Artificial pozzolanas

Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH- to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

Diatomaceous earth is composed of the siliceous skeletons of microorganisms. It is pozzolanic, but its use in concrete is much restricted by its very high specific surface area, which greatly increases the water demand. Some clays react significantly with lime at ordinary temperatures, but while this property can be of value for soil stabilization, their physical properties preclude their use in concrete. Many clay minerals yield poorly crystalline or anrorphous decomposition products at 600-900 C (Section. 3.3.2), and if the conditions of heat treatment are properly chosen, these have enhanced pozzolanic properties. Heat-treated clays, including crushed bricks or tiles, can thus be used as pozzolanas in India, they are called surkhi. Other examples of natural rocks that have been used as pozzolanas, usually after heat treatment, include gaize (a siliceous rock containing clay minerals found in France) and moler (an impure diatomaceous earth from Denmark). The heat-treated materials are called artificial pozzolanas, and this term is sometimes used more widely, to include pfa. 

The Romans replaced also the natural pozzolana by the ground roofing-tiles, bricks and porcelain. Lea states that the name cement in the Late-Latin or Old-French languages was for the first time used to determine the materials which now are called artificial pozzolanas [1]. Later on this name was used for mortar produced from three components, and only recently the to-day mining was adopted. 

In Poland the artificial pozzolana i.e. fly ash is the most important among the pozzolanic additions, application of which in cement production is rational both from the economic and ecological reasons. 

Table 7.1 summarizes the typical composition and properties of a representative series of mineral additions used in blended cements. Binders based on granulated blast furnace slag and related materials are discussed in mote detail in Chapter 8, and those based on natural and artificial pozzolanas in Chapter 9. Cements containing calcium caibonate ate discussed in section 2.2.14. 

Unlike natural pozzolanas, artificial pozzolanas may be produced from inactive starting materials, or those with a limited reactivity, by thermal treatment. The required temperature varies in different materials, and ranges between about 400 and OfiO C. 

As in the pozzolanic reaction free calcium hydroxide is consumed, and is replaced by phases of extremely low water solubility lime mortars combined with natural or artificial pozzolanas attain a high degree of durability and water resistance if allowed to be precured for a sufficiently long time. Thus, not surprisingly, many stractures built in ancient times—especially by the Romans—using these binders have been preserved until the present day, even when constracted to be used as aqueducts. 

Galal, A.F. et al. (1990) Hydraulic reactivity and microstructure of artificial pozzolana-lime pastes, iaProceedings 12th International Conference on Cement Microscopy, Vancouver, pp. 135-153. 

Norsy, M.S., Galal, A.F., and Abo-El Enein, S.A. (1998) Effect of temperature on phase composition and microstructure of artificial pozzolana-cement pastes containing burnt kaolinite clay. Cement and Concrete Research 28, 1157-1164. 

Pozzolana. A material that, when ground and mixed with lime and water, will react with the former to produce compounds having hydraulic properties. There are both natural and artificial pozzolanas. The original Natural Pozzolana was a volcanic tuff worked at Pozzoli (or Pozzuoli), Italy, in Roman times. One of the principal types of Artificial Pozzolanas is produced by firing clay or shale at 6fJtt-1000°C. 

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