Origin of Portland Cement
Although the history of Portland Cement is comparatively short, the use of the structural binding agent dates back thousands of years. One of the earliest examples is that of the water tanks at Aden which were constructed 6000 B.C. and are still in use today. Analysis of the mortar used by the Egyptians in the construction of the “Pyramid of Cheops” in about 3600 B.C. show that they possessed a good practical knowledge of the subject at the time.
The Greeks, at a very early period of their civilization, used compositions of lime as a base to cover walls. According to Plinius, the walls of the palace of Croesus were also protected and ornamented in this manner.
In Italy, the first people to employ mortar in their buildings were the Etruscans. It was from them that the Romans derived their knowledge of the art. It says much for their ability as builders that the dome of the Pantheon of Rome, constructed with a type of concrete, is still in an excellent state of preservation today.
With the fall of the Roman Empire, all knowledge of cement seemed to have vanished and nothing more was heard of it until the early part of the eighteenth century when its use could be traced to England, in almost exactly the same form as that used by the Egyptians and Romans!
During the first part of the eighteenth century very little progress was made with the evolution of cement. In 1756, an Engineer named John Smeaton gave serious attention to the principle of setting lime under water.
After considerable research, Smeaton found that by mixing lime and pozzolana, a substance was produced which, became hard and solid, with the addition of water. Although this was not Portland Cement, it was a vast improvement over lime mortars. The fact that Smeaton’s lighthouse, which he built with his own product, stood for 120 years on the Eddystone Rock is evidence of his success.
Very little notice was taken of Smeaton’s discovery at the time but 50 years later, the French Chemist, Vicat, went a step further by burning pulverized chalk and clay together in the form of a paste. His product, like Smeaton’s received very little attention. They even carefully picked out and discarded the very portion of the burnt material which would have given them Portland Cement! It was not until 1824, that Joseph Aspdin, a Leeds bricklayer, discovered what is known as Portland Cement.
Aspdin found that by mixing finely pulverized clay in specified proportions, burning them to a high temperature and then grinding the resultant clinker, he was able to produce a hydraulic binding material far superior to any product known at the time. It was Aspdin who named it “Portland Cement” because when set it looked like “Portland Stone.”
Present-day Method of Manufacture
Since Aspdin’s discovery, the production of cement has increased immensely and much improvement in the standards of quality. Today, the manufacture cement differs greatly from that of early cements. “Portland Cement” as we know it, is an active combination of silicates, aluminates, and ferro aluminates of lime, obtained by grinding and mixing lime, silica, iron-oxide and alumina, burning the mixture to incipient vitrification, and then grinding the resultant clinker to a fine powder with the addition of a small percentage of gypsum in order to control or regulate the setting time.
The Caribbean Cement Company
For nearly a quarter of a century before the year 1947 (the date of incorporation of Caribbean Cement Company), several groups of the investors had investigated the feasibility of manufacturing cement in Jamaica but no one had come forward to build a factory until 1950.
In 1944?, Jamaica used 54,000 tons of cement, and it appeared that for a long time in the future Jamaica was destined to import all its cement requirements from England.
Then in 1949 a group of men, among them Sir William Stephenson and Sir Neville Ashenheim, established the Caribbean Cement Company Limited and secured from the Government a licence under the Cement Industry (Encouragement and Control) Law which conferred on the company the exclusive right to manufacture and sell ordinary Portland Cement in Jamaica. Cement was manufactured in Jamaica for the first time on January 23,1952, in a plant with a rated capacity of 100,000 tons. This satisfied Jamaica’s needs at the time, and allowed the company to even compete in the export market.
In 1964, with the completion of the second expansion, the plant’s rated capacity was 400,000 tons per annum.
In 1988, there was a third expansion which took place with the introduction of a dry process kiln and a Cement Mill with a rated capacity of 120 tons per hour. The capacity of the company was increased to 600,000 tons per year. Cement production increased to 435,124 tons in 1990.
Manufacture of Portland Cement
A calcareous and an argillaceous material in the approximate proportion of 80% of the former and 20% of the latter are ground together to produce a slurry or a raw meal. The slurry or raw meal is then homogenized and burnt in rotary kilns at a very high temperature. The resultant clinker with an addition of about 5% of gypsum are ground together to a specific fineness to produce Portland Cement.
Basic Raw Materials
The raw materials used by Caribbean Cement Company for the manufacture of Portland Cement are limestone, shale and gypsum. All of which are in great abundance in Jamaica.
The limestone used in the manufacture of cement contains between 85% and 95% calcium carbonate, and small quantities of magnesium carbonate, silica, alumina and iron. At Carib Cement, it is obtained from a quarry situated close to the plant. The quarrying is done by ripping and blasting and the limestone is transported to the crushing plant by rear- dump trucks. Here the limestone is dumped onto a wobbler feeder which separates the fine particles and feeds the coarse rock into a hammer mill, where the material is reduced to minute 11/2” size particles. The crushed material together with the fine particles from the wobbler is transferred to the limestone storage silos or to the limestone stock-pile.
Shale is crushed in the same way as the limestone. At the crushing stage 2% of red mud can be added to enrich the iron content of the raw mix.
Gypsum is extracted from deposits in the Eastern parishes of the Island, mainly St. Thomas. These mines are owned by Caribbean Cement Company’s subsidiary Jamaica Gypsum and Quarries and it provides all the gypsum for the manufacture of cement in Jamaica and exports to several countries regionally.
Production, Chemical Adjustment and Storage of Slurry
The partly crushed limestone and shale are simultaneously introduced in regulated quantities into the slurry grinding mills. Water is added to maintain the mixture in a pumpable density. These mills reduce the raw materials to a thick slurry, which is then pumped into the storage silos for homogenization, chemical analysis and blending.
Production of Clinker
The calcination of the slurry is one of the most important phases of the entire cement manufacturing process and takes place in rotary kilns that transform the raw materials into clinker. The kilns are the largest and heaviest pieces of moving machinery in the factory.
The kilns are set above ground, sloping gradually down from the feeding end to the firing zone and revolving slowly at a rate of 1 to 1.33 revolutions per minute. The interior of the kiln is lined with special types of heat resistant bricks. The heat required in the burning zone is between 1350 degrees and 1500 degrees c and the flame generated by the burning of fuel oil or coal. In the wet kiln process the slurry is fed to the kiln by being pumped to a feed controller which is regulated to give a constant and correct feed. It then passes through a series of chains loosely suspended in the interior of the kiln in order to provide a large heat exchange with the material entering the kiln. After the material exits the kiln, the burnt material is then referred to as clinker. The clinker is partially cooled by blowing air through the hot bed of the clinker as it leaves the kiln via a cooler The air, contains clinker particles, which would normally be expelled through the smoke stack is collected by electrostatic precipitators and a silicone treated fiberglass bag dust collector. The dust collected falls into dust hoppers and is reused in the process.
The dry process is different from the wet process in that it involves a pneumatic transport medium instead of the water. In the dry process, the limestone and shale/red mud are crushed in a Roller Mill.
(The basic principle of operation for a Roller Mill involves feed entering the mill on a rotating table where the raw material is crushed as it passes under the heavy rollers which are on a fixed shaft.)
An air current is passed through the mill which takes the crushed raw mix into a separator where a centrifugal force is in operation. The coarse particles are forced out of the air current and fall back on the rotating table to be recrushed. The finer particles flow out with the air current.
The raw mix is then passed into a blending silo to allow homogeneity of the raw mix before it passed into raw meal storage silo.
The transformation of raw meal into clinker is carried out by a dry process kiln. Attached to this kiln is a four stage cyclone pre-heater which is used to preheat the raw meal before entry to the kiln.
As with the wet process kiln, the interior is lined with special types of heat resistant bricks and the material is burnt at approximately the same temperature. This flame is also generated by burning either coal or fuel oil through a special burner.
The raw meal is pumped to the kiln via a weighfeeder for constant measurement and a series of chemical and physical changes take place until the material leaves the kiln as red hot clinker. The clinker is cooled by passing air through the clinker in a clinker cooler.
Production of Cement
Clinker and gypsum are ground together in ball mills similar to the slurry grinding mills with the exception now that the operation is now dry grinding. These mills have three compartments and carry charges of assorted sizes of steel grinding media (balls) weighing about 100 tons for the smaller chamber and 600 tons in the larger chamber. Gypsum is added in order to regulate the setting time of the cement which would otherwise set almost immediately when mixed with water. The cement is ground to a fineness of not less than 3000 square cm. Per gram as determined by measurement of the specific surface.
Storage and Packaging
The cement from the mills is pumped by pneumatic conveyors into storage silos, ready for shipment in bags or in bulk. Bagging is done by automatic packers which in automatically fill each bag of cement to its 42.5 kg weight limit. The plant can deliver in bulk form by means of a bulk loading system to special trucks and to ships for export.
Routine checks and tests are carried out by Laboratory Technicians to each phase of the production process so as to ensure that a consistent high quality is maintained.
Cement quality is monitored by the Jamaica Bureau of Standards to ascertain that the cement conforms to the Jamaican Standard (JS32 1974).
In addition, the cement at C.C.C is manufactured to conform to the ASTM standard for Portland type I cement. Exported cement is tested in order to comply with the ASTM standard.