Cements and concretes

Concrete is a composite material made from cement paste and aggregate (a coarse, stony material). The composition of concrete varies widely and depends on the intended application but always contains cement, water and aggregate. The mixture of cement, water and sand is called mortar, whereas cement and water alone constitute cement paste. 

Early forms of cement, which were composed mainly of calcium hydroxide [Ca(OH)2] partly transformed to calcium carbonate (CaC03) predate Roman times. The material was made from limestone, [impure calcium carbonate (CaC03)] heated or burned to give quicklime or burnt Ume, calcium oxide (CaO). 

Quicklime reacts with water to release considerable heat, a process called slaking, to give slaked Ume, calcium hydroxide [Ca(OH)2]  

Chemical name Mineral name Chemical formula Shorthand notation Typical composition/wt% 

Calcium sulphate dihydrate Gypsum CaS04-2H20 CSH2 2-5 

Lime kilns are frequently associated with cement-making7 The lime-sand cements in use since Roman times gain mechanical strength from the slow reaction of Ca(OH)2 with CO2 of the air to form interlocking crystals of CaCOs- The sand acts primarily as a matrix around which this process occurs. 

Abrasive products (a) section of silicon carbide wheel, unetched (50x) (b) aluminum oxide wheel, unetched (lOOx). In both, light area is grain, gray area bond, and dark area porosity. 

Simplified model of Portland cement paste structure showing needle or platelet gel particles and capillary cavities C. 

In the gel phase itself, there are pore spaces between the individual gel particles. The excess water, when it leaves by evaporation, gives rise to large residual pores. 

Interlocking crystalline network in pottery plaster, CaS04-2H20 (courtesy W. Gourdin). (From W. D. Kingery et al.. Introduction to Ceramics, 2nd Edn., John Wiley, New York, 1976, 

In addition to the cement gel, there is present in concrete an aggregate of crushed stone that acts as a filler material. The Portland cement paste serves to bond together the aggregate particles in much the same way as a bond material present in a refractory brick or an abrasive wheel. 

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Welan has similar properties to xanthan gum except that it has increased viscosity at low shear rates and improved thermal stabiUty and compatibihty with calcium at alkaline pH (90). The increased thermal stabiUty has led to its use as a drilling mud viscosifter especially for high temperature weUs. The excellent compatibihty with calcium at high pH has resulted in its use in a variety of specialized cement and concrete appHcations. 

Castings Investment Casting Institute 8521 Clover Meadow Dallas, Tex. 75243 American Die Casting Institute 2340 Des Plaines Ave. Des Plaines, lU. 60018 American Poundrymen s Society Gulf and Wolf Roads Des Plaines, lU. 60016 Steel Pounders Society of America 455 State Street Des Plaines, lU. 60016 Cement and Concrete Cement Statistical and Technical Association Mahno Sweden American Concrete Institute P.O. Box 19150 Detroit, Mich. 48219... 

T. D. Robson, High-Mlumina Cements and Concretes,]ohxi Wiley Sons, Inc., New York, 1962. 

Guide to Compounds ofilnterest in Cement and Concrete Kesearch, Special Eeport 127 Highway Research Board, National Academy of Sciences, Washington, D.C., 1972. 

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

Table 1.3 shows a rough breakdown of material prices. Materials for large-scale structural use - wood, cement and concrete, and structural steel - cost between UK 50 and UK 500 (US 75 and US 750) per tonne. There are many materials which have all the other properties required of a structural material - nickel or titanium, for example - but their use in this application is eliminated by their price. 

Today, cement and concrete replace stone in most large structures. But cement, too, is a ceramic a complicated but fascinating one. The understanding of its structure, and how it forms, is better now than it used to be, and has led to the development of special high-strength cement pastes which can compete with polymers and metals in certain applications. 

Cement and concrete are used in construction on an enormous scale, equalled only by structural steel, brick and wood. Cement is a mixture of a combination of lime (CaO), silica (SiOj) and alumina (AI2O3), which sets when mixed with water. Concrete is sand and stones (aggregate) held together by a cement. Table 15.4 summarises the most important facts. 

Low-grade ceramics - stone, and certain refractories - are simply mined and shaped. We are concerned here not with these, but with the production and shaping of high-performance engineering ceramics, clay products and glasses. Cement and concrete are discussed separately in Chapter 20. We start with engineering ceramics. 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

Make a list, based on your own observations, of selected examples of components and structures made from cement and concrete. Discuss how the way in which the materials are used in each example is influenced by the low (and highly variable) tensile strength of cement and concrete. 

Each chapter is designed to provide the content of a 50-minute lecture. Each block of four or so chapters is backed up by a set of Case Studies, which illustrate and consolidate the material they contain. There are special sections on design, and on such materials as wood, cement and concrete. And there are problems for the student at the end of each chapter for which worked solutions can be obtained separately, from the publisher. In order to ease the teaching of phase diagrams (often a difficult topic for engineering students) we have included a programmed-learning text which has proved helpful for our own students. 

Acid water (pH <6.5) Cement and concrete Slow disintegration... 

Diverse techniques have been employed to identify the sources of elements in atmospheric dust (and surface dust) (Table V). Some involve considering trends in concentration and others use various statistical methods. The degree of sophistication and detail obtained from the analyses increases from top left to bottom right of the Table. The sources identified as contributing the elements in rural and urban atmospheric dusts are detailed in Table VI. The principal sources are crustal material, soil, coal and oil combustion emissions, incinerated refuse emissions, motor vehicle emissions, marine spray, cement and concrete weathering, mining and metal working emissions. Many elements occur in more than one source, and they are classified in the... 

Into this category come the water-based plasters, mortars, cements and concretes which set at room temperature as the result of a chemical reaction between water and a powder. Some of these have been known... 

Beaudoin, J. J. Feldman, R. F. (1975). Mechanical properties of autoclaved calcium silicate systems. Cement and Concrete Research, 5 (2), 103-18. 

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