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under the same conditions takes 438 years to dissolve completely. One should also remember that chrysotile being a sheet silicate with a misfit between the octahedrally coordinated magnesiumcontaining layer and the tetrahedrally coordinated silica layer curls up into a tube forming fibrils with the octahedral layer on the exterior (see Section 2.5.2 on Surface Area and Fig. 2.11). Thus the dissolution of a mineral of this structure can be quite different from one that forms a uniform chemistry across a solid fiber.

Concrete, Cement, Limestone, Marble, and Clay

All the building materials contain some lime, several being mostly calcium carbonate (limestone and marble). Thus these calcareous materials are attacked by acids. Generally, any aqueous liquid with a pH<6 will exhibit some attack. The corrosion of concrete takes place by the leaching of water-soluble salts that are formed by the reaction of the acid and the calciumcontaining compounds of the concrete [5.41]. For example, the reaction of calcium carbonate with SO2 containing acids* forms calcium sulfate or gypsum. The gypsum, being much more soluble than the carbonate, is then washed away. Run-off water has been shown to be more important than the pH of the solution for pH>3. This causes etching of the surface in addition to pitting and scaling. Webster and Kukacka gave the following mechanisms for the dissolution of concrete:

1.Dissolution of hydrated cement compounds

2.Dissolution of anhydrous cement compounds

3.Dissolution of calcareous aggregates in the mix

4.Deposition of soluble sulfate and nitrate salts

5.Formation of new solid phases within pores

6.Production of stresses from numbers 4 and 5

*Sulfuric acid has been reported to make up 60–70% of the acidity in acid rain for the northeastern United States.

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Generally, a combination of the above parameters is operative. In addition, there is the deterioration of any reinforcing steel.

The most common soluble salts that are involved in efflorescence are the alkali and calcium and magnesium sulfates, chlorides, nitrates, and carbonates. Sodium sulfate generally leaves a fluffy deposit behind, whereas gypsum deposits are more adhesive and scaly. The chlorides are much more mobile in solution than the sulfates and therefore tend to penetrate deeper into the corroding material. The disruptive action of these soluble salts is mostly through hydration-dehydration- rehydration. Thus the damage caused by these salts results from their having several hydration states. Therefore, sodium chloride is not applicable to this type of disruption because it expands only during dehydration [5.42]. Sodium exists in two hydrate forms, the monoand the deca-hydrate. Amoroso and Fassina [5.42] reported that the mono-hydrate exists only above 32°C, with the deca-hydrate being the stable form below that temperature. A hepta-hydrate may also exist at intermediate temperatures.

Amoroso and Fassina discussed the mechanism of limestone deterioration by sulfate pollutant attack. This deterioration starts by the deposition of either dry or wet SO2. They discussed the effects of atmospheric conditions (i.e., wind, etc.) and the limestone surface characteristics upon the degree and rate of deterioration. At low relative humidity, the reaction favors the formation of calcium sulphite according the following equations:

(5.7)

or

(5.8)

When water is present, the sulfite is oxidized to the sulfate according to:

(5.9)

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This later reaction is one thought to be not of major consequence since a large amount of water would not allow a sufficient deposition of dry SO2 to occur. Oxidation of SO2 in an atmospheric aqueous phase (i.e., rain, fog, etc.) to sulfuric acid that will then react with the limestone to form gypsum is most likely the predominant mechanism of deterioration.

Rain water can dissolve carbon dioxide of the atmosphere forming the weak carbonic acid:

(5.10)

This then becomes dissociated into and , which is controlled by the carbonic acid and hydrogen ion concentrations. If the CO2 concentration increases, increasing the concentration of carbonic acid, the hydrogen ion concentration must also increase. This, in turn, will alter the pH of the solution. The weak carbonic acid solution reacts with the carbonates of limestones, marbles, and mortars to form the more soluble bicarbonates. Amoroso and Fassina [5.42] have reported that calcium bicarbonate is about 100 times more soluble than the carbonate. The presence of impurity ions will influence what phase precipitates from the solution. Kitano [5.43,5.44] reported that the presence of Mg2+, Sr2+, and ions supports the formation of aragonite rather than calcite.

Nitrogen oxides when in contact with water form nitrous and nitric acids. Although nitric acid is a weaker acid than sulfuric acid, it can cause more extensive damage to concrete because it reacts with Ca(OH)2 to form the more soluble calcium nitrate.

The deterioration of brick masonry is a combination of corrosive attack upon the brick and the mortar that is used to bond the brick together. Mortars are generally of a calcareous type, and as such, their corrosion is very similar to that described above for concrete and limestone. The actual corrosion of the mortar depends upon the type used: claybased, lime sand, or Portland cement. The most susceptible to acid rain are those containing calcium of one form or another. The formation of gypsum from the reaction of acid rain upon

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calcium carbonate produces local stresses sufficient mechanically to disrupt the mortar. Any calcium hydroxide present from the hydration of Portland cement is especially susceptible to acid attack. Additional reactions such as the reaction of calcium sulfate with tri-calcium aluminate forming ettringite (3CaO·Al2O3· 3CaSO4·32H2O) also produces large local stresses because of the associated expansive crystallization.

It has been noted that 12th century Venetian bricks were more durable than modern ones, although they were fired at lower temperatures. This has been attributed to a very uniform microstructure, the lack of a highly vitrified surface, and a larger pore size (avoiding the <1-µm critical pore size) when compared to modern bricks [5.45]. Bricks may also contain Na2SO4 if fired in a kiln using sulfur-rich fuel and at a sufficiently low temperature. The sodium sulfate, dissolved by water, can recrystallize after evaporation forming the anhydrous salt, thenardite, or the decahydrate, mirabilite. The specific environmental conditions of temperature and relative humidity will change the degree of hydration and the amount of mechanical disruption.

Zirconia-Containing Materials

The hydrothermal effect of water upon the dissolution of yttria (14 mol%)-stabilized zirconia (YSZ) single crystals was investigated by Yoshimura et al. [5.46]. They found four regimes of behavior for YSZ treated at 600°C and 100 MPa for 24 hr, depending upon the pH of the solution. In alkali solutions (those containing LiOH, KOH, NaOH, or K2CO3), partial decomposition and dissolution/precipitation were found, with yttria being the more soluble component. In acidic solutions (those containing HCl or H2SO4), rapid dissolution of yttria occurred forming an interface of polycrystalline monoclinic ZrO2. In reactions with H3PO4 solution, the interface layer formed was ZrP2O7. In neutral solutions, the dissolution was minimal.

Copyright © 2004 by Marcel Dekker, Inc.