Cement
| Infobox on Cement | |
|---|---|
| Example of Cement |  |
| Facts | |
| Origin | - |
| Stowage factor (in m3/t) |
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| Humidity / moisture | See text |
| Ventilation | - |
| Risk factors | See text |
Cement
Description
Cement is a grey dusty cargo.
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together.
Cement used in construction is characterized as hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g., lime and gypsum plaster) must be kept dry in order to retain their strength.
The most important use of cement is the production of mortar and concrete—the bonding of natural or artificial aggregates to form a strong building material that is durable in the face of normal environmental effects.
Cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement', the most commonly used type of cement (often referred to as OPC).
Portland cement is a basic ingredient of concrete, mortar and most non-specialty grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white.
Concrete should not be confused with cement because the term cement refers to the material used to bind the aggregate materials of concrete. Concrete is a combination of a cement and aggregate.
Cement sets or cures when mixed with water which causes a series of hydration chemical reactions. The constituents slowly hydrate and crystallize; the interlocking of the crystals gives cement its strength. Maintaining a high moisture content in cement during curing increases both the speed of curing, and its final strength. Gypsum is often added to Portland cement to prevent early hardening or "flash setting", allowing a longer working time. The time it takes for cement to cure varies depending on the mixture and environmental conditions; initial hardening can occur in as little as twenty minutes, while full cure can take over a month. Cement typically cures to the extent that it can be put into service within 24 hours to a week.
Shipment / Storage / Risk factors
Check temperature before loading as the product may have recently been in a kiln.
Cement is shipped in bulk or, alternatively, in 50 kg paper sacks or one to two tonne polypropylene bags. Bagged cement for export is packed in multi-ply bags with up to five layers, one of which can be made from a damp-proof or water-proof material. Kraft paper sacks have a gummed seal which effectively guarantees water tightness. Cement deteriorates with age because of absorption of moisture and/or carbon dioxide from the air. The deterioration may not affect the appearance of the cement, but can, over a long period of time, significantly reduce its performance. Sugar should never be stored over cement as sugar contamination seriously affects the setting and hardening performance.
Cement Packing Bags is a kind of flexible transport container, also known as Cement Big Bags or Cement Sling Bags. PP and PE is its main material. It applies to ship bagged cement for long distance or ocean transport. The main specification for short distance transport is 1 Ton, 1.5 Ton and 2 Ton, FIBC Bags for export ocean transport. Waterproof, dust-proof and , radiation-resistant bags are the ideal products for cement, clinker and other industries with characteristics such as 5-5.7 times safety factor, high strength structure, easy handling and operation,etc.
Bulk cement in specially designed vessels will present few problems provided that holds are initially dry and properly closed and condensation does not occur. If moisture reaches the cement, a hard crust or lumps will result and must be discarded. Over a normal voyage period the powder will be perfectly usable even if a crust has formed. Stowage factor for bulk cement is 0,61-0,64 m3/t.
Bagged cement when stowed should be protected from water or damp and preferably stowed on a flat surface to prevent splitting and breakages. The use of pallets and shrink wrapping helps in this respect. If bags have been badly wetted by the ingress of sea water or by heavy rain, hard lumps may develop, usually at the corners of the bags. The powdered cement not affected by water may be sound and usable.
Softer lumps formed without contact with water (airset cement) or cement compacted by its own weight when stacked high or on an exceptionally long shipping period (packset cement) are usable provided that the lumps are not too hard. The normal test involves crushing between the finger and thumb. The powder from split bags should be carefully swept up, excluding paper dunnage or extraneous matter, and rebagged. Extra empty bags are usually provided for the purpose. Rebagged cement is usable for all but the most critical work. Cement clinker is the semi-manufactured material, usually very hard grey granules between 1 and 20 mm in diameter. It is substantially unaffected by water but should be kept dry to avoid caking. Contamination with sea water will increase the chloride content of the clinker and the cement produced from it.
Bags of cement routinely have health and safety warnings printed on them because not only is cement highly alkaline, but the setting process is exothermic. As a result, wet cement is strongly caustic and can easily cause severe skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation. Cement users should wear protective clothing.
For the most part large parcels of cement are carried in bulk with suitable mechanical loading and discharging facilities. When pumped into vessel through hoses, cement flows like a liquid whilst in motion, and it is important that the ship remains upright at all times. On completion cargo should be reasonably level. A period of time should be allowed for the cement to settle and any air to escape. Once settled cement is not liable to shift and shifting boards, or bagging, is not considered necessary. Discharge normally by elevator or grab.
Should sugar have been recently carried in a compartment into which cement is to be loaded, the cleaning of floors, brackets, stringers, etc., should be thorough, and an officer should inspect every part of the compartment before loading commences. Sugar to the extent of only .001 per cent mixing with cement renders the cement worthless as a binding mixture. Sugar should never be stowed on top or above, cement, neither is cement to be stowed with or near Ammonia or its Sulphate as their fumes or gas alters the character of the cement to a quick setting cement.
It not infrequently happens, when cement is loaded direct into ship alongside factory's premises that, the cement, having but recently passed through the kilns, is shipped at a high temperature, cases being known where the temperature varied between 160 and 165 degrees C. Exception to such a condition need not be taken when loading a full cargo of cement, but, if only parcels of cement are received, it becomes necessary to avoid stowing any other cargo in the same compartment or in one connecting with same which will be affected by the high hold temperature thus artificially produced. Before receiving cement the holds should be well swept (and the limber boards made dust tight) to enable cement siftage, of which usually there is a great deal, to be recovered as clean as possible.