Definition
Cement is a hydraulic mineral binder, produced by firing limestone-clay raw materials into clinker and grinding it with additives, which hardens after mixing with water and binds aggregate into concrete; in bulk transport it has a bulk density of around 1100–1600 kg/m³ and is classified per PN-EN 197-1 as CEM I to CEM V.
Cement is not a cargo that passes through our terminal — in Chorula we work with plastic granulates and other bulk materials that flow well and are not dangerous goods. We describe it here as a model example of a mineral material transported by silo, because the principles of bulk-material physics — bulk density, hygroscopicity, compartment cleanliness — are common to cement and granulate alike, even though the consequences of ignoring them can differ. Cement is also one of the most mass-produced bulk materials in the world, so understanding its logistics organises the whole body of knowledge about bulk transport.
What cement is and how it is made
Portland cement is produced in the rotary kiln of a cement works, where a mixture of ground limestone and clay (or marl) is fired at a temperature of about 1450 °C. The product of the firing is clinker — hard, sintered granules of calcium silicates and aluminates. The clinker is then ground with the addition of a setting-time regulator (gypsum) and, depending on the type of cement, with mineral additives: blast-furnace slag, fly ash, pozzolana or ground limestone.
After grinding, cement takes the form of a very fine, homogeneous dust with a particle size in the range of tens of micrometres and a specific surface usually of 3000–4500 cm²/g (Blaine). It is precisely this fineness that makes it a reactive material — the large contact surface with water drives the hydration reaction by which cement paste sets and hardens. The same fineness makes cement a difficult dust to handle: it generates dust, is hygroscopic and requires sealed installations.
Types of cement per PN-EN 197-1
The European standard PN-EN 197-1 divides common-use cements into five main types according to composition — the proportion of clinker and mineral additives:
| Symbol | Name | Main composition |
|---|---|---|
| CEM I | Portland cement | clinker ≥ 95% |
| CEM II | Portland cement with additives | clinker + slag/fly ash/limestone/pozzolana |
| CEM III | blast-furnace cement | clinker + a large proportion of blast-furnace slag |
| CEM IV | pozzolanic cement | clinker + pozzolana/fly ash |
| CEM V | composite cement | clinker + slag + pozzolana |
To the type symbol is added the strength class: 32.5 / 42.5 / 52.5 (compressive strength after 28 days in MPa) and a letter for the rate of early strength gain: N (normal), R (rapid) or L (low). For example, CEM I 42.5 R is pure Portland cement of class 42.5 with high early strength, while CEM III/A 32.5 N is a blast-furnace cement with normal strength gain.
From a logistics perspective these designations are to cement what the melt flow index is to granulate: a batch identifier that must not be confused. Mixing CEM I with CEM III in a single silo renders the material non-compliant with the customer’s specification — which is why traceability and compartment cleanliness are guarded here just as vigilantly as with polymer granulates.
The bulk density of cement
The most important transport and storage parameter of cement is bulk density — the mass of material per unit of loose volume, including the air between particles. For cement it depends strongly on the state of aeration:
| State of cement | Bulk density [kg/m³] |
|---|---|
| fresh, aerated (after discharge) | 1100–1300 |
| working value for calculations | ~1300 |
| settled / caked | 1500–1600 |
Freshly transloaded, air-“fluffed” cement is light (closer to 1100 kg/m³), but after settling in the silo it thickens — the particles pack more tightly and the air escapes. For example, product data sheets give a bulk density of around 1330 kg/m³ for CEM III/A and even approx. 1420 kg/m³ for CEM I 42.5 R. For designing silo capacity and tanker payload, a working value of about 1300 kg/m³ is usually assumed, with a margin for compaction.
It is worth distinguishing bulk density from the specific density of cement grains, which is approx. 3050–3150 kg/m³. The latter matters in concrete mix design, the former in logistics. Cement is a material much heavier by volume than plastic granulate (0.5–0.6 kg/l, i.e. 500–600 kg/m³), so in its transport the limit is usually the permissible gross weight of the combination, not the tank volume — exactly the opposite of light, “volumetric” PE granulate. This shows why there is no single universal tanker for everything: silo trailers are matched to the density of the specific material.
Why cement is carried by silo
Cement is transported in bulk by cement tankers (silo tankers) with pneumatic discharge. The reason is both economic and qualitative: concrete plants and cement works receive cement in quantities of hundreds of tonnes, and 25 kg bags or big-bags only make sense for small, retail deliveries. In bulk the material is cheaper, delivery is faster, and the risk of contamination and losses is lower.
The discharge itself is a classic pneumatic discharge: the tanker’s compressor creates an overpressure in the compartment (usually up to approx. 2 bar) that aerates the cement and conveys it in a stream of air through the discharge pipe into the customer’s silo. Cement, as a very fine dust, fluidises exceptionally well — when aerated it behaves almost like a liquid, which makes pneumatic transport the natural choice for this material.
And here an important difference emerges from what we do in Chorula with granulate. For cement, pneumatics is an advantage — the material is dust anyway, so there is nothing to “damage” mechanically. For plastic granulate the opposite is true: accelerating the pellets in pipes under pressure causes electrostatic charging, dust and angel hair, which is why for sensitive granulates we use transloading without pneumatics — a gentle, gravity chute. The same silo tanker therefore serves both worlds, but the working method is matched to the nature of the material.
The couplings and fittings of cement tankers are, incidentally, common to the whole family of silo tankers: Storz couplings (along with PERROT and Camlock) remain the standard, and the compartments are fitted with fluidising cone bottoms that assist emptying.
Hygroscopicity and caking
The most dangerous enemy of cement is moisture. Cement is hygroscopic — it absorbs water vapour from the air on its own, and on contact with water it begins the hydration reaction, the very setting that is supposed to occur only in the concrete. Damp cement cakes, loses flowability and drops in strength class; in extreme cases it hardens in the silo or tanker compartment.
Hence the iron rule of cement transport and storage: everything must be dry. The tanker compartment before loading should be dry and clean, the compressor air for discharge dried, and the storage silo sealed, with a vent filter protecting against damp air from outside. This is exactly the same discipline of dryness we apply to granulates: although plastics are less sensitive to moisture than cement, moisture on the surface of pellets also impairs processing. The difference is one of scale, not of principle.
Caked cement is at the same time the reason why bulk density increases with settling — a compacted, damp mass packs more tightly. For a silo operator this means that old, unused cement may be not only heavier by volume but also partly worthless.
Tanker cleaning and cleanliness
After carrying cement, the tanker is prepared for the next load. Cleaning is done at a tank cleaning station — most often dry (thorough emptying, blowing through with compressed air), and when changing to a different type of material also wet, with mandatory drying of the compartment after washing. Cement residue in a damp compartment would harden and contaminate the next load — and for a tanker later carrying food granulate or chemicals such contamination is unacceptable.
Compartment cleanliness is confirmed by an EFTCO ECD (European Cleaning Document) — a certificate standard in the tanker industry describing what the tanker was previously loaded with and how it was cleaned. This is the same mechanism that protects against cross-contamination at every change of load, regardless of whether the previous material was cement, urea or plastic granulate. In terminal practice we treat the compartment’s history like a record card: a sensitive material is not carried in a tanker whose previous loads and cleaning cannot be documented.
Storing cement in bulk
Cement in bulk is stored in silos — vertical steel or concrete tanks with a conical bottom and a discharge system. Two elements are key: bottom aeration (nozzles or fluidising mats blowing in air that loosens the cement and eases its flow from the hopper) and a vent filter at the top, which releases the air displaced during filling while retaining the dust and protecting against damp outside air.
Without aeration, fine cement is prone to forming arches (bridging) and dead hoppers in the silo — the material hangs up instead of flowing. This is a problem common to all fine, cohesive powders. That is why cement silos are designed with steep hoppers and flow assistance, and filling and emptying operations are planned so that the material does not sit too long and absorb moisture.
The general principles of storing bulk materials — silos, buffers, protection from moisture and batch traceability — are described in more detail in the article on storing bulk materials. At our terminal in Chorula we apply them to granulates: a buffer store of 2000 big-bags and a throughput of 200 tonnes per day are the same logic of managing a bulk material, only in flexible packaging instead of dust silos.
Cement and the work of the SMIALA terminal
It must be said plainly: we neither carry nor transload cement. The profile of the SMIALA terminal is non-dangerous bulk materials — PE, PP, PVC, PET, ABS, PS granulates and the like, which we transfer from big-bags into silo trailers using a technology that protects the quality of the granulate. We describe cement in this encyclopaedia because it is a textbook example of a bulk material transported by silo and helps to understand the mechanics of the whole industry: bulk density, fluidisation, hygroscopicity, compartment cleanliness.
This knowledge is not detached from our work — quite the opposite. Every bulk material is governed by the same laws of physics and differs only in its sensitivity to particular hazards. Cement fears moisture and requires pneumatics; plastic granulate fears pneumatics and contamination. An operator who understands cement also understands why we choose a different method for granulate. The full offer of transporting and transloading bulk materials is run by the PHS Magnum network, and the gentle transloading technology itself is described on the big-bag to silo trailer transloading page.
Related topics
It is worth setting cement alongside the silo trailer as the tool of its transport and pneumatic discharge, which is the natural method of emptying it. We develop the logistics of carrying by silo tankers in the article on silo transport, and the rules of bulk storage in the text on storing bulk materials. You will find the network’s transloading services on the storage page of the SMIALA terminal.
Sources
- PN-EN 197-1 — cement: composition, specifications and conformity criteria for common-use cements (classes CEM I–V).
- Technical and product data sheets for cements (bulk density, strength classes).
- Industry guidelines on tanker cleanliness (EFTCO ECD — European Cleaning Document).
- Operational practice of the SMIALA terminal, Chorula — Aleksy Pasternak.
