Calcium carbonate, reaction interface

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Other reactions of probable less importance than those above leading to undersaturated conditions with respect to calcium carbonate near the sediment-water interface include nitrate reduction and fermentation (e.g., Aller, 1980). Such reactions may also be important near the sediment-water interface of continental shelf and slope sediments, where bioturbation and bioirrigation can result in enhanced transport of reactants. Generally, as water depth increases over continental slope sediments, the depth within the sediment at which significant sulfate reduction commences also increases. It is probable that the influence of reactions other than sulfate reduction on carbonate chemistry may increase with increasing water depth. 

Reddy M.M. and Gaillard W.D. (1981) Kinetics of calcium carbonate (calacite)-seeded crystallization Influence of solid/solution ratio on the reaction rate constant. J. Colloid Interface Sci. 80, 171-178. 

Dissociation of Calcium Carbonate A number of studies of this decomposition have concluded that Ea is similar to the dissociation enthalpy for equilibrium conditions, about 173 kJ mol 1. A study of the reaction in vacuum, below I 0-2 Pa (17), however, measured the activation energy as 205 kJ mol-1, or about 30 kJ mol-1 greater than the enthalpy of the dissociation process these values have been supported by subsequent studies (78). This study investigated the reaction between 934 and 1013 K through measurements, by a quartz microbalance, of the constant rate of mass loss during inward interface movement from a single flat reactant crystal face. 

Nakatsuka, T, Kawasaki, H., Itadani, K., and Yamashita, S. (1981) Topochemical reaction of calcium carbonate and alkyl dihydrogenphosphate. /. CoUoid Interface Sci 82 (2), 298-306. 

Stability of Acid-in-Diesel Emulsions. Al-Anazi et al. [14] showed that acid-in-diesel emulsion is stable for more than three days at room temperature. However, at high temperatures it breaks down and an aqueous (acidic) phase was noted at the bottom of the test tube. Figure 4 depicts the volume of the separated aqueous phase as a function of time at 96 °C. The aqueous phase first appears after 85 minutes. The volume of separated acid gradually increases until complete phase separation occurs after nearly 220 minutes. In the presence of reservoir rock, the aqueous phase appears after approximately 20 minutes, and complete phase separation takes place after an hour. These results indicate lower emulsion stability in the presence of calcite. The acid reaction with the carbonate rock produces water (which causes the pH to rise) and calcium chloride. It appears from these results that the surfactant moves away from the acid-diesel interface as the pH or ionic strength increases, which causes the emulsion to break. 

These techniques are bas not only on the principle that lead-containing phosphates with the apatite structure are highly insoluble, but also that rapid reactions occur with apatite and lead ions at the sohd/aqueous solution interface [12, 13, 15, 20, 29, 48, 53, 56]. Removal of lead from aqueous solutions using synthetic hydroxyapatite gives aqueous lead concentrations below the maximum contamination level after Ih [12, 53]. Other workers [9] observed the formation of calcium-lead apatite solid-solutions after 3 mins contact between synthetic hydroxyapatite and aqueous solutions containing lead, and no lead was detected in the aqueous solution after 24 h contact. However, the efficiency of lead removal depends on the characteristics of the phosphate rock employed [15]. It has been shown that the composition and crystallinity of the phosphate influence the speed of the surface reactions [4, 44]. More highly crystalline solids have lower solubilities and dissolution rates, making the apatite less reactive [4]. The presence of fluoride in the hydroxyapatite structure decreases its solubility and dissolution rate, while the presence of carbonate decreases structural stability, and increases solubility and the dissolution rate [4, 35]. 

The pore solution is effectively neutralized by this reaction. Carbonation damage usually appears as a well-defined front parallel to the outside surface. Behind the front, where all the calcium hydroxide has reacted, the pH is reduced to around 8, whereas ahead of the front, the pH remains above 12.6. When the carbonation front reaches the reinforcement, the passive film is no longer stable, and active corrosion is initiated. Figure 1.14 shows that active corrosion is possible at the reduced pH level. Damage to the concrete from carbonation-induced corrosion is manifested in the form of surface spalling, resulting from the buildup of voluminous corrosion products at the concrete-rebar interface (Fig. 1.15). 

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