Limestone surface

Limestone is an ossified form of calcium carbonate, CaCC>3. Limestone surfaces soon become slippery and can be quite dangerous if... 

In wet FGD processes, either DA or limestone slurry, the combined effects of calcium and magnesium actually determine the limestone dissolution rate. Sjoberg s results(fi) indicated that Ca2+ can inhibit the CaCO dissolution rate much more effectively than Mg2+ by the same surface adsorption phenomenon. The combined effects of Ca2+ and Mg2+ can be described as competitive adsorption, and the limestone surface will act as an ion-exchanger. The fraction of surface occupied by adsorbed Ca2+ and Mg2+ can be expressed as ... 

Mossotti, V.G., Lindsey, J.R. Hochella, M.F. Jr. (1987) Effect of acid rain environment on limestone surfaces. Materials Performance 26, 47-52. 

Most analytical techniques require the state of chemical equilibrium. At equilibrium, the rate of a forward process or reaction and that of the reverse process are equal. The photo at left shows the beautiful natural formation called "Frozen Niagra in Mammoth Cave National Park in Kentucky. As water seeps over the limestone surface of the cave, calcium carbonate dissolves in the water according to the chemical equilibrium... 

Plate 4.1 Red bauxite-bearing oxisol (ferralsol) overlying Tertiary limestone (white) in south Jamaica. The soil is piped into solution-enlarged hollows of the limestone surface. Cliff face approximately 6 m high. Photograph courtesy of J. Andrews. 

Sulfur deposition and subsequent remobilization by incident rain for a limestone surface differs from that exhibited by a marble surface. This difference may be the result of larger porosity values of limestone compared to marble. Larger limestone porosity adsorbs a larger portion of incident rain than marble. Thus, fewer low-rain events occur for limestone. Where small rain-amount data are available for both limestone and marble, excess sulfate values consistently are lower for limestone runoff than for marble runoff. This observation suggests that limestone surface-accumulated sulfate is less mobile than marble surface-accumulated sulfate. 

Marble and limestone surfaces were exposed to atmospheric conditions at four eastern U.S. sites and were monitored for changes in surface chemistry, surface roughness/re-cession, and weight. The effect of acid deposition, to which calcareous materials are especially sensitive, was of particular interest. Results are described for the first year of testing, and aspects of a preliminary equation to relate damage to environmental factors are discussed. Thus far, findings support that acid deposition substantially damages marble and limestone surfaces. 

Figure 7 e and f. Results of ion chromatographic analyses of limestone surfaces (e) from control samples not exposed to out-dorr conditions and (f) from back surface of briquette from District of Columbia. 

The results of this investigation show that CaCC>3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO2 hydrolysis reaction. 

By reaction stoichiometry, the flux of calcium from the limestone surface must be equal to the flux of total C02 ... 

Above pH 6.0, CaC03 dissolution is controlled mostly by diffusion of OH- and HCO3- from the limestone surface. The driving force for this diffusion is determined to a large extent by the equilibrium ... 

Figures 5, 6, and 7 demonstrate the effect of organic acids on the dissolution rate at 25°C. Additives that provide buffer capacity between the bulk solution pH (4.0 to 5.5) and the pH at the limestone surface (5.5 to 8.0) enhance dissolution rate by providing an additional means of diffusing acidity to the limestone surface. Figure 5 shows that at pH 5.0, 3 mM total acetic acid enhances the dissolution rate a factor of 7. This enhancement is somewhat greater at higher pH, where H+ diffusion is much more limited.

Organic buffer additives such as acetic, adipic, acrylic, and sulfosuccinic acid enhance the rate of calcite dissolution by providing for mass transfer of acidity to the limestone surface. 

The dissolution of limestone is known to be controlled by both diffusion of ions in solution and surface reaction rates. The pH value influences which of these steps dominates in the limestone dissolution. For example, at pH values less than 5 the diffusion process dominates the dissolution with little dependence on surface reaction. On the other hand, at pH values greater than 7 the reaction at the limestone surface begins to dominate the dissolution process. In the pH range between 5 and 7 both dissolution steps can influence the overall rate. 

An initial and important distinction needs to be made between two microenvironments - exposed and sheltered. Exposed surfaces have water flow across their surfaces either from direct rainfall or from runoff or both. However the magnitude and frequency of this flow may vary between surfaces. Sheltered surfaces do not experience any water flow products of degradation cannot, therefore, be removed from the surface by water flow. This can result in the build-up of a crust of degradation products that could protect the limestone surface from further alteration. On an exposed surface, the build-up of any degradation products is likely to be temporary. Microenvironmental variations in exposure begin to define the type of degradation forms that can be expected to develop on different parts of the building. 

For degradation forms to develop, however, it is necessary for reactions to occur that can alter or cause stress in the stone. Delivery of potential reactants to a limestone surface can be by two pathways, wet or dry. Wet deposition occurs when gaseous and particulate pollutants are incorporated into water droplets, falling as rain. This solution is usually acidic natural rainfall having a about pH of about 5.6 due to weak carbonic acid urban rainfall falls as... 

Karst is a term describing the features produced in limestone and at limestone surfaces by weathering. 

Moropoulou A, Kefalonitou S (2002) Efficiency and countereffects of cleaning treatment on limestone surfaces—investigation on the Corfu Venetian Eortress. Build Environ 37(11) 1181-1191. doi 10.1016/80360-1323(01)00059-2... 

Dissolution rate of limestone in sulfuric acid is compared with that in sulfurous acid in Fig. 3 in which the rates are plotted against acid concentration [4], As seen in this figure, the dissolution rate of limestone is expressed by the mass transfer of sulfurous acid. The difference between sulfuric acid and sulfurous acid is mostly due to the stoichiometry as seen in Eqs. 1 and 2. Additionally, the rate in sulfuric acid is different from that in sulfurous acid in zone m, because the crystallization did not take place on the limestone surface in contrast to the limestone dissolution in... 

The data presented suggest that the primary chemical weathering process occurs by the reaction of SO2, either in gaseous form or dissolved in rain-water, with the limestone surface. This leads to a build up of sulphur rich layer at the surface which is shown to be predominantly S04 ". A more detailed study of this reaction mechanism should allow identification of reaction intermediates such as SO3. The formation of chlorine and nitrogen compounds appears to be a secondary process for the samples studied. The data presented and the results of the experiment described in section 5 suggest that subsequent dissolution of sulphate, by rain-water, is a significant part of the weathering mechanism. 

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