|COMPERATIVE ANALYSIS OF DIFFERENT PORTLAND CEMENT
(A CASE STUDY OF SOKOTO, ASHAKA AND ELEPHANT CEMENT)
Cement is a finely ground inorganic material which when mixed with water forms a paste which sets hardens by means of hydration reactions and processes in which after hardening retains its strength and stability even under water (Mehta and Montero 1993).
Also Cement is a substance applied to the surface of solid bodies to make them cohere firmly’ or, more specifically, a powdered substance which, made plastic with water, is used in a soft and pasty state (which hardens on drying) to bind together bricks, stones, etc in building (lea 1970). The raw material used in the manufacture of ordinary Portland cement is limestone. Limestone is composed of calcium carbonates and mechanically admired clay and sand.(Moriki 2017).
TYPES OF PORTLAND CEMENT
Portland cement is a closely controlled chemical combination of Calcium, Silicon, Aluminum, iron and small amounts of other compounds, to which Gypsum is added in the final grinding process to regulate the setting time of the concrete. Some of the raw materials used to manufacture cement are limestone, shells, and chalk or marl, combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore. Lime and silica make up approximately 85 percent of the mass. (Mehta and Montero 1993).
TYPE I (Ordinary Portland cement) is a general purpose Portland cement suitable for all uses where the special properties of other types are not required. It has fairly high C3S content for good early strength development. It is used where cement or concrete is not subject to specific exposures, such as Sulfate attack from soil or water, or to an objectionable temperature rise due to heat generated by hydration. Its uses include pavements and sidewalks, reinforced concrete buildings, bridges, railway structures, tanks, reservoirs, culverts, sewers, water pipes and masonry units. (Morike 2017)
TYPE II (Modified cement) Portland cement is used where precaution against moderate Sulfate attack is important, as in drainage structures where sulfate concentrations in ground waters are higher than normal but not unusually severe. Type II cement will usually generate less heat at a slower rate than Type I it has Low C3A content. With this moderate heat of hydration (an optional requirement), Type II cement can be used in structures of considerable mass, such as large piers, heavy abutments, and heavy retaining walls. Its use will reduce temperature rise especially important when the concrete is placed in warm weather. (Moriki 2017).
TYPE III (Rapid hardening cement). Is a high-early strength Portland cement that provides high strengths at an early period, usually a week or less It is used when forms are to be removed as soon as possible, or when the structure must be put into service quickly it has Low content of C3S (<50 %) and C3A. In cold weather, its use permits a reduction in the controlled curing period. Although richer mixtures of Type I cement can be used to gain high early strength, Type III, high early-strength Portland cement, may provide it more satisfactorily and more economically. (Moriki 2017).
TYPE IV (Low heat cement) is a low heat of hydration cement for use where the rate and amount of heat generated must be minimized. It develops strength at a slower rate than Type I cement. Type IV Portland cement is intended for use in massive concrete structures, such as large gravity dams, where the temperature rise resulting from heat generated during curing is a critical factor. (Moriki 2017).
TYPE V (Sulfate-resisting cement) used only in concrete exposed to severe sulfate action principally where soils or ground waters have a high sulfate content (Mehta and Montero 1993).
HISTORY OF CEMENT
Cement has been around for at least 12 Million years when the earth it self undergoing geological changes natural cement was being created. It was this natural cement that human first put to use. Eventually they discovered how to make cement form other materials (Mehta and Montero 1993). In 1200000 BC, reaction between limestone and oil shale during spontaneous combustion occurred in Palestine to form a natural deposit of cement compounds. The deposits were characterized by the geologist in the 1960s and 1970s. In 300BC Egyptians used mud mixed with straw to bind dried bricks. They also used gypsum mortar and mortar of lime in the pyramid. In the same year Chinese used cementations materials to hold the bamboo together in their boats and in the great walls.In 800BC Greeks and Cyprus used lime mortars which were much harder than later Roman mortars. In 1200 BC the quality of cementing materials is deteriorated. (Moriki 2017).
In 1838, young chemical engineer, Isaac Johnson, burned the raw cement material at a high temperature until the mass was nearly vitrified .In 1840 Joseph aspirins son William aspirin bottle kiln plant in north fleet England.
In 1890 the addition of gypsum when grinding clinker to act as retardant to the setting was introduced in (U S A) In the same year ball mills were used for grinding Portland cement.
In 1960 XRF for chemical analysis was introduced.
In 1970 fiber reinforcement was introduced in concrete. In 1980 high efficient separator was introduced for cement grinding also pre calciner process was developed (Alkourd and hammad 2009).
HYDRATION OF CEMENT
Cement derives it is strength from chemical reactions between cement and water the process known as hydration. Cement grains get into contact with water and may begins to form a strong network structure which is responsible for strength when Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical reaction with water is called hydration (Mehta and Montero 1993). Each one of these reactions occurs at a different time and rate. Together, the results of these reactions determine how Portland cement hardens and gains strength
• Hydration starts as soon as the cement and water are mixed.
• The rate of hydration and the heat liberated by the reaction of each compound is different.
• Each compound produces different products when it hydrates.
Cement Consists Of The Following Major Compounds
• Tri-Calcium Silicate (C3S). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of C3S will exhibit higher early strength. Tri-calcium aluminates (C3A). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately and contributes somewhat to early strength. Gypsum is added to Portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause Portland cement to set almost immediately after adding water.
• Dicalcium Silicate (C2S). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.
• Tetra Calcium Aluminoferrite (C4AF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to C4AF. (Mehta And Montero 1993).
Stages of cement hydration reactions; all, five distinct stages have been identified:
Stage 1: Pre-induction (initial stage)
Stage 2: “Dormant” (induction) period
Stage 3: Acceleration
Stage 4: Deceleration
Stage 5: Steady state
The particle size distribution of the Portland lime-stone cement, as well as the fineness of clinker and limestone, is strongly connected with the limestone content and the fineness of the cements.
The limestone cement indicates satisfactory strength and generally demands less water then the relative pure cements.
The limestone addition improves the clinker reactivity and the exploitation of its hydraulic potential.
The Portland limestone indicates competitive concrete properties and improves the durability of the concrete. (Mehta And Montero 1993).
Lime stone is an important factor in the hydration of C3A, as well as C3s and β-C2S in the presence of CaSO4 and lime.
Limestone fills the pores between the cement particles due to the formation of carbonaluminate phase.
Limestone addition leads to an increase of lime librated from cement pastes.
The replacement of OPC by 20 wt % of Homra increases the compressive strength. (Heikal,El-didamony,morsy).
AIM AND OBJECTIVES OF THE RESEACH
The aim of this research is to compare the physical, chemical strength and hardened of Sokoto, Ashaka, and Elephant cement.
The objectives are as follows:
(1). To determine the comparatives strength between cement the three different portland cement product.
(2). To compeer the one with higher quality then the other one’s
Properties of Portland cement
Strength is the most important of all properties of cement.
The strength of cement is measured on mortar specimens made of cement and standard sand (silica)
Standard sand is used for finding the strength of cement. Standard sand should be pure free from any organic matter, it should have at least 98 % silica and 0.2 % moisture, free from silt content, shall be light, grey or white in color, it shall be 100 % retained on 90 macron sieve and shall be of quartz.(Moriki 2017).
Aim – Determination of compressive strength of a given cement sample.
Apparatus – Automatic mixer jolting machine prism mould scrapper glass plate mixing bowl spoon measuring cylinder weighing balance and compressive strength testing machine.
– weigh 1350 +- 5g of the standard sand
– weigh 450 +- 2g of the cement sample
– weigh 225 +- 1g of clean water
– mount the prism mould on jolting apparatus and screw it.
– mixed the above weighed proportions (sand cement and water) on mixing bowl using an automatic mixer
Transfer the half of mixture into three compartments of the prism mould And jolt for one minute (this is done to reduce the air bubbles on the prism cubes)
Transfer the remaining mixture in to the compartments and jolt again for one minute.
Level the mixture on the prism mould with a trowel.
Cover the specimen with glass plate and cure in 24H curing chamber for 24 hours.
Demould and cure in water at different periods that is 2 days, 7 days and 28 days ( this is done to allow the hydration to continue and to observe the development of strength gain of the mortar cubes over a time) (Habib And Vikingson 2003).
LOW STRENGTH OF MORTAR CUBES IS CAUSED BY
incorrect mix, most commonly high water cement ratio
incorrect sampling or molding
According to standard the strength for 2.7 and 28 days should not be less than 10N/MM2, 16.0N/MM2 and 42.5N/MM2 respectively. (BS 1998)
In actual construction dealing with cement, mortar or concrete, certain time is required for mixing, transporting and placing. During this time cement paste, mortar, or concrete should be in plastic condition (BS 1998).
Clinker minerals react very fast after Mixing with water in order to avoid early reaction gypsum (CaSO4.2H2O) was introduced in 1890 for the purpose of controlling the early reaction of cement (Al kourd and hammad 2009).
Setting time is the time required for stiffening of cement paste to a defined consistency (BS 1998).
Consistency is defined as the percentage of water by weight of cement which produces a consistency which will allow a v cat plunger to penetrate the test block up to a depth of 5 to 7 mm. If the penetration is below 5 mm it shows that the water is too much and when the penetration is above 7mm it shows that the water is not enough. There for 5 to 7mm penetration Is necessary to obtained a normal consistency of standard cement paste.
Consistency = quantity of water added to the cement/ weight of cement 100. Example : 0…equation( 2.1) (BS 1998).
Aim – determination of setting time of cement (initial and final)
weigh 400 g of cement sample in a mixing bowl
measure clean water using measuring cylinder
add water and mix ( take the time and note that the entire process of mixing and filling up the mould should not exceed 4 minutes)
transfer the content into vicat mould
Place the plunger mounted to the vicat apparatus and allows the plunger to penetrate the paste freely.
take the plunger reading when it stops (note that the plunger reading must be between 5 to 7 mm. Below 5 mm it indicates that the water content is too much so reduce the water content and above 7 mm indicates that the water is not enough so increase the water content. If the 5 to 7 mm is achieved the paste is called standard cement paste. You should now record your water consumption which can now be expressed to percentage. The percentage of water consumed by the paste to obtain the standard cement paste is called consistency (Habib And Vikingson 2003).
DETERMINATION OF INITIAL SETTING TIME:
Place the standard cement paste under the initial setting time needle attached to the vicat apparatus and lower the needle until it comes in contact with the surface of the cement paste and gently release to allow it to penetrate freely into the cement paste and observe the penetration.
Repeat this test at appropriate time until the needle fails to pierce the test block to a depth of 5 to 7 mm from the bottom of the mould and note the time.
Initial setting time will then be equal to 6:00 AM minus 5:00 AM which is equal to 60 minutes.
5:00 AM refers to the time when the water was first added to the cement
6:00 AM refers to the time when 5 to 7 mm penetration was observed (BS 1998).
Initial setting time is the period elapsing between the time when the water is added to the cement and the time at which a needle of 1mm square section fails to pierce the test block to a depth of 5 to 7 mm from the bottom of the mould. According to standard the initial setting time should be more than one hour.
DETERMINATION OF FINAL SETTING TIME
Final setting time is determined on the similar cement paste on the vicat mould prepared for the determination of initial setting time.
In this test 1mm square section needle (initial setting time needle) is replaced by a needle with annular attachment (final setting time needle).
After the elapse of initial setting time, observations with annular attachment are made at appropriate time interval until the needle make an impression on the surface of the cement paste while annular attachment fails to make it and note your time.(BS 1998)
Final setting time will then be equal to the difference between the time the moment water is added to the cement and the time at which annular attachment fails to make an impression on the surface of the cement paste.
8:00 AM minus 5:00 AM which is equal to 3 hours
5:00 AM refers to the time when water is added to the cement
8:00 AM refers to the time when the annular attachment fails to make an impression on the surface of the cement paste (BS 1998).
Final setting time – is the period elapsing between the time when the water is added to the cement and the time at which the needle makes an impression on the surface of the cement paste while annular attachment fails to make it.
According to standard the final setting time of cement should not be more than ten hours.
CAUSE – – Drying of gypsum: when hydrous gypsum (CaSO4.2H2O) inter ground with hot clinker above 100 degree it loss 75 % of its water and formed gypsum hemihydrates (CaSO4.1/2H2O) and if temperature of clinker increases gypsum will lose all water in its composition and formed gypsum anhydrite (CaSO4).
CaSO4.2H2O—-> CaSO4.1/2 H2O) —–> CaSO4(Moriki 2017).
Any structure has to have longer life that is it has to be durable, durability depends on how sound is the material used may be cement sand aggregate. As for cement is concerned the soundness depends on its ingredient especially excess of unborn lime, magnesia and gypsum present in cement cause unsoundness. These materials expand in the structure and thus the concrete or mortar also expands causing disintegration. (Moriki 2017)
The hydrating speed of the over burnt CaO or MgO is slow, CaO or MgO begins to hydrate after cement hardening and causes the hardened cement to expand and crack. When added too much, gypsum continues to react with calcium aluminates hydrate to form calcium sulfoaluminate hydrate, whose volume increase 1.5 times as big as gypsum and causes the hardened cement paste to crack. At this time, sulfoaluminate hydrate is called cement bacillus (Michael and John 1999).
Causes of expansion
1- FREE LIME CaO If the raw materials fed into the kiln contain more lime that can combine with the acidic oxides, or if burning or cooling are unsatisfactory, the excess lime will remain in a free condition. This hard-burnt lime hydrates only very slowly and, because slaked lime occupies a larger volume than the original free calcium oxide, expansion takes place. Cements which exhibit this expansion are described as unsound. CaO + H2O → Ca(OH)2
2- Free MgO Cement can also be unsound due to the presence of MgO, which reacts with water in a manner similar to CaO. However, only Pericles, that is, ‘dead-burnt’ crystalline MgO, is deleteriously reactive, and MgO present in glass is harmless, because it hydrates quickly transforming to the stable state in the hardened paste. MgO + H2O → Mg(OH)2 Up to about 2 per cent of Pericles, (by mass of cement) combines with the main cement compounds, but excess Pericles generally causes expansion and can lead to slow disruption.
3- Calcium sulfates (gypsum) Gypsum added to the clinker during its grinding in order to prevent flash set, but if gypsum is present in excess of the amount that can react with C3A during setting (Michael and John 1999).
Aim – determination of expansion of cement
Equipments – lichatelier apparatus measuring cylinder mixing bowl weighing balance spoon glass sheets water bath and ruler.
Apparatus – lechatelier apparatus, glass plates, weighing balance, measuring cylinder, water bathe and ruler.
Weigh 200 g of cement sample
Measure water using measuring cylinder
Mix to obtain standard cement paste
Transfer the paste into lechatelier apparatus and cover with a glass sheets
Measure the distance between the two tails of the lechatelier apparatus
Record your reading as L1
Submerge the whole assembly in water for 24 hours (this is done to allow the slaking of Lime and Magnesia to take place)
Boil for one hour using water bathe (so that any tendency to expand is speeded up or is accelerated)
Cool and measure the distance between the two tails
Record your reading as L2
Expansion = L2 – L1 (mm)……. (2.2)
(Habib And Vikingson 2003).
According to standard the expansion of cement should not be more than 10 mm.
The last steps in the manufacture of cement are the grinding of clinker mixed with gypsum.
Strength development of concrete is the result of the reactions of water with cement particles. The reactions always start with the cement available at the surface of the particles. Thus larger the surface area available for reaction greater the rate of hydration; Rapid development of strength requires grater degree fineness. However too much fineness is also considerable, Finer cement deteriorates more quickly when exposed to air and is likely to cause more shrinkage, but less prone to bleeding.(Moriki 2017).
LOST ON IGNITION (LOI)
LOI is a test design to measure the amount of impurities or impurities lost when the sample is ignited under a specified condition.
It represents the % weight loss suffered by a sample of cement after heating to 1000 degree. Any water bonded to hydrate, volatile substances will escape above this temperature until it’s mass ceases to change. The test is typically consist of placing a few grams of sample in a pre ignited platinum crucible and determine it’s mass by placing it in a temperature controlled furnace for a set time of 30 minutes.
Scope – this method is used for determination of moisture or impurities in a material
Apparatus – weighing balance spatula platinum crucible and furnace
Ignite an empty crucible for 5 minutes in the furnace (1000 degree) and let it cool down in a desiccators to ambient room temperature.
Weigh the empty crucible
Weigh 4 to 5g of the sample into the crucible
Ignite the sample in a furnace at 1000 degree for 30 minutes and allow it to cool down to ambient temperature inside the desiccators and weigh again
Calculation LOI= W3-W4 ———————–(2.3)
CEMENT CHEMICAL ANALYSIS: X-RAY FLUORESCENCE (XRF)
The basic principle of XRF analysis is simple. (The physics of what happens is complex, but we won’t concern ourselves too much with the details.)
If we zap a sample of cement with a beam of X-rays, the X-ray beam will cause other X-rays to be generated within the cement. Some of these X-rays escape from the cement and are collected by a suitably-positioned X-ray detector. Many of these collected X-rays have energies which are characteristic of the atomic number of the atom in which they were generated. (Moriki 2017)
The specimen of cement or other material may be in the form of a glass bead Beads are made by heating the specimen together with a flux, typically lithium borates, at about 1100 C to form a glass. This approach has the advantage that the specimen is then a homogeneous material, allowing more accurate X-ray analysis.
XRF is at the heart of the control of the production process in any modern cement works and is central to the ability of the cement maker to produce a consistent product.
Aim – determination of elemental composition of cement clinker raw meal and limestone
Apparatus claisse machine, XRF, furnace, platinum crucible, weighing balance spatula etc
Reagent – lithium borates
Prepare LOI using LOI procedure above
Weigh 1.2 g of sample into crucible
Weigh 8.4 g of lithium borates
Mix the weighed sample and lithium borates
Make a glass bead using claisse machine
Analyze the glass bead using XRF analyzer.
A alkourd and Adel hammadislamic university of gaza, Cement and concrete technology
2009 Pg. 11-12.
Annual Book of ASTM Standards, Volume 4, published by the American Society for Testing and Materials 2000
BS 4551. Methods of testing mortars, screeds and plasters.Part 1.Physical testing. 1998. British Standards Institution.
Cement operations course CCNN Sokoto 2009 by isaksontaylor page 7.
Cement production course. milling technology CCNN sokoto 2003 p1 – 6.
Concrete- Microstructure, Properties and Materials”, P.K. Mehta and Palulo J.M. Monteiro, Tata Mcgraw Hill
Habib S,and J vikingson. CCNN laboratory manual 2003.
Lea, F. M.; The Chemistry of Cement and Concrete (3rd edition); Edward Arnold (Publishers) Ltd.; 1970
. Mehta and P.K Monteiro P.J.M Concrete: Structure, Properties, and Materials, Prentice-Hall, Englewood Cliffs, NJ, 2nd Ed., 1993.
Moriki J Cemenrt and its analysis CCNN Sokoto 2017 page 1-50