Portland Cement: What We Build With - Essay Example

? Portland Cement: What We Build With Your Full Table of Contents Content Page Number ………………………………………………………… 3 Introduction ……………………………………………………… 4 History of Cement ……………………………………………….. 4 Chemistry of Cement …………………………………………….. 5 Types of Cement …………………………………………………. 6 Cement Manufacture …………………………………………….. 7 Uses of Cement …………………………………………………... 10 Discussion Environmental Issues …………………………………………. 10 Health Issues ………………………………………………….. 13 Conclusion ……………………………………………………….. 14 References ……………………………………………………….. 15 Abstract Cement has been a part of almost everything that man has built to improve his life. The chemistry and technology involved in its production has significantly contributed to the development of the world that we have today. On the other hand, environmental and even occupational issues comes hand-in-hand with this industry. Hence, safety measures and standards have been formulated to be able to keep the balance between the benefits and disadvantages that are gained from the cement industry. Portland Cement: What We Build With Cement is a commodity which is derived from processing of limestone and other minerals in high temperature then cooled afterwards and reduced in size until it becomes a fine powder (Environmental Protection Agency [EPA], n.d.). It has become part of man’s lives since the ancient civilizations have used mortars and binders to be able to construct different structures including the great pyramids of Egypt. Through time and innovations, different types of cements have been formulated to be able to meet the demands of engineering (Cement History, 2013). And with these innovations, issues have also risen especially in terms of the generation of wastes and the safety of the people involved in the manufacture of cement. This paper talks about Portland cement, its chemical composition, applications and the issues concerning the cement manufacture industry. History of Cement The actual time when binders were used in structures cannot be clearly identified, but early civilizations like the Egyptians have used burned lime since 2600 B.C., while the Mesopotamians started to use the same binder in 2000 B.C. In 1000 BC, the Phoenicians were the civilization to make a binder that can set underwater. They were able to make it so by mixing ground volcanic rocks or brick with mortars. The Greeks were also able to make structures of their own in 150 BC by using, emplekton; which is a mixture of stones and mortar (Cement history, 2013). Romans were the first to actually discover the technology of using cement as binder in making structures. During the early times, Romans mixed Calcium carbonate (CaCO3) or lime with volcanic ash, and from then on was used in the construction of the Colosseum and the Pantheon. However, the technology that was established by the Romans, died with the empire and was rediscovered only in the 1600’s (Wansbrough, n.d.). In 1810, an English man named Edgar Dobbs was able to patent a cement which used clay, road dust and lime as its raw materials; but when the patent expired in 1824, Joseph Aspdin grabbed the opportunity to improve on the technology to produce the cement was the person who introduced the cement we currently know as Portland cement. The name of the cement was derived from the Portland stone which is quarried in the island of Portland in England (Van Oss, 2005). Chemistry of Cement The most common type of cement now used all over the world is Portland cement. It is made with mixtures of the oxidized forms of calcium, silicon, aluminum and iron. These materials are subjected to very high temperature reaching up to 1400?C. The binding effect of cement is attributed to the Si-O-Si bonds that are formed between the cement particles when water is added to the cement. The length of reactions upon cement application varies from one cement composition type to another. Initially, the cement expands because of the formation of a colloid, which shrinks eventually when excess moisture is lost (Wansbrough, n.d.). Table 1 presents the primary raw materials for cements and their respective chemical formula. Of these compounds, the strength of cement depends on the composition of C3A and C3S. There is an inverse relationship between the amount of C3S used in the cement with the strength and heat generation properties of the product. High concentrations of C3S (with low C2S) in the cement give the type, which is high in strength but also produces high amounts of heat during setting. The other combination, which is the low C3S – high C2S mixture, produces cements which take slower to develop strength, but produces lower amount of heat (Wansbrough, n.d.). Table 1. The Four Major Minerals in Cement Production (Wansbrough, n.d.). Compound Abbreviation Chemical Formula Composition Dicalcium silicate C2S 2CaO•SiO2 10 – 20% Tricalcium aluminate C3A 3CaO•Al2O3 5 – 10% Tricalcium silicate C3S 3CaO•SiO2 60 – 70% Tetracalcium alumino-ferrate C4AF 4CaO•Al2O3•Fe2O3 3 – 8% The correct composition of compounds is a very important key in the manufacture of cement to be able to generate good quality of products in terms of the performance characteristics. With this in mind, it is then important that an on-site laboratory is present from the beginning of the manufacture of cement to ensure that good quality and the correct type of raw materials are used in the process, and that finished products meet the standards for cement. Analyses involved in cement manufacture vary from physical property to chemical properties like air permeability analysis or X-ray fluorescence (Wansbrough, n.d.). Types of Cement The American Society for Testing and Materials [ASTM] is the organization that provides the standards for the specifications of different materials used in almost all types of industries. The ASTM C150 / C150M – 12 is the Standard Specification for Portland Cement and this standards classifies ten types of Portland cements, categorized to five general types and their subtypes. Table 2 presents the types of Portland cement based on the ASTM standard. The A subtype stands for the air-entrainment capacity of the cement, and the MH is the subtype that has moderate heat of hydration property (ASTM, 2011). Air-entraining additives create thousands of bubbles in the concrete when the cement is mixed, this improves the concrete’s resistance to cracking brought about by changes in temperature especially during winter seasons in cold regions (Van Oss, 2005). Table 2. Types of Portland Cement Based on ASTM C150/C150M -12 (ASTM, 2011; Mehta and Monteiro, n.d.) Type Description Uses Subtypes I General Purpose Common use IA II Moderate Sulfate Resistance General construction IIA, II(MH), II(MH)A III High Early Strength Precast, winter construction IIIA IV Low Heat of Hydration Mass concrete None V High Sulfate Resistance Sewers None The ratios of the cement minerals vary from one type of Portland cement to another. Specific ratios have to be set in order to meet the requirements specified by ASTM. Table 3 shows the general composition percentages of the cement minerals for each type of Portland cement (Mehta and Monteiro, n.d.). Cement Manufacture There are six steps in the manufacture of cement, namely: quarrying, blending and grinding, preheater tower, kiln, clinker and bagging (Portland Cement Association [PCA], 2013). Raw materials for the cement may be taken from rocks which have been blasted from the quarry and is brought to the primary crusher where the rocks are crushed to pieces similar to the size of a baseball. These rocks are subjected another crusher called the hammer mill to reduce the size further to about the size of a gravel (PCA, 2013). Table 3. Chemical Compositions for the Different ASTM Types of Portland Cement (Mehta and Monteiro, n.d.) ASTM Type Percent Composition (%) C3S C2S C3A C4AF I 45 – 55 20 – 30 8 – 12 6 – 10 II 40 – 50 25 – 35 5 – 7 6 – 10 III 50 – 65 15 – 25 8 – 14 6 – 10 V 40 – 50 25 – 35 0 - 4 10 - 20 The laboratory analyzes the raw materials and when they pass the requirements, they are blended according to the required proportion and ground again in a heavy wheel-type roller on a rotating table. The blended powder is now transferred to a preheater tower either as slurry or in powder form. The tower is equipped with vertical chambers which are cyclone-shaped and this is where the raw materials pass through before entering the kiln. Some cement plants subject the raw materials to preheating before entering the kiln to conserve energy. The hot gas rises up the tower heating the materials as they go down (PCA, 2013). The kiln is the heart of the cement plant. It is an enormous horizontal furnace which turns at a rate of about three revolutions per minute and is lined with a firebrick. The kiln is considered to be the largest dynamic industrial equipment with its diameter sizing up to about 12 feet. The raw materials tumble inside the kiln and progresses along the horizontal direction where the temperature increases. The kiln is fueled with natural gas and powdered coal for spontaneous ignition and this helps the kiln to reach temperature to about 1870?C. The hottest part of the kiln partially melts the raw materials and induces the physical and chemical changes where the oxides of calcium and silicon are converted to calcium silicates. This hot new particles are referred to as the clinker (PCA, 2013). The clinkers are like marbles which are grayish-black in color. Aluminum and iron, or ferrite, is added as fluxing agent to reduce the melting point of silica (“Composition of Cement”, n.d.). Table 4 below presents the temperature regions where physical changes and chemical reactions take place in the kiln. Table 4. Reactions and Processes Occurring in the Kiln (Wansbrough, n.d.; Van Oss, 2005). Zone Temperature Range (?C) Processes Involved Chemical Equation involved 1 < 100 – 200 Dehydration; Water is removed from the slurry and the raw materials become dry None 2 200 – 550 Removal of bonded water from the raw materials None 3 800 – 1100 Decarbonation, Formation of oxides, Melting of Al2O3 and Fe2O3 CaCO3 + heat ? CaO + CO2 (g) 4 1100 - 1300 Exothermic reactions, C2S is formed 2CaO + SiO2 + heat ? 2CaO•SiO2 5 1300 – 1450 Sintering, Formation of clinkers, C3S and ferrite is formed 2CaO•SiO2 + CaO + heat ? 3CaO•SiO2 6 Read More