Industrial Glass and Ceramics

COMPOSITION AND PROPERTIES

Portland cement.

Portland cement is made up of four main compounds: tricalcium silicate (3CaO {dot}

SiO

), dicalcium silicate (2CaO {dot}

SiO

), tricalcium aluminate (3CaO {dot}

), and a tetra-calcium aluminoferrite (4CaO {dot}

). In an abbreviated notation differing from the normal atomic symbols, these compounds are designated as C {sub 3}

S, C

A and C

AF, where C stands for calcium oxide (lime), S for silica, A for alumina, and F for iron oxide. Small amounts of uncombined lime and magnesia also are present, along with alkalies and minor amounts of other elements. The composition of portland cements falls within the range of 60 to 67 percent lime, 19 to 25 percent silica, 3 to 8 percent alumina, and 0.3 to 6 percent iron oxide together with 1 to 3 percent sulfur trioxide, derived mainly from the added gypsum, 0.5 to 5 percent magnesia, and 0.3 to 1.3 percent alkalies. Titanium oxide is usually present to the extent of 0.1 to 0.4 percent. Manganese oxide is usually present only in small amounts except when blast-furnace slag is used as a raw material; then it may rise to 1 percent, giving the cement a brownish tinge rather than the normal gray colour.

The strength developed by portland cement depends on its composition and the fineness to which it is ground. The C {sub 3} S is mainly responsible for the strength developed in the first week of hardening, and the C {sub 2} S for the subsequent increase in strength. The alumina and iron compounds that are present only in lesser amounts make little direct contribution to strength. Set cement and concrete can suffer deterioration from attack by some natural or artificial chemical agents. The alumina compound (C {sub 3} A) is the most vulnerable to chemical attack in soils containing sulfate salts or in seawater, while the iron compound (C {sub 4} AF) and the two calcium silicates are more resistant. Calcium hydroxide is released during the hydration of the calcium silicates, and this is also vulnerable to attack. Cement liberates heat when it hydrates; and consequently concrete placed in large masses, as in dams, can cause the temperature inside the mass to rise as much as 72 {degree} F (40 C) above the outside temperature. Subsequent cooling can be a cause of cracking. The highest heat of hydration is shown by C {sub 3} A, followed in descending order by C {sub 3} S, C {sub 4} AF, and C {sub 2} S.

Five types of portland cement are standardized in the U.S.: ordinary (Type I), modified (Type II), high-early-strength (Type III), low-heat (Type IV), and sulfate-resisting (Type V ;see Table ). In other countries Type II is omitted, and Type III is called rapid-hardening. Type V is known in some European countries as Ferrari cement. Typical compositions are shown in the table below. These various types are differentiated both by requirements as to their composition and by various tests. Thus in Type V, the content of C {sub 3} A is limited to 3.5 to 5 percent in different countries, and in Type IV to a slightly higher value. The content of C {sub 3} S ranges from about 40 to 65 percent except for Type IV, where it is usually below 30 percent. The content of C {sub 2} S varies more or less inversely to that of C {sub 3} S, falling as low as 10 percent in rapid-hardening cement and rising as high as 60 percent in low-heat cement. The C {sub 3} A content usually falls between 6 and 15 percent in Types I and III, and that of C {sub 4} AF between 5 and 10 percent. In Types IV and V, the C {sub 4} AF content is usually higher and may exceed 15 percent.

There also are various other special types of portland cement. Coloured cements are made by grinding 5 to 10 percent of suitable pigments with white or ordinary gray portland cement. Air-entraining cements are made by the addition on grinding of a small amount, around 0.05 percent, of an organic agent that causes the entrainment of very fine air bubbles in a concrete. This increases the resistance of the concrete to freezing and to the action of calcium chloride or common salt spread on concrete roads to melt snow and ice. The air-entraining agent can alternatively be added as a separate ingredient to the mix when making the concrete. Low-alkali cements are portland cements with a total content of alkalies not above 0.6 percent. These are used in concrete made with certain types of aggregates that contain a form of silica that reacts with alkalies to cause an expansion that can disrupt a concrete. Masonry cements are used primarily for mortar. They consist of a mixture of portland cement and ground limestone or other filler together with an air-entraining agent or a water-repellent additive. Waterproof cement is the name given to a portland cement to which a water-repellent agent has been added. Hydrophobic cement is obtained by grinding portland cement clinker with a film-forming substance such as oleic acid in order to reduce the rate of deterioration when the cement is stored under unfavourable conditions. Oil-well cements are used for cementing work in the drilling of oil wells where they are subject to high temperatures and pressures. They usually consist of portland or pozzolanic cement (see below) with special organic retarders to prevent the cement from setting too quickly.

Slag cements.

The granulated slag made by the rapid chilling of suitable molten slags from blast furnaces producing pig iron, forms the basis of another group of constructional cements. A mixture of portland cement and granulated slag, containing up to 65 percent slag, is known in the English-speaking countries as portland blast-furnace (slag) cement. The German Eisenportlandzement and Hochofenzement contain up to 40 and 85 percent slag, respectively. Mixtures in other proportions are found in French-speaking countries under such names as Ciment Portland de Fer, Ciment métallurgique mixte, Ciment de haut fourneau, and Ciment de liatier au clinker. Properties of these slag cements are broadly similar to those of portland cement, but they have a lower lime content and a higher silica and alumina content. Those with the higher slag content have an increased resistance to chemical attack.

Another type of slag-containing cement is a supersulfated cement consisting of granulated slag mixed with 10 to 15 percent hard burned gypsum or anhydrite (natural anhydrous calcium sulfate) and a few percent of portland cement. The strength properties of supersulfated cement are similar to those of portland cement, but it has an increased resistance to many forms of chemical attack. Pozzolanic cements are mixtures of portland cement and a pozzolanic material that may be either natural or artificial. The natural pozzolanas are mainly materials of volcanic origin but include some diatomaceous earths. Artificial materials include fly ash, burned clays, and shales. Pozzolanas are materials that, though not cementitious in themselves, contain silica (and alumina) in a reactive form able to combine with lime in the presence of water to form compounds with cementitious properties. Mixtures of lime and pozzolana were used by Roman engineers 2,000 years ago and still find some application but largely have been superseded by the modern pozzolanic cement. Hydration of the portland cement fraction releases the lime required to combine with the pozzolana.

High alumina cement.

High alumina cement is a rapid-hardening cement made by fusing at 2,732 {degree}

to 2,912

F (1,500

to 1,600

C) a mixture of bauxite and limestone in a reverberatory or electric furnace or in a rotary kiln. It also can be made by sintering at about 2,282 {degree}

F (1,250

C). Suitable bauxites contain 50 to 60 percent alumina, up to 25 percent iron oxide, not more than 5 percent silica, and 10 to 30 percent water of hydration. The limestone must contain only small amounts of silica and magnesia. The cement contains 35 to 40 percent lime, 40 to 50 percent alumina, up to 15 percent iron oxides, and preferably not more than about 6 percent silica. The principal cementing compound is calcium aluminate (CaO {dot}

). This cement gains a high proportion of its ultimate strength within 24 hours and also has a high resistance to chemical attack. It also can be used in refractory concretes for furnaces. A white form of the cement, containing minimal proportions of iron oxide and silica, has outstanding refractory properties.

Expanding and nonshrinking cements.

Expanding and nonshrinking cements expand slightly on hydration, thus offsetting the small contraction that occurs when fresh concrete dries for the first time. Expanding cements were first produced in France about 1945, but their present manufacture is confined largely to the U.S. and Russia. The U.S. type is a mixture of portland cement and an expansive agent made by clinkering a mix of chalk, bauxite, and gypsum.

Other cements.

Gypsum plasters are used for plastering, the manufacture of plaster boards and slabs, and in one form of floor-surfacing material. These gypsum cements are mainly produced by heating natural gypsum (CaSO {sub 4}

O) and dehydrating it to give calcium sulfate hemihydrate (CaSO {sub 4}

1/2 H

O) or anhydrous calcium sulfate. Gypsum and anhydrite obtained as by-products in chemical manufacture also are used as raw materials. The hemihydrate (plaster of paris) sets within a few minutes on mixing with water; for building purposes a retarding agent, normally keratin, a protein, is added. The anhydrous calcium sulfate plasters are slower setting, and often another sulfate salt is added in small amount as an accelerator. Flooring plaster, originally known by its German title of Estrich Gips, is of the anhydrous type.

Magnesium oxychloride (Sorel cement) is the cementing agent in magnesite flooring. This is made from a mixture of lightly burned magnesia, magnesium chloride solution, and an inert filler. Magnesium oxysulfate is similarly used as a binder in some wood-wool slabs.

Plastics cements find a use in construction as binding materials. Epoxy resin cements are used as a jointing material between structural units, as binders for the repair of concrete, and in protective surface coatings. Polyester and some other resins are similarly used. Synthetic polymer emulsions such as polyvinyl acetate and acrylic resins, natural rubber latex, and bituminous emulsions find use as additives to concrete mixes for flooring, jointing, and lining where special properties are required.

There are many other materials--protein glues, vegetable gums, rubber solutions and emulsions, and synthetic resins solutions and emulsions--that are used as adhesives in thin films; but these are not classed as constructional cements.