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2372 results in Chemical engineering

Page 24 of 119

  • 5 - Engineering Materials

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 307-351

  • Summary

    Cement

    Introduction

    Cement is a binder, a substance that sets and hardens independently, and can bind other materials together. It is used in construction for binding materials like bricks, stones and tiles, and is also used for flooring and plastering purposes.

    Cements used in construction can be either hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) set and harden because of hydration and hydrolysis reactions between the anhydrous cement powder and water.

    Non-hydraulic cement such as slaked limes (calcium hydroxide mixed with water) harden due to the carbonation reaction by carbon dioxide present naturally in the air.

    Cement alone cannot be used for construction work. It is used as a mixture with sand and crushed stones. It may be used as mortar or concrete. Mortar It is a mixture of cement, sand and water which is used for binding bricks together and for plastering.

    Concrete It is a mixture of cement, gravel (crushed stones), sand and water. It is used in flooring works, columns, beams, foundations, etc. If instead of cement, lime is used, then the concrete is known as lime concrete.

    Reinforced concrete construction (RCC) It is concrete reinforced with steel rods or heavy wire mesh. Reinforced concrete is used in floors, beams, girders, columns, slabs, bridges, etc. It imparts high tensile strength to structures.

    Portland cement was discovered by Joseph Aspdin in 1824. The name Portland cement comes from the fact that the colour and quality of the resulting concrete are similar to Portland stone, a kind of limestone found in England.

    Types of cements

    The different types of cements based on Indian Standards are as follows:

  • Ordinary Portland cement It is the most important type of cement and is commonly called OPC. It is made by grinding and mixing clay and lime bearing minerals in proper proportions and then heating the mixture to nearly 1400 °C. It is not in much use these days, having been replaced by Portland pozzolana cement (PPC).

  • Portland pozzolana cement It can be fly ash-based or calcined clay-based. PPC is obtained by mixing OPC with suitable pozzolans at a certain temperature. Pozzolans are deposits of volcanic ash produced by rapid cooling. When mixed with slaked lime, pozzolans show properties similar to cement when it comes in contact with water. PPC is commonly used for various construction activities.

  • 14 - Surface Chemistry

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 757-792

  • Summary

    Introduction

    Various physical and chemical reactions occur at the interfaces or boundaries of two different phases like solid–liquid interface, solid–solid interface, solid–gas interface, liquid–gas interface, liquid–solid interface and liquid–liquid interface. These reactions have been of great interest to the chemists worldwide as they have important applications in industrial and domestic field.

    All these changes occurring at the boundaries of different phases are studied under a branch of chemistry called surface chemistry. Surface chemistry not only deals with the reactions occurring at the interfaces but also studies the nature of surfaces and the reactions taking place at the surfaces. Adsorption and colloidal properties are some of the important effects that occur at the solid – liquid interfaces.

    Adsorption

    The surface of solid or liquid has a tendency to attract and retain molecules of other species (gas or liquids). This phenomenon of attracting and retaining molecules on the surface is termed as adsorption (latin word surbere; to suck up). The material on the surface of which adsorption occurs is termed as adsorbent and the substance adsorbed on the surface is called adsorbate. Now, the question of why adsorption occurs arises. Adsorption occurs because of the presence of unbalanced forces at the surface of liquids and solids. In a solid or liquid, the molecules present in the bulk are attracted equally from all sides and the net force experienced by the molecule in the bulk is zero. However, the molecule that lies near the surface (known as surface molecule) is being attracted only by the molecules below it because there are no molecules above it (Fig. 14.1). Hence, a net downward force is experienced by the surface molecule. Because of this downward force, the surface molecule tends to attract and retain other species (a gas or a dissolved substance). These species do not penetrate inside the molecule but remain on the surface. Hence, their concentration is higher on the surface than in the bulk.

    Common examples of adsorption are

  • (i) Finely divided activated charcoal adsorbs a number of gases like ammonia, sulphurdioxide, chlorine, etc.

  • (ii) Animal charcoal adsorbs dyes and is used for decolorising a number of organic substances.

Engineering Chemistry
  • Fundamentals and Applications
  • 2nd edition
  • Shikha Agarwal
  • Published online:
    08 May 2019
    Print publication:
    16 May 2019
  • Gain a detailed understanding of the fundamental concepts of chemistry and their engineering applications with this fully revised second edition. Catering to the needs of first and second semester undergraduate students from all branches of engineering taking courses on engineering chemistry, it offers new material on topics such as periodic properties, structure and bonding, gaseous states, ionic equilibrium, oxidation and reduction, Werner's coordination theory, Sidgwick coordination theory, valence bond theory, crystal field theory, bonding in coordination compounds, and isomerism in coordination compounds. Lucid language and an easy-to-learn approach help students to understand the basic concepts, use them to construct engineering materials, and solve problems associated with them. Each chapter is further strengthened by numerous examples and review questions.

Page 24 of 119