Case Study: Ordinary Portland Cement INTRODUCTION •Cement –Is a material with adhesive and cohesive properties which is capable of bonding mineral fragments into a compact-solid whole •Ordinary Portland Cement (OPC) –Is a hydraulic cement. It is used in the making of concrete with property of setting and hardening, of which when the chemical properties reacts with water. OPC Does not disintegrate in water as it sets and hardens in water •Reason of widely usage in Malaysia –Is because raw material used in the making of Portland cement can be easily found and is available all over the world.
Apart from that OPC is the cheapest among all other cementitious material available. Malaysia does not have much problem of existing sulfates in soil or groundwater, therefore usage of OPC is optimum to eliminate unnecessary cost spending ORDINARY PORTLAND CEMENT PROPERTIES The properties of concrete depend on the quantities and qualities of its components. Cement is the most active component of concrete and usually has the greatest unit cost; therefore, its selection and proper use are important in obtaining most economically the balance of properties desired for any particular concrete mixture.
Table 1: Properties of cement according to their types Cement typeUse I1General purpose cement, when there are no extenuating conditions II2Aids in providing moderate resistance to sulfate attack IIIWhen high-early strength is required IV3When a low heat of hydration is desired (in massive structures) V4When high sulfate resistance is required IA4A type I cement containing an integral air-entraining agent IIA4A type II cement containing an integral air-entraining agent IIIA4A type III cement containing an integral air-entraining agent (Source: Cement-Part 1:
Compositions, Specifications and Conformity Criteria for Common Cements; Malaysian Standard, MS 522: Part 1:2007) Table 2: Effects of cements on concrete properties Cement PropertyCement Effects PlaceabilityCement amount, fineness, setting characteristics StrengthCement composition (C3S, C2S and C3A), loss on ignition, fineness Drying ShrinkageSO3content, cement composition PermeabilityCement composition, fineness Resistance to sulfateC3A content Alkali Silica ReactivityAlkali content Corrosion of embedded steelCement Composition (esp. C3A content)
OPC is Type I Portland cements. It provides adequate levels of strength and durability. The Cement composition and fineness play a major role in controlling concrete properties. Fineness of cement affects the placeability, workability, and water content of a concrete mixture much like the amount of cement used in concrete does. Finer powder is better than coarse as it helps to coat the surface of grains of sand or other inert material completely. The Cement composition affects the permeability of concrete by controlling the rate of hydration.
However, the ultimate porosity and permeability are unaffected. The More finely cement used and the greater surface been exposed in proportion to its mass, the more rapid the rate of hydration and the greater react of the proportion of cement. •Soundness Soundness refers to Cement paste that has set(hardened cement), and the expansion occurrence need to be determine. The Cement paste should not undergo a large change in volume and Sometimes after a period of time (month or years have elapsed), it will undergo a change in volume.
This May cause cracks due to the slow hydration or due to reaction of cement paste with foreign compound present •Setting time (change of cement from the fluid state to a rigid stage) •Workability of cement is affected by the water content, temperature and admixture used in concrete mix. There are two stages of Setting time which are (i) initial set (ii) final set. •For initial set, paste’s degree of stiffness point must be determined to establish the length of time for paste to still be in plastic and workable state. For final set, point condition of paste from fluid changing to rigid phase need to be determined to measure the strength development. •The mechanical strength (hardened cement) •The mechanical strength of cement co-related with the concrete strength for structural use. Mortar test is used to determine the compressive strength of cement (mortar test; cement and sand ratio of 1:3). The mass of water use for the mix is 10% of the mass of dry materials. Manufacturing Process of Ordinary Portland Cement
The Portland cement is made primarily from calcareous material, such as limestone, and from alumina and silica found as clay or shale. Figure 1: Samples of materials used in the making of clinker. Taken at CMS Cement Sdn Bhd Bintulu laboratory (23. 09. 09) the Raw materials (clinkers) are mixed in certain proportions, heated and burned in a large rotary kiln. Figure 2: Samples of materials used in processing cement Taken at CMS Cement Sdn Bhd Bintulu laboratory (23. 09. 09) The kiln resembles a large horizontal pipe with a diameter of 10 to 15 feet (3 to 4. meters) and a length of 300 feet (90 meters) or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures of 1450? c. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite).
For each ton of material that goes into the feed end of the kiln, two thirds of a ton (calcium silicates ; 3CaO. SiO2 and 2CaO. SiO2), then the end discharge, is called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. A small amount of gypsum is added during the grinding process to control the cement’s set or rate of hardening. In controlling the quality of OPC, processing are divided into three categories, which are clinker, gypsum and limestone. Clinker covers about 90% of the cement material.
It is then tested for quality control for first phase before being sent to the clinker shed, then the weighfeeder. Gypsum covers about 5% of the material. It is tested for second phase of quality control before being sent to gypsum shed then the weighfeeder. Limestone on the other hand covers 5% of the material. It is placed at the storage yard and then tested for quality control in the weighfeeder. These three materials are then being sent to the mill for in process cement with fourth phase of quality control then sent off to the cement silo before packing it for sale.
Physical and Chemical Characteristics of Ordinary Portland Cement Mainly the physical characteristics of OPC are: –Grey colour –Very fine –React with water •OPC acts as the main binder in concrete blocks, roofing tiles, grouts, fiberboard, mortar, tile adhesives, etc. •Disadvantages of OPC include low early strength and high shrinkage on drying. In the most general sense of the word, a cement binder, a substance which sets and hardens independently, and can bind other materials together. The most important use of cement is the production or mortar and concrete, the bonding of natural or artificial aggregates to form a strong building material which is durable in the face of normal environmental effects. The clicker contained in the ordinary Portland Cement normally contains oxide compounds such as CaO, SiO2, Al2O3, Fe2O3, and small quantities of other materials. Thus this clinker mixture is a homogeneous. The clinker is also a hydraulic component containing at least two-thirds by the mass of calcium silicates (3CaO•SiO2 and 2CaO•SiO2) and the remainder containing other onsisting of aluminum and iron. The ratio of the (CaO)/(SiO2) mass shall not be less than 2:0 while the content of magnesium oxide (MgO) shall not exceed the total mass by 5%. (Malaysian Standard, 2007). Granulated Blast Furnace Slag •The granulated blast furnace slag is made by rapidly cooling the slag melt of suitable composition like the obtaining of iron ore in a blast furnace. It must contain at least two-thirds of glassy-slag and possesses hydraulic characteristics when activated suitably.
The majority of chemical compounds available in the slag would be Calcium Oxide (CaO), Magnesium Oxide (MgO) Silicon Dioxide (SiO2) and a small amount of Aluminum Oxide (Al2O2) with other compounds. Pozzolanic Materials •Pozzolanic Materials are classed as natural substances of siliceous or silico-aluminous composition or a combination of both. Although they themselves do not harden with the mix with water but they are able to form strength-developing calcium silicate and calcium aluminate with the mix of dissolved calcium hydroxide (Ca(OH)2) in water.
Pozzolanic Materials can be divided into two (2) categories, which are natural pozzolana and natural calcined pozzolana. Natural pozzolana have its origins derived from materials from volcanic origins or sedimentary rocks with suitable chemical or mineralogical composition while the natural calcined pozzolana are the also the volcanic materials, clays, shales or sedimentary rocks activated by thermal treatment. Fly Ash •Fly ashes are obtained through electrostatic or mechanical precipitation of dust-like particles from the flue gases from fired furnaces with pulverized coal.
Fly ash is siliceous or calcareous in nature, therefore it has both pozzolanic properties and the hydraulic properties. •Siliceous Fly Ash consists essentially of reactive silicon dioxide (SiO2), Aluminum Oxide (Al2O3) and Iron Oxide (Fe2O3). The Calcareous Fly Ash on the other hand consists of reactive Calcium Oxide (CaO), reactive Silicon Dioxide (SiO2), Aluminum Oxide (Al2O3) and a small amount of Iron-Oxide (Fe2O3). Limestone •The limestone properties that are used in OPC should consequently meet the below three (3) requirements. Calcium Carbonate (CaCO3) content obtained from the calculating of Calcium Oxide must be at least 75% by mass. •The Clay content, determined using the methylene blue test shall not exceed 1. 20 g/100 g. •The total organic corbon (TOC) should not exceed 0. 20 % or 0. 50% by mass when tested in accordance with the EN 13639:1999. Silica Fume •Silica Fumes originates from the reduction of high purity quartz with coal in electric arc furnaces during the production of silicon and ferrosilicon alloys and consists of extremely fine particles containing at least 85 % by mass of amorphous silicon dioxide.
For the intergrinding with the clinker and calcium sulfate, the silica fume may be in its original state, compacted or pelletized (with water). SUMMARY: The properties of concrete depend on the quantities and qualities of its components. This is Because cement is the most active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining most economically the balance of properties desired for any particular concrete mixture.
OPC is the most economical of all types of cement available and is suitable for Malaysia’s environment which does not necessarily require expensive cementitious material. Type I can provide adequate levels of strength and durability, are the most popular cements used by concrete producers. Early hydration of cement is principally controlled by the amount and activity of C3A, balanced by the amount and type of sulfate interground with the cement.
C3A hydrates very rapidly and will influence early bonding characteristics. Abnormal hydration of (C3A) and poor control of this hydration by sulfate can lead to such problems as flash set, false set, slump loss, and cement-admixture incompatibility. The Development of the internal structure of hydrated cement occurs after the concrete has set and continues for months (and even years) after placement. The microstructure of the cement hydrates will determine the mechanical behavior and durability of the concrete.
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