Pozzolanas

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

Fig. 20.1. A pozzolana cement. The lime (C) reacts with silica (S) in the ash ta give a banding layer af tabamarite gel C3S2H3.

Concretes are cements containing a large proportion of gravel. Hydraulic cements are cements that set (harden) in wet environments, as required when building structures submerged in water. Like all other cements used in ancient times, hydraulic cements were also composite materials in which one particular component, such as pozzolana in ancient Rome (see text below), endowed the cement with the property of setting in wet environments (Gani 1997 Akroyd 1962). 

Celsius (°C) A scale for measuring temperatures, also known as the centigrade scale, where the freezing point of water is 0°C and the boiling point 100°C. cement A natural or artificial fluid or semifluid substance, or mixture of substances, that hardens to act as an adhesive for binding solid surfaces together, cement, hydraulic A type of waterproof stony cement that sets even under water see pozzolana. 

Stokely, M. E. Newland, L. 1992. Pozzolana-induced reductions in the environmental availability of cadmium from fly and bottom ash. Chemosphere, 24, 1591-1596. 

The Romans pioneered the use of hydraulic, or water-cured, cement. Its unique chemical and physical properties produced a material so lasting that it stands today in magnificent stmctures like the Pantheon. Yet the formula was forgotten in the first few centuries after the fall of the Roman Empire and wasn t rediscovered until 1824 as Portland cement. One Roman version was based on a burned mixture of two major components volcanic ash—called pozzolana—from Mount Vesuvius, which destroyed Pompeii and nearby towns in... 

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. 

Various types of cement and pozzolanas (e.g., coal burning fly ash, lime, blastfurnace slag and similar materials) are mostly used as the stabilizing matrix. That stabilization technique is used for the immobilization of inorganic or organic waste. 

Pozzolanic activity of the used fly ash sample (Table 6), determined by the test with the hydrated lime according to the mentioned standard, is for the P5 pozzolana class which means that it is convenient for the cement production as the mineral admixture [38-41]. 

Most natural pozzolanas are of volcanic origin, though some are sedimentary. Some clays and other materials that are unsuitable for use in concrete in their natural state become usable as pozzolanas if heat treated. Both natural pozzolanas and heat-treated materials have been used with lime since ancient times, but today they are mainly used as constituents of pozzolanic cements. Several reviews are available (M81,M82,M83). 

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. 

Early studies, reviewed by Malquori (M81), showed that natural pozzolanas take up CH, including that produced by Portland cement, with the formation of products similar to those formed on hydration of the latter material. They also showed that the zeolites present in many of them were at least as reactive in this respect as the glassy constituents. Zeolites are cation exchangers, but the amounts of CaO they take up are much greater than can be thus explained moreover, cation exchange could not explain the develop-... 

Table 9.7 Chemical compositions of some natural pozzolanas... 

Costa and Massazza (C44) concluded from a study of natural pozzolanas of varied types that reactivity in mixtures with CH at w/s = 2 and 40 C depends during the first 28 days on the specific surface area and at later ages on the contents of Si02 and AI2O3 in the active constituents. A comparative study of five natural pozzolanas and three low-CaO pfas in pastes with cement showed that the CH contents of the pozzolanic cements were considerably lower than those of the pfa cements at 3-60 days, but virtually the same at 90 days, the pozzolanas thus appearing to react more rapidly than the pfas at early ages but more slowly later. Determinations of the unreacted mineral admixture in pastes with CH showed that at 90 days 23-30% of the natural pozzolana had reacted, compared with 11-15% for the pfas. The similarity in CH contents suggests, however, that these values may not apply to mixtures with cement. 

X-ray microanalyses of CjS-pozzolana pastes showed a gradual decrease in Ca/Si ratio on passing from regions near the iinreacted CjS to ones near the pozzolana (021). TMS studies of pastes of C3S, (3-C,S and cement with and without natural pozzolanas (U9,M44) showed that, in the presence of the latter, formation of polymeric anions is accelerated and their mean molecular weight is increased. TMS results and determinations of combined water showed that the hydration of P-CjS is almost completely suppressed in the presence of a pozzolana and that, in pastes with CjS, 16-29% of the pozzolana had reacted in 180 days (M44). The eflect on P-CjS hydration is similar to that found using QXDA for pfa (D12). Chemical extraction showed 10-45% of the pozzolanas in pastes with C3S to have reacted in 28 days, compared with under 10% for a pfa (U9). 

Kollek el al. (K68) extended the parallel studies on expansion and pore solution compositions mentioned in Section 12.4.2 to mortars containing pfa, slag or natural pozzolanas. For each mix, several contents of the... 

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