclosed grab or pneumatic suction discharge to contain dust.
• Enclosed spaces: respiratory protection is required for any personnel entering a cargo hold containing ground limestone immediately after opening for discharge, because fine dust that settled on the cargo surface can be disturbed. Allow ventilation with the hold ventilators open for at least 15 minutes before personnel entry.

The IMSBC Code’s provisions in Section 3 (Assessment of acceptability of consignments) and Section 5 (Trimming procedures) do not impose any special requirements on limestone beyond those applicable to all Group C cargoes, but the dust notation in the schedule triggers the general obligation to follow safe handling practices.

Fire and explosion: not applicable to limestone

Limestone dust, unlike coal dust, grain dust, or sulphur dust, does not form explosive mixtures with air. CaCO3 is non-combustible and does not support oxidation reactions. Hold atmosphere monitoring for oxygen depletion or toxic gases is not required. This is a practical advantage on vessels carrying mixed cargo parcels or switching between commodity types: a hold cleaned after limestone does not require pre-entry gas testing for explosive atmosphere.

Cargo properties and stowage

Bulk density and stowage factor

Standard crushed limestone has a bulk density of approximately 1,300 to 1,600 kg/m3 at rest. For loading-plan purposes, a design value of 1,500 kg/m3 is commonly used for crushed grades (5 to 50 mm), giving a stowage factor of approximately 0.67 m3/t. Coarser lump stone (50 to 200 mm) has slightly lower bulk density in the 1,250 to 1,450 kg/m3 range because of the larger void spaces between lumps, giving a stowage factor of approximately 0.69 to 0.80 m3/t.

The difference between loaded bulk density and in-situ solid density (approximately 2,600 to 2,700 kg/m3 for limestone) reflects the void fraction of roughly 40% in a loose-poured crushed cargo. Compaction under ship vibration and wave motion can reduce the void fraction by 2 to 5 percentage points on a voyage, meaning the cargo surface drops 2 to 5 cm per meter of hold depth. This compaction is not a hazard in itself, but it affects the reconciliation of loaded and discharged tonnage in a draft survey.

Angle of repose and trimming

Crushed limestone has an angle of repose of approximately 30 to 38 degrees, varying with particle size and moisture. Finer, drier material has higher repose angles; coarser, wetter material tends to flow slightly. The practical consequence for loading is that limestone in crushed grades self-trims adequately in holds with standard batter angles: the cargo pile redistributes itself under the conveyor loading point without requiring mechanical trimming in most cases.

Coarser lump limestone (above 50 mm) does not self-trim. The cargo builds a pile under the hatch opening and does not spread laterally without mechanical assistance. A bulldozer on the cargo or a trimming conveyor inside the hold is required to distribute lump stone across the full hold width. This is important for stability: an untrimmed lump limestone cargo concentrated near the keel centerline creates a high KG (the center of gravity above the keel) in the early stages of loading, and the excess vertical concentration also stresses the inner bottom locally.

Structural loading considerations

Limestone at 1,300 to 1,600 kg/m3 bulk density is heavier than grain (approximately 750 to 850 kg/m3) and similar in density to iron ore pellets (approximately 1,600 to 2,200 kg/m3). A standard Handysize vessel of 28,000 DWT with three or five holds must distribute limestone across all holds to avoid exceeding allowable inner-bottom pressure and shear force limits.

Inner-bottom pressure limits for most Handysize vessels are 10 to 15 t/m2 under a full head of cargo. Limestone at 1,500 kg/m3 in a hold with a 10 m cargo column exerts approximately 15 t/m2 at the inner bottom, which is at or near the design limit for some older vessels. Vessels carrying heavy limestone cargoes should review the loading manual, which typically provides a maximum allowable filling level for each hold. This is not unique to limestone but is particularly relevant because limestone is heavy enough to load a Handysize vessel to DWT on a shorter (5,000 to 8,000 m3) hold volume than many other cargoes.

Bulk carriers built under SOLAS Chapter XII (SOLAS Chapter XII: Additional Safety Measures for Bulk Carriers) are designed with double-bottom structures and flooding protection for high-density cargoes. Vessels without double-bottom protection should be careful not to fill a single hold to its volume limit with limestone, as the center-of-gravity effect of a full hold adjacent to an empty hold can exceed shear-force limits.

Hold preparation

Cleanliness requirements

Limestone is not chemically aggressive to steel structure, but its end users often impose strict purity requirements. A cement kiln’s quality control system will detect elevated sulphur, chloride, or organic carbon at the 0.01% level. A steel mill’s flux specification limits MgO, SiO2, and alkalis precisely. Any residue from a previous cargo that introduces contaminants at the parts-per-thousand level may result in a cargo rejection or quality claim.

The standard required by limestone receivers is a hold that is swept and washed, with no residue from the previous cargo, no rust scale that would dissolve into contact with wet limestone and introduce iron contamination, and no bilge water carrying traces of previous cargo. The specific pre-load cleanliness check needed varies with the previous cargo:

• Previous cargo: coal or grain. Coal residue introduces combustible carbon and sulphur that contaminate limestone for cement or chemical use. Grain residue introduces organic nitrogen compounds. Both require hold washing, high-pressure fresh-water rinse, and drying before loading. A surveyor’s hold inspection certificate is common on coal-to-limestone transitions.
• Previous cargo: sulphur, fertilizer, or chemical cargo. These cargoes contaminate limestone for almost all end uses. High-pressure wash and multiple rinse cycles are required. Some receivers require fumigation or deodorizing after a sulphur cargo.
• Previous cargo: iron ore, manganese ore, or heavy-mineral cargo. Iron contamination from ore residue introduces elevated Fe2O3 into the limestone analysis, which can affect glass and chemical uses. Hold should be swept and washed.
• Previous cargo: another limestone or neutral cargo (sand, aggregate, clean ballast hold). Minimal preparation beyond sweeping and bilge clearance is typically acceptable.

For full guidance on hold preparation procedures, the cargo hold preparation standards article covers the applicable requirements across the range of bulk cargoes.

Bilge system

The bilge strainer covers must be intact and positioned before loading. Limestone fines can block bilge suctions if they migrate through damaged or missing strainer covers. This is particularly relevant for ground limestone cargoes where fines are plentiful. Once limestone paste accumulates in the bilge well, it sets as calcium carbonate cement when it dries and is extremely difficult to remove without chipping. Bilge clearing before and during the voyage is essential for ground-limestone cargoes.

Hatch covers and weathertightness

Limestone does not require weathertight hatch covers for its own protection during the voyage. The cargo is not harmed by rain or seawater ingress in terms of its material properties: wet limestone is simply heavier. However, water ingress that raises the moisture content of ground limestone above the TML threshold converts a Group C cargo into a Group A condition in the affected hold. For finely ground limestone cargoes, weathertight hatches are therefore a precaution against creating a liquefaction risk during the voyage, not a cargo-protection measure.

For coarse crushed limestone (above 5 mm), hatch weathertightness is a routine operational standard but not a specific safety requirement related to cargo behavior.

Draft survey for limestone

The cargo draught survey is the standard quantity-determination method for limestone bulk shipments, particularly on voyages where the discharge terminal does not have calibrated weighbridge or conveyor belt scales.

Limestone’s physical properties create two draft-survey considerations that surveyors note in practice:

Cargo compaction. As noted above, the limestone cargo mass compacts under vibration during loading. If a draft survey is taken mid-way through loading (a running draft check), the density of cargo already in the hold may be slightly higher than the as-loaded value used in the loadmaster’s calculation, because compaction at the lower levels occurs during subsequent loading. This introduces a small positive bias into tonnage estimates made from running drafts. A final survey taken after loading is complete should use the surveyor’s direct observation of draft marks and a fresh wedge-volume calculation, not an extrapolation from mid-load readings.

Wedge and free surface calculation. Limestone in holds of standard Handysize or Supramax bulk carriers presents a roughly flat cargo surface. The wedge correction to trim is straightforward. Where the cargo has been unevenly distributed, or where a different number of holds are loaded than are symmetric about the midships, the longitudinal trim correction requires the full five-hold algorithm. Errors here on limestone cargoes are a common source of draft-survey disputes.

Freshwater allowance. A limestone shipment from a tropical river port where the water density is below 1.025 t/m3 requires a freshwater allowance correction to the draft readings. The allowance is standard but must be applied from the surveyor’s reading of the dock water density, not assumed at 1.025 t/m3.

Loading and discharge operations

Loading: conveyor and grab

Most limestone loading terminals use shore-based conveyor systems feeding a shiploader boom or a fixed wharf spout. Loading rates at established limestone terminals range from 1,500 to 5,000 t/h for crushed grades. The main operational considerations during loading are:

• Self-trimming vs. manual trimming. Crushed grades (5 to 50 mm) self-trim adequately in holds with normal hatch-to-hold width ratios. Lump grades require a bulldozer for shoulder trimming. The IMSBC Code Section 5 specifies that Group C cargoes must be trimmed reasonably level when loading is complete to avoid cargo shifting; for crushed limestone this is normally achieved through the loading pattern alone.
• Hold filling sequence. Heavy limestone cargoes should be loaded symmetrically across holds to maintain vessel trim and avoid exceeding shear force limits in intermediate stages. The master’s loading plan should sequence fills to keep the bending moment below the permissible value at each hold combination.
• Dust at loading. Water spray at the shiploader spout is standard practice. If the terminal does not provide spraying, the mate should request it, and if refused, should record the refusal in the Statement of Facts (SOF).

Some ports load limestone by bulldozer-and-crane method (stockpile-to-grab-crane-to-hold), particularly in smaller terminals. Loading rates by this method are 200 to 600 t/h. The grab tends to shatter lump stone further on impact, generating additional fines and dust in the hold.

Discharge: grab crane

Limestone discharge is almost universally by shore-based grab cranes at the receiving terminal. Discharge rates of 500 to 2,500 t/h are typical. Residue after grab discharge is heavier for coarse lump material than for self-trimming crushed material: lump stone leaves irregular piles at the hold corners that the grab cannot reach cleanly, requiring a dozer pass or hand-shoveling for the final 100 to 400 t per hold.

For cement-plant discharges, some terminals use belt conveyors from the ship’s hold, where hoppers at the hold bottom feed a take-away conveyor. This system can achieve lower residue than grab, but it requires appropriate hold structure and is less common than grab for limestone.

For agricultural lime and FGD-grade limestone (very fine product), pneumatic suction discharge is used at some specialized terminals. This method controls dust well but has relatively low capacity (200 to 600 t/h) and is not applicable to coarser grades.

Dust control: operational measures

The most effective dust control during limestone loading is reducing the vertical drop of cargo onto the pile. A shiploader boom positioned to minimize free-fall, with a telescoping chute that follows the cargo pile surface, can reduce dust generation by 50 to 70% compared to a fixed-position spout at full drop height. At terminals without telescoping chutes, water spray is the principal control.

During discharge by grab crane, the main dust sources are the open hatch (dust rising from the disturbed cargo surface) and the shore-side unloading hopper (dust from cargo falling into the hopper). Spraying the cargo surface in the hold between grab cycles reduces the first source. Terminal-side water curtains or enclosed hoppers address the second. Port-state control inspections at dust-regulated ports (Rotterdam, Hamburg, Antwerp, and several US East Coast ports have elevated fugitive-dust requirements) check for effective dust control in limestone operations.

Related carbonate cargoes in the IMSBC Code

Limestone is one of several calcium-bearing or carbonate-mineral cargoes in the IMSBC Code schedule. Understanding the distinctions prevents cargo declaration errors:

Calcium carbonate (IMSBC Code entry). The dedicated CALCIUM CARBONATE entry covers refined or synthetic CaCO3 products, including PCC from paper mills and fine ground limestone. It is Group C. The schedule entry is narrower than LIMESTONE and applies to products where the calcium carbonate content is the marketed commodity (purity above 90% CaCO3 is typical), rather than a quarried rock that contains limestone as the dominant mineral.

Dolomite. Dolomite (CaMg(CO3)2) is the magnesium-carbonate analogue of limestone. It has similar physical properties to limestone (bulk density approximately 1,400 to 1,600 kg/m3 crushed) and is also Group C under the IMSBC Code. It’s used in steel-making as a dolomitic flux, in glass manufacture, and as construction aggregate. The key difference from limestone is its MgO content (approximately 20%), which disqualifies it for cement manufacturing where MgO above 2% causes long-term dimensional instability. Dolomite does not have its own named IMSBC schedule entry; it is typically shipped under the LIMESTONE or a related schedule, or declared under the MHB provisions if it contains significant impurities.

Gypsum. Calcium sulfate dihydrate (CaSO4·2H2O). Also Group C, also a cement additive (as a set retarder), but chemically distinct. Gypsum is softer and more soluble than limestone, and requires different hold preparation. It does not substitute for limestone in the kiln. The IMSBC schedule for GYPSUM is a separate entry.

Cement clinkers. The product of limestone calcination in the kiln: calcium silicates and aluminates, Group C, but with a significant exothermic hydration hazard if the holds are wetted and inadequately ventilated. Not a substitute for limestone; it is the processed product of limestone.

Cement. Ground clinker with gypsum added. Group C but with a serious health hazard: Portland cement dust is caustic (pH above 12 in solution) and causes severe chemical burns to skin and eyes. Loaded and discharged at much lower rates than limestone, typically in bags or under cover. Requires stringent respiratory and skin protection during hold cleaning.

Phosphate rock. Calcium phosphate mineral (Ca3(PO4)2), mined and shipped as Group C. Higher bulk density (1,400 to 1,800 kg/m3) than most limestone grades, and finely crushed. Phosphate rock fines have the same Group A fines risk as ground limestone at high moisture content. The phosphate content makes it toxic to aquatic environments if spilled in significant quantity.

Sand. Silica-dominant material, Group C, but entirely different chemistry and higher dust health risk owing to crystalline silica content. No carbonate chemistry.

Marine incidents and claims history

Why limestone generates few casualties

The historical record of vessel losses attributable to limestone is short. The IMO’s Global Integrated Shipping Information System (GISIS) database records no vessel total losses attributed to limestone liquefaction among bulk carriers lost from 1980 to 2024, in contrast to the documented losses from nickel ore and iron ore fines. The reasons are structural:

Limestone is quarried in countries with mature mining and port infrastructure: the United States, Bahamas, Turkey, Norway, Vietnam, and Egypt. These origins are generally subject to better cargo certification and port-state control than some of the developing-country origins associated with liquefaction-prone cargoes. The cargo’s coarse size in most trade routes means the liquefaction risk is genuinely low. And the cargo is often measured by weight at both ends of the voyage using calibrated port scales, which limits the incentive to load above the certified condition.

Claims that do occur

The claims that P&I clubs and cargo insurers handle for limestone are predominantly:

Cargo shortage. The gap between the bill of lading tonnage and the draft survey or port-scale tonnage at discharge. Compaction of the cargo during the voyage (genuine physical reduction in void volume) contributes to a small, systematic shortage of approximately 0.3 to 0.8% on longer voyages. Claims above 1% typically involve either draft survey errors, shore scale calibration issues, or pilferage at discharge.

Contamination. Limestone cargoes rejected at the steel mill or cement plant because of elevated silica, sulphur, iron, or organic content tracing to the vessel’s previous cargo. These claims depend on the adequacy of hold preparation: a proper pre-load inspection with a surveyor’s certificate provides the shipowner’s primary defense.

Moisture damage in adjacent cargoes. On a vessel carrying limestone in some holds and a moisture-sensitive cargo (such as bagged cement or steel coils) in others, water absorbed by the limestone from tropical rain and then released through hold-to-hold condensation pathways has caused damage claims. This scenario is unusual but has occurred in short-sea trades where hatch coaming seals were not maintained.

Dust damage to machinery and deck equipment. Fine limestone dust deposited on winch drums, capstans, and navigation instruments during loading causes accelerated wear and corrosion when the dust combines with moisture. Operators of vessels regularly trading in fine limestone should establish a deck-cleaning routine after each loading, covering exposed machinery with tarpaulins during operations and washing down with fresh water afterward.

Personal injury from dust exposure. Stevedores or crew members handling limestone without respiratory protection at terminals with inadequate dust control have raised occupational disease claims, particularly in markets where crystalline silica content in the limestone is elevated. These claims are typically handled under workers’ compensation frameworks rather than P&I, but the ship’s P&I club may be drawn in if the terminal points to inadequate deck-crew oversight.

Limitations of this article

This article describes the IMSBC Code schedule for LIMESTONE and CALCIUM CARBONATE as these are defined in the Code through Amendment 07-23 (mandatory from 1 January 2025). The Code is subject to amendment every two years; readers should verify the current schedule text against the edition published by the IMO, not against reproductions in commercial databases. The IMSBC Code is available from the IMO Publications catalogue.

Physical property ranges given here (bulk density, angle of repose, stowage factor) are from industry practice and published engineering data, not from a single authoritative source. Actual values vary substantially with quarry origin, production process, particle size specification, and moisture condition. The shipper’s cargo declaration is the contractual statement of properties for any specific shipment.

The fines and moisture risk discussion describes the physical mechanism based on established soil mechanics principles applied to calcium carbonate, and on P&I club published guidance. It is not a substitute for a certified TML laboratory test where that test is indicated by the cargo’s particle size and moisture condition.

The trade volume and route data in this article are from public industry sources (USGS Mineral Commodity Summaries; IMO GISIS; global cement production statistics from the Global Cement and Concrete Association) and are approximate. Limestone trade data are not reported with the precision of iron ore or coal volumes because limestone is a lower-value, widely produced commodity for which no single industry body collects complete statistics.

Applicable regulations beyond the IMSBC Code include SOLAS Chapter VI (cargo information and securing), SOLAS Chapter XII (bulk carrier safety measures, relevant for structural loading calculations), and port-state regulations on dust emissions, which vary by jurisdiction. This article does not cover port-specific dust regulations. Masters and operators should consult port authority requirements for each limestone discharging port.

See also

• IMSBC Code
• IMSBC Code Group C Cargoes
• IMSBC Code Group A Cargoes
• Gypsum: IMSBC Code Schedule and Carriage
• Cement: IMSBC Code Schedule and Carriage
• Cement Clinkers: IMSBC Code Schedule and Carriage
• Phosphate Rock: IMSBC Code Schedule and Carriage
• Sand: IMSBC Code Schedule and Carriage
• Cargo Draught Survey for Bulk Carriers
• Cargo Hold Preparation Standards
• SOLAS Chapter XII: Additional Safety Measures for Bulk Carriers