Hydrated lime and quicklime — properties, bulk density, bulk transport

Definition

Hydrated lime (slaked lime, calcium hydroxide, Ca(OH)2) is a fine, white powder produced by the reaction of quicklime CaO with water. Quicklime, in turn, is calcium oxide CaO, the product of firing limestone at a temperature of around 900–1100 °C. Both forms are strongly alkaline, hygroscopic and — most importantly from a logistics standpoint — exceptionally light and dusty, which completely changes the way they are transported in bulk compared with polymer granulates.

Lime is a special case among dusty cargoes: a material in which physics — low density and chemical reactivity — dictates the entire organisation of transport. This article organises the practical knowledge about lime: the difference between the quicklime and slaked forms, the standard, the bulk density and its consequences for the trailer, the requirement for tightness and occupational safety.

Quicklime (CaO) versus hydrated lime (Ca(OH)2)

The technological chain of lime is simple and worth knowing, because it explains everything else.

Limestone (calcium carbonate, CaCO3) is fired in a kiln. Carbon dioxide is released, leaving quicklime CaO — calcium oxide, a reactive material “hungry” for water. When CaO comes into contact with water, it reacts violently, releasing considerable amounts of heat (an exothermic reaction) and turns into hydrated lime Ca(OH)2 — calcium hydroxide, that is slaked lime.

From the carrier’s perspective the differences are crucial:

A third form worth mentioning is lime milk — a suspension of Ca(OH)2 in water. This, however, is not carried by a powder silo trailer but by a liquid tanker, so it falls outside the scope of bulk-material transloading.

It is worth understanding that commercial naming can be misleading. “Lime” in everyday usage sometimes means the quicklime form, sometimes the slaked one, and sometimes even calcium carbonate in meal form. From a logistics perspective the decisive question is: does the material react with water violently (CaO) or is it already hydrated (Ca(OH)2), and what are its bulk density and dustiness? These three pieces of information determine the choice of tanker, the discharge parameters and the dryness regime. That is why with every order we demand an unambiguous product designation consistent with the standard and a current safety data sheet — this rules out a mistake in selecting the trailer and preparing the installation.

The PN-EN 459-1 standard

Building lime is classified by the European standard PN-EN 459-1 (“Building lime — Definitions, specifications and conformity criteria”). It divides lime into two main families:

• Calcium lime (CL) — with a high content of calcium oxide/hydroxide, with classes CL 90, CL 80, CL 70 (the number corresponds to the minimum CaO+MgO content in percent).
• Dolomitic lime (DL) — with the addition of magnesium oxide, classes DL 90, DL 85, DL 80.

A form suffix is added to the designation: Q (quicklime), S (slaked/hydrate — hydrated powder) or S PL (lime putty). Hence the typical commercial designation CL 90-S reads as: calcium lime, minimum 90% CaO+MgO, in the form of a hydrated powder. For us, the terminal, the standard designation is important not for chemical but for logistical reasons — different classes have a similar bulk density but a different dustiness and flowability, which affects the pace of loading and discharge.

The standard also regulates the requirements for hydraulic lime (NHL), but this is a material with a different binding characteristic and we leave it aside here.

Bulk density and its consequences for the trailer

This is the most important practical parameter. Hydrated lime has a loose bulk density of around 390–490 kg/m³ — one of the lightest materials carried by silo trailer. For comparison: PE/PP granulate has about 500–600 kg/m³, cement about 1000–1400 kg/m³, and sand far more. Ground quicklime is somewhat heavier than hydrated lime, but still light.

What does this mean in load practice? That the tank volume runs out much earlier than the permissible gross weight of the combination. A powder silo trailer with a capacity in the range of 55–65 m³ filled with hydrated lime will reach its full volume at a load mass that does not use up the tonnage limit. We therefore carry lime “by volume” — it is a classic light, bulky cargo, where the volume, not the weight, is the limiting factor.

The practical conclusion: when planning the transport of lime, we first calculate the tanker capacity and only then check the tonnage. Ignoring this relationship is a common mistake in quoting and planning fleet rotation. The mechanism is identical here to that for other light powders — we described the same “volume vs mass” logic in more detail in the context of bulk transport of materials by silo.

Low bulk density has one more consequence, felt only on the road. A light, dusty load readily fluidises from the vibrations of driving — the particles “float” in the tank. On the one hand this eases the later discharge, on the other it requires disciplined driving and monitoring, so that excessive compression of the material in the cone bottom does not occur. For a driver handling lime the difference compared with heavier cement is perceptible: the tanker is light, but the load “lives”. This is one of the reasons we entrust lime to an experienced crew familiar with the specifics of powders of extremely low density.

Hygroscopicity and reactivity — the requirement for tightness

Hydrated lime is chemically active in two ways at once:

1. It reacts with moisture. It absorbs water from the air, cakes, loses flowability and part of its reactivity. Caked material can block the fluidisation of the tanker bottom and hinder pneumatic discharge.
2. It binds carbon dioxide (CO2). In contact with air, lime slowly carbonates — Ca(OH)2 reacts with CO2 and returns to the form of calcium carbonate CaCO3. This process is irreversible under storage conditions and means a loss of the product’s usefulness.

Hence the iron rule: lime is transported and stored in a sealed and dry system. A powder silo trailer must have functioning seals on its manholes and valves, a checked vent filter and dry air in the pneumatic installation. Every leak is a risk of the load caking en route. At the customer’s end the material goes directly into a closed silo, without contact with the atmosphere — analogously to cement.

For the same reason lime is never suitable for gravity transloading from big-bags without a closed system — its dustiness and reactivity rule out an open transfer. The materials we transfer without pneumatics are granulates with a larger grain; the mechanism of the difference is described in the entry on transloading without pneumatics. Lime belongs to the second category — exclusively a closed, pressure system.

Bulk transport and pneumatic discharge

Lime is carried by powder silo trailers (tankers for powders) — aluminium tanks with a fluidised, conical bottom and a compressed-air installation. In our fleet such materials are served by the powder tankers from our pairs of ~60 m³ silo trailers; the same type handles cement, soda or limestone meal.

The cycle looks as follows:

• Loading — by gravity through the top manhole from the manufacturer’s installation or a silo, while maintaining dryness.
• Transport — in a sealed system, with seal monitoring; because of the low density the load is light, so driving the combination is comfortable, but care must be taken with shifts of the dusty material.
• Pneumatic discharge — compressed air fluidises the tank bottom, the material passes into a state resembling a liquid and is conveyed by pipeline into the customer’s silo. This is the classic pneumatic discharge of a silo trailer, not a gravity transfer.

We provide couplings and connections to the standard found on the dusty-materials market; the choice of fitting is agreed with the installation of the customer’s silo. The discharge time depends on the pressure, the length of the pipeline and the state of the material — caked lime discharges much more slowly, which once again underlines the importance of dry transport.

Applications of lime

Lime is a material with a very broad spectrum of customers, which translates into steady demand for transport:

• Construction — masonry and plastering mortars, soil and road sub-base stabilisation (lime binds moisture in the soil and improves bearing capacity).
• Road construction / asphalt — hydrated lime as a filler in mineral-asphalt mixes improves the adhesion of the binder to the aggregate and the resistance of the surface to rutting.
• Water treatment — pH correction, water softening, precipitation of contaminants.
• Power generation — flue gas desulphurisation (semi-dry method) — lime binds sulphur dioxide, reducing SO2 emissions from boilers.
• Environmental protection — neutralisation of acidic wastewater, hygienisation of sewage sludge.
• Agriculture — soil liming, pH regulation, improvement of soil structure.

This diversity means that lime customers range from cement works and bituminous-mix producers to municipal plants and power generation — each with a different type of silo and different requirements as to the cleanliness of the delivery.

Occupational safety — alkaline dust

Lime is a strongly alkaline (basic) material. Its dust irritates the respiratory tract, eyes and skin, and on contact with damp skin can cause irritation and chemical burns. Quicklime CaO is particularly aggressive in this respect because of its reaction with the water contained in tissues.

During loading, discharge and tanker cleaning, protective measures therefore apply: a dust mask of an appropriate class, safety glasses or goggles, gloves resistant to alkalis and clothing covering the skin. We run loading and discharge stations so as to limit dusting — sealed connections and extraction. The specific hazard and first-aid data are always given by the manufacturer’s safety data sheet (SDS), and that is the overriding document.

Tanker cleaning after lime

After carrying lime, the tanker is cleaned dry — by blowing through with compressed air and mechanically removing the dust deposit. Washing with water is risky: lime residue in contact with water forms a caked, hard-to-remove deposit that can block valves and the bottom fluidisation. Only after verifying that the tank is dry and free of residue is the tanker suitable for the next load.

Cleanliness control is especially important if a material with high purity requirements is to follow the lime — then we document the state of the tank and issue a cleanliness certificate. In practice we keep lime in tankers dedicated to mineral and dusty materials, separated from the fleet handling polymer granulates, so as to rule out cross-contamination between product groups. We described the principles of trailer selection and cleaning in more detail in the context of silo trailers.

Related topics

Lime fits into the wider group of mineral materials transported in bulk. If you are interested in how we carry similar powders and how we organise the terminal:

• Cement — the closest “cousin” of lime in dusty transport.
• Calcium carbonate — the raw material for lime production.
• Silo trailers — tankers for bulk and dusty materials.
• Silo transport — the logistics of carrying materials in bulk.

We handle the transport and transloading of bulk materials at the SMIALA terminal in Chorula by the A4. For details, see the services of big-bag to silo trailer transloading and storing bulk materials. You will find the network’s full transport offer at magnumchorula.pl/transport/.

Sources

• PN-EN 459-1 “Building lime — Part 1: Definitions, specifications and conformity criteria”.
• Safety data sheets (SDS) from manufacturers of quicklime and hydrated lime.
• Practice of the SMIALA bulk-materials terminal, Chorula — over 30 years of experience in bulk transloading and transport.
• Expert commentary: Aleksy Pasternak — a terminal practitioner, co-owner of PHS Magnum.