Chemically Bonded Phosphate Ceramics

Twenty-First Century Materials with Diverse Applications

Arun S. Wagh

• Publisher: Elsevier
• Year: 2016
• ISBN:
  9780081003961
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Scienza dei materiali

Chemically Bonded Phosphate Ceramics brings together the latest developments in chemically bonded phosphate ceramics (CBPCs), including several novel ceramics, from US Federal Laboratories such as Argonne, Oak Ridge, and Brookhaven National Laboratories, as well as Russian and Ukrainian nuclear institutes. Coupled with further advances in their use as biomaterials, these materials have found uses in diverse fields in recent years. Applications range from advanced structural materials to corrosion and fire protection coatings, oil-well cements, stabilization and encapsulationof hazardous and radioactive waste, nuclear radiation shielding materials, and products designed for safe storage of nuclear materials. Such developments call for a single source to cover theirscience and applications. This book is a unique and comprehensive source to fulfil that need. In the second edition, the author covers the latest developments in nuclear waste containment and introduces new products and applications in areas such as biomedical implants, cements and coatings used in oil-well and other petrochemical applications, and flame-retardant anti-corrosion coatings.

• Explores the key applications of CBPCs including nuclear waste storage, oil-well cements, anticorrosion coatings and biomedical implants
• Demystifies the chemistry, processes and production methods of CBPCs
• Draws on 40 years of developments and applications in the field, including the latest developments from USA, Europe, Ukraine, Russia, China and India

Chapter 1

Introduction to Chemically Bonded Ceramics

Abstract

A brief overview of chemically bonded ceramics (CBCs) is provided. These are ceramics or cements formed by chemical reactions at ambient or near-ambient temperatures, in which acid-base cements (ABCs) are a class of solids formed by reaction of a mild acid with a base. Chemically bonded phosphate ceramics (CBPCs) are ABCs in which the acid component is orthophosphoric acid or a soluble orthophosphate and the resulting ceramic is an insoluble orthophosphate. Phosphate bonds are found in nature as one of the fundamental blocks of DNA structure and in bones, and their use in the production of CBPCs for various technological applications is detailed in this book.

Keywords

Chemically bonded ceramics; Carbon footprint; Mineral accretion process; Geopolymers; DNA

Matter consists of atoms and molecules, which are bonded by chemical and physical forces to form solids, liquids, and gases. Interatomic and chemical bonds play a crucial role in the physical properties of matter, while the chemical nature of the atoms and molecules themselves decide the chemical properties. While the chemical nature of the atoms and molecules dominate in the discussion of gases, in which molecules are not bonded intimately, solids at the other extreme are highly dependent on the nature of these bonds, and solution chemistry plays a major role by dissociating these atoms and molecules and reforming new solid products. This engineering of a chemically bonded ceramic (CBC) may be done in two ways. The desired components can be heat treated to high temperatures and fuse grains into a solid with desired properties. Forming alloys from metals, or packing oxide powders and sintering them at high temperature and obtaining a ceramic are examples of high-temperature treatment. This approach is energy intensive, and in the modern world produces an undesirably high carbon footprint, and yet it is an indispensable process, because the bonds between atoms and molecules are so strong that other methods cannot achieve the same strength, density, and other characteristics that these products offer.

An alternative, wherever feasible, is to dissolve the component solids in a solvent and then reassemble the dissolved species into a new solid. While there are a large number of organic solvents available for such applications, water is the best and most plentiful solvent available. If one can produce new solids with desired properties using water as solvent, synthesize new solids with desired properties in ambient conditions without application of any heat at all, or use heat treatment only moderately, that would be the best way to produce solids of desired properties. The process is chemical and hence the candidate materials selected are those that allow dissociation of the bonds and reaction of the dissociated atoms or molecules. Among inorganic materials, phosphates allow such dissociation and bonding, hence this book is based on the nature of phosphate bonds, how to exploit phosphate materials to form new products of technological importance, the parameters involved in the processes and how to control them. The result is a new class of materials, namely chemically bonded phosphate ceramics (CBPCs).

The discussion in this book is limited to inorganic materials only, which allow one to produce products with low carbon footprint, the fewest emissions of hazardous pollutants, and very low after-use impact on the environment. In particular, the inorganic materials discussed in this book are oxides or oxide minerals or phosphates that are commonly available. The net result is that the end products are ceramics or cements.

1.1 Ceramics and Cements

Ceramics and cements are two major classes of inorganic solids that are man made and in common use [1]. Most cements used in bulk amounts are water based and hence are called hydraulic cements. Ceramics are formed by compaction of powders and their subsequent fusion at high to very high temperatures, ranging anywhere from ≈ 2000 to 3000°C. Once fused, the resulting ceramics are hard and dense, and exhibit excellent corrosion resistance. These materials have found applications in bricks; pottery; refractory products of alumina, zirconia, and magnesia; and high-temperature superconductors. There are porous ceramics such as filters and membranes that are also fabricated by the sintering process, but porosity is introduced in them intentionally. Ceramics, in general, are highly crystalline with some glassy phase. If glassy phase dominates, then they are called “glass ceramics.”

Hydraulic cements are another class of technologically important material. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed in this way set into a hard mass that has sufficient compression strength for load-bearing applications and hence can be used as structural materials. Their structure is generally noncrystalline.

Hydraulic cements are excellent examples of accelerated chemical bonding. Hydrogen bonds are formed in these materials by chemical reaction when water is added to the powders. These bonds are distinct from the bonds in ceramics in which high-temperature interparticle diffusion leads to consolidation of powders.

Portland cement is the most common hydraulic cement. It is formed by clinkering a mixture of powders of limestone, sand, iron oxide, and other additives at a very high temperature (≈ 1500°C). It is mixed with water to form hydrated bonding phases of dicalcium and tricalcium silicates (Ca₂SiO₄ and Ca₃SiO₅), dicalcium aluminate (Ca₂Al₂O₆), and calcium aluminoferrite [Ca₄(Fe_{1 − x}Al_x)O₅]. When this cement is mixed with sand and gravel, it bonds them to form cement concrete that is used in construction. Typically, initial bonding occurs in a few hours, but slow curing takes place for weeks to gain full strength.

The preparation of calcium aluminate cements is similar. Here, instead of calcium and silica, calcium and alumina react with water to form hydrated calcium aluminate [2] as the bonding phase. The initial strength gain for this material is faster than that for Portland cement.

Intense research into hydraulic cements has resulted in a wide range of blends that are used in various applications. Accelerated setting formulations have been developed to gain early high strengths. Additives to reduce water demand have been used to develop macrodefect-free (MDF) cements [3] in which large-sized pores are eliminated. Pumpable versions of Portland cement for oil drilling applications [4] are common. All the modifications, however, depend on the primary bonds formed by chemical reactions among silica, calcium oxide, alumina, and iron oxide.

The main distinction between ceramics and cements is thus how they are produced. Objects that go through intense heat treatment for their consolidation are ceramics, while those formed by chemical reaction at room temperature are cements. Because of the cost involved in high-temperature processes used in forming ceramics, and also because of the raw materials costs, ceramics are used to add value (where the benefits outweigh the cost) as compared to cement, which is used in bulk applications.

The difference between ceramics and cements, however, goes beyond this definition. From a structural viewpoint, the distinction between ceramics and cements concerns the interparticle bonds that hold them together and provide the necessary strength. Hydraulic cements are bonded by van der Waals forces, while ceramics are formed by either ionic or covalent bonds between their particles. The nature of these specific bonds will be discussed in detail in Chapter 8. Because covalent and ionic bonds are stronger than van der Waals bonds, ceramics have better strength than cements.

Another major distinction between ceramics and hydraulic cements is the porosity. Ceramics are made dense unless their application requires some degree of porosity. Hydraulic cements, however, are inherently porous. Porosity is < 1 vol% for the best ceramics, but typically 15–20 vol% is common for cements. Ceramics tolerate very high temperatures, and are corrosion resistant over a wide range of pH, while cements are made for use at ambient temperatures and are affected by high temperature as well as acidic environment. Compared to cements, ceramics are more expensive; thus cement is produced in high volume while ceramics, except for a few products such as bricks, are specialty products.

1.2 Chemically Bonded Ceramics as Intermediate Products

The distinctions made between ceramics and hydraulic cements do not cover many products that have been produced by materials research in the last 50 years. Some of these products are made by partial heat treatment first and then set like cements. There exist products that are made like cements, but exhibit a structure like that of ceramics, because the bonding mechanism in them is covalent and ionic. They have much higher compressive strength compared to hydraulic cements and they are less corrosion resistant. Some set much more rapidly than hydraulic cements.

Refractory cement is a good example [2]. High-alumina cement paste is mixed with refractory powders such as alumina in the form of corundum (Al₂O₃) that is cast into position, dried and hardened, then fired to make a ceramic. Thus a chemical route is employed to generate early strength and then ceramic bonds are developed upon firing. Another product that lies between cements and ceramics is the FUETAP cement [5] used for nuclear waste encapsulation. This is dense and is formed by hot pressing. To define such intermediate products, the name chemically bonded ceramics (CBCs) was coined by Roy [6].

Geopolymers are another type of intermediate product that lie between cements and ceramics [7]. A geopolymer is made by pyro-processing naturally occurring kaolin (alumina-rich clay) into meta-kaolin. This meta-kaolin is then reacted with an alkali hydroxide or sodium silicate to yield a dense rock-like hard mass. Thus a chemical reaction, which is being studied extensively, is employed to produce a hard ceramic-like product. Though this product is produced like cement, its properties are more like a sintered ceramic.

The examples given above are only some of the several CBC materials that can be synthesized without heat treatment. A much wider range of such materials that share attributes of cements and ceramics are formed by acid-base reactions. These cements are discussed below.

1.3 Acid-Base Cement CBCs

Acid-base cements are a class of CBCs that are formed at room temperature but exhibit properties like those of ceramics. They are formed by reaction of an acid with a base. Normally this reaction produces a noncoherent precipitate. If, however, the reaction rate is controlled properly between certain acids and bases, coherent bonds can develop between precipitating particles that will grow into crystalline structures and form a ceramic. The acidic and alkaline components neutralize each other rapidly, and the resulting paste sets rapidly into products with neutral pH.

Much of the initial development in CBCs occurred because of the need for rapid-setting dental cements. Wilson and Nicholson [8] pr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• About the Author
• Preface to the Second Edition
• Abbreviations
• Chapter 1: Introduction to Chemically Bonded Ceramics
• Chapter 2: Chemically Bonded Phosphate Ceramics
• Chapter 3: Raw Materials
• Chapter 4: Phosphate Chemistry
• Chapter 5: Dissolution Characteristics of Metal Oxides and Kinetics of Ceramic Formation
• Chapter 6: Thermodynamic Basis of CBPC Formation
• Chapter 7: Oxidation and Reduction Mechanisms
• Chapter 8: Crystal Structure, Mineralogy of Orthophosphates
• Chapter 9: Magnesium Phosphate Ceramics
• Chapter 10: Zinc Phosphate Ceramics
• Chapter 11: Aluminum Phosphate Ceramics
• Chapter 12: Iron Phosphate Ceramics
• Chapter 13: Calcium Phosphate Cements
• Chapter 14: Chemically Bonded Phosphate Ceramic Matrix Composites
• Chapter 15: Chemically Bonded Phosphate Ceramic Coatings
• Chapter 16: Chemically Bonded Phosphate Ceramic Borehole Sealant
• Chapter 17: Chemically Bonded Phosphate Ceramic Nuclear Shields
• Chapter 18: Applications of CBPCs to Radioactive and Hazardous Waste Immobilization
• Chapter 19: Chemically Bonded Phosphate Bioceramics
• Chapter 20: Environmental Implications of Chemically Bonded Phosphate Ceramic Products
• Appendix A: Thermodynamic Properties of Selected Materials
• Appendix B: Solubility Product Constants
• Appendix C: List of Minerals and Their Formulae
• Index