Water For Mixing Concrete

Water for Mixing Concrete Almost any natural water that is drinkable and has no pronounced taste or odor can be used as mixing water for making concrete. Some water which may not be suitable for drinking may still be safe for mixing concrete. Pipe born drinking water supplies are generally safe for making concrete. Water of doubtful quality can be simply tested by making two sets of cubes or cylinders of the same mix, one with the doubtful water, and the other (used as a reference) set with distilled water, purified water, tap water, or other drinkable water of good quality. If the suspected water produces concrete of 28 day compressive strengths at least 90% of the strength of the companion (reference) set, it can be considered suitable. Salt or brackish water can cause dampness of the concrete, efflorescence (white deposits of precipitated salts on the surface of the concrete), increased risk of corrosion (rust) damage to embedded reinforcement, and damage to paint systems. It is therefore advisable not to use such water for durable concrete work, and its use is generally avoided. (Concrete which has to be placed permanently under sea water would obviously not require such precautions). Typical limits of chemicals allowed in mixing water for concrete are specified in ASTM C94 92*: which provides a useful guide as to allowances that have worked in practice.

Some General Precautions 1. 2. 3.

4.

Avoid using wastewater from tanneries, from industrial, chemical and metals related plants (e.g. galvanizing plants, battery making plants); as some salts of manganese, tin, zinc, copper and lead can cause significant reductions in strength and large variations in the setting time of concrete. Avoid waters from abattoirs, chicken processing plants, etc. impurities in such waters may have severe effects on concrete. Avoid waters from swamps, marshes etc., which may contain organic impurities in amounts sufficient to interfere with the setting and hardening reactions of the cement. Avoid water containing algae; algae can cause excessive reductions in strength by influencing cement hydration or by causing a large amount of air to be entrained in the concrete mix. Algae may also be present on aggregate surfaces, in which case they reduce the bond between the aggregate particles and the cement paste and so will reduce concrete strengths. 5. Sugar. This is to be avoided at all costs *ASTM C94 92 "Standard Specification for Ready Mixed Concrete". 99 de Relatively small amounts of sugar can cause problems of non-setting and non hardening of concrete in quantities as low as 0.25% of the mass of the cement.

The Proportioning of the Concrete Mix The amount of the various ingredients (aggregate(s), cement, water, admixtures and/or additives) in the concrete, and not simply the fact that they are present, is obviously important. The activity of deciding the mix proportions is called mix design, and on large jobs is usually the responsibility of materials engineers or technicians, or is left to ready mixed concrete companies who sell concrete to such projects. (On smaller jobs, such as house building, the building or small walk ways, drains, aprons, etc., concrete is often proportioned by volume measurements). The main purpose of mix design is to decide on a set of proportions which will enable a workable practical concrete to be made, having sufficient strength and durability. It involves choosing a suitable water cement ratio, and a proportioning of aggregate which will enable all the stone particles to be coated with sufficient water / cement paste and be able to be packed into a solid mass, relatively free from air spaces (voids). The following equations summarize what takes place:

1. 2.

3.

Water + Cement = cement paste (the "glue" of concrete) Cement paste + fine aggregate (sand) = mortar Mortar + coarse aggregate (gravel or crushed rock) = concrete A practical concrete mix must have enough "mortar" to fill in the spaces between the large aggregate particles when they are packed together, plus a little "spare" mortar for narrow spaces, corners, and such like.

Making Concrete and Using it in Construction (The basics of concrete technology and practice) Before discussing some basic Do‘s and Don‘ts of good concrete practice, it is useful to have a general definition of concrete. ("Concrete" as discussed here is taken to mean Portland cement concrete, as distinct from asphaltic or biluminous concrete, which is mostly used for road pavements, Portland cement concrete is also suitable for making roads, airport runways, and such the like).

Concrete may be seen as a man made rock, similar to so called conglomerates (cemented gravels) which occur in nature. it is made up of a system of rock particles or fragments of different sizes (the so called "aggregate") which are packed together, and fastened (cemented) by a hardened paste of a hydraulic cement (usually Portland cement with or without other materials) and water. In one form or the other, cementitious concrete is the material most used in structural (i.e. Load bearing) applications in construction, sometimes competing with steel, sometimes working together with it, in so called reinforced concrete is known for its Compressive Strength (which is much better than its tensile strength) and for its Durability (some existing Roman concretes are nearly 2,000 years old and are still in good condition) and for its ability to be molded into nearly any desired shape, on or off the construction site. This latter quality makes it adaptable to an extremely wide range of construction applications.

The effects of materials and processes on concrete The main factors which determine the character and performance of concrete are: 1. 2.

The properties of the materials which are combined to make it; The proportions in which these materials are mixed together, and;

3.

The effect of any processes which are applied to the mixture so produced.

These points will now be briefly examined: The final properties of hardened concrete are greatly affected by its behaviour in the "wet‘ or freshly mixed condition and by the processes it undergoes while in that state; it is therefore essential that the making and handling of concrete be guided by a proper understanding of this behaviour, if its full potential is to be realized in practice. The properties and performance of concrete can be affected by any or all of the following *

Choice of Ingredients: The choice of materials greatly influences the type and character of the concrete which is produced; some of the main considerations are the following: The Type(s) and Origin(s) of the aggregate(s) selected, this affects the strength, durability and reactivity of the aggregates, which typically make up between 65% and 75% of the volume of an "average" concrete, and so exert a considerable influence on the concrete in both the freshly mixed and the hardened states. The Grading Of The Aggregates (this is a measure of how much of each of the different sizes of stone particles exists in the particular aggregates, and depends on the aggregates chosen and in what amounts they are mixed). Grading affects how much water and cement ("cement paste") is required to cover all the stone particles and enable a practical concrete to be made. The Prevailing Shape And Texture Of The Aggregate Particles. This is usually a result of two things: (a) the type of (so called ‘parent") stone or rock from which the aggregate was removed; and (b) The natural as well as man made processes to which these stone fragments were subjected. These factors considerably affect how the aggregate will behave while the fresh concrete is being handled and molded. The Porosity of the Aggregate depends largely on its type and origin, and can greatly affect its strength and durability; depending on its state of dryness, it can also greatly affect the so called "workability" (the ease or difficulty with which the concrete can be ‘worked" or molded into various shapes) of the fresh concrete. The Type, Amount and Fineness of the Cement chosen. Cement must not be confused with concrete; cement is the (usually) grey powder which when mixed with water hardens into a stone like mass. It is the chemically active and an essential binding agent for the making of concrete.

The manufacture of Portland Cement 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.

Limestone

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 reardump 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 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 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.

Wet Process 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.

Dry 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. Quality Control 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.