Ordinary Portland Cement | The Civil Engineering World

OPC: Composition | Properties | Uses | Manufacturing Definition of OPC Cement can be defined as the bonding material having cohesive & adhesive properties which make it capable of uniting the different construction materials and forming the compacted assembly. Ordinary/Normal Portland cement is one of the most widely used types of Portland Cement. The name Portland cement was given by Joseph Aspdin in 1824 due to its similarity in color and its quality when it hardens like Portland stone.Portland stone is white-gray limestone in the island of Portland, Dorset. Composition of OPC The chief chemical components of ordinary Portland cement are: Calcium Silica Alumina Iron

Calcium is usually derived from limestone, marl or chalk while silica, alumina, and iron come from the sands, clays & iron ores. Other raw materials may include shale, shells and industrial byproducts. Basic Composition: Contents % CaO 60-67 SiO2 17-25 Al2O3 3-8 Fe2O3 0.5-6.0 MgO 0.5-4.0 Alkalis 0.3-1.2 SO3 2.0-3.5 The chief compound which usually forms in process of mixing: 1-triclcium silicate (3CaO.SiO2) 2-Dicalcium silicate (2CaO.SiO2) 3-tricalcium aluminates (3CaO.Al2O3) 4-tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3) Production & Manufacturing: Today, Ordinary Portland cement is the most widely used building material in the world with about  1.56 billion tons produced each year. Annual global production of Portland cement concrete is around 3.8 million cubic meters per year. In Pakistan; cement production will go beyond 45 million tons per year in the next two years Manufacturing >Raw Materials Calcareous (material having content of lime) Argillaceous (material having contents of silica & alumina) Gypsum >Process Cement is usually manufactured by two processes: Wet process Dry process These two processes differ in operation but fundamentals of both these processes are same.  In Pakistan, most of the factories use Wet Process for the production of cement.  There are five stages in the manufacturing of cement by wet process: Crushing and grinding of raw material Mixing the material in proportion Heating the prepared mixture in rotary kiln Grinding the heated product known as clinker Mixing and grinding of cement clinker with gypsum Crushing and Grinding: In this phase, soft raw materials are first crushed into the suitable size.  This is done usually in cylindrical ball or tube mills containing the charge of steel balls Mixing the Material: In this part, the powdered limestone is mixed with the clay paste in proper proportion  (75%=lime stone; clay=25%)  The mixture is then grounded and made homogeneous by mean of compressed gas.  The resulting material is known as slurry having 35-40% water. Heating the slurry in rotary kiln: The slurry is then introduced in a rotary kiln with help of conveyor. The rotary kiln consists of large cylinders 8 to 15 feet in diameter & height of 300-500 feet.  It is made with steel & is usually lined inside with fire bricks. Kiln rotates at the rate of 1-2 revolution per minute.  In the rotary kiln, the slurry is passed through different zones of temperature.  This whole process in kiln usually covers 2 to 3 hours. Different temperature zones are as under: Preheating Zone: In this zone, the temperature is kept at 500 degree Celsius & usually the moisture is removed & clay is broken into silica, aluminum oxide, iron oxide. Decomposition Zone: Temperature is raised up to 800 degree Celsius. In this zone, lime stone decomposes into lime and  CO2. Burning Zone In this zone, the temperature is maintained up to 1500 degree Celsius and the oxides formed in above zones combine together and form respective silicate, aluminates & ferrite.{loadposition articlemid} Cooling Zone This is the last stage where the whole assembly cooled is up to 150 to 200 degree Celsius. Clinker Formation The product which is obtained from the rotary kiln is known as the cement Clinker. Clinker is usually in the form of greenish black or gray colored balls. Grinding the Clinker with Gypsum The Cement Clinker is then air cooled. The required amount of Gypsum (5 %) is ground to the finepowder and then mixed with the Clinker. Finally, cement is packed in bags and then transported to the required site. Setting and Hardening: When ordinary Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to set. These chemical reactions all involve the addition of water to the basic chemical compounds. This chemical reaction with water is called "hydration". Each one of these reactions occurs at a different time and with different rates. The addition of all these reactions gives the knowledge about how Ordinary Portland cement hardens and gains strength. Those compounds and their role in hardening of cement are as under: Tricalcium silicate (C3S): Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Ordinary Portland cement with higher percentages of C3S will exhibit higher early strength. Dicalcium silicate (C2S): Hydrates and hardens slowly and is largely responsible for strength increases beyond one week. Tricalcium aluminate (C3A): Hydrates and hardens the quickest. It liberates a large amount of heat almost immediately and contributes somewhat to early strength. Gypsum is added to Ordinary Portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause ordinary Portland cement to set almost immediately after adding water. Tetracalcium alumino- ferrite (C4AF): Hydrates rapidly but contributes very little to strength.  Most ordinary Portland cement color effects are due to C4AF. Uses of OPC (Ordinary Portland Cement): It is used for general construction purposes where special properties are not required.  It is normally used for the reinforced concrete buildings, bridges, pavements, and where soil conditions are normal. It is also used for most of the concrete masonry units and for all uses where the concrete is not subject to special sulfate hazard or where the heat generated by the hydration of cement is not objectionable.  It has great resistance to cracking and shrinkage but has less resistance to chemical attacks. Tests On Ordinary Portland Cement Fineness test Soundness test Setting time test Strength tests Compressive strength test Tensile strength test Flexural strength test Specific gravity test Consistency test Heat of hydration test Loss of ignition test ENVIRONMENTAL IMPACT OF Ordinary Portland Cement: There are lots of environmental impacts of Cement on our ecology. One of the major problems is the emission of CO2 from the Cement industry. It is found that the world yearly 1.6 billion tons production of cement covers 7% of carbon dioxide's yearly production. As CO2 is harmful to human health and also for the wildlife. It causes many respiratory problems like asthma, bronchitis, and nasal infections. The cement manufacturing industry is labor intensive and uses the large-scale and potentially hazardous manufacturing processes. The industry experiences accident rates that are high compared with some other manufacturing industries. There are a number of hazards inherent to the cement production process. Some examples for health hazards are: Exposure to dust and high temperatures; Contact with allergic substances; and Noise exposure And some examples for safety hazards: Falling/impact with objects Hot surface burns It is the responsibility of the government to implement the laws as described in EPA (Environmental Protection Agency) rule to protect both the environmental and human health.