Hydration of anhydrite

Hydration is the incorporation of water mole-cule(s) into a mineral, which results in a structural as well as chemical change. This can drastically weaken the stability of a mineral, and make it very susceptible to other forms of chemical weathering. For example, hydration of anhydrite results in the formation of gypsum ... 

The reverse reaction, the hydration of anhydrite, has been observed in cores from regions where the most recent geologic event has been erosion (Murray, 1964). 

Hydration of anhydrite is, perhaps, the simplest reversible substitution type. The result of anhydrite hydration is gypsum according to reaction... 

The rate of hydration differs in the various calcium sulfates whereas calcium sulfate hemihydrate and anhydrite-111 hydrate rapidly, the hydration of anhydrite-II progresses... 

Calcitization of the sulphates can be a multi-stage process (Scholle et al., 1992), which begins with (1) dissolution (or at least corrosion) of anhydrite, (2) hydration of anhydrite and gyjjsum formation, (3) dissolution of gypsum (this process can be accompanied by the formation of collapse breccia), and afterwards (4) precipitation of calcite inside free spaces and pores arisen after leached sulphates. Sometimes sulphur is the secondary product of calcitization of sulphates (see fig. 18.). 

Apparent decrease of Sr concentration occurs during rock transformation in the op>en system with unbounded circulation of the solution in free pore spaces, whereas the residual products of these transformations are often enriched in strontium. During the hydration of anhydrite, gypsum shows limited ability of Sr ions incorporation into its crystal lattice and is not able to incorporate them completely. Dissolution and recrystallization purify gypsum and anhydrite from impurities, and activate strontium lowering its content in newly created mineral comparing to the primary mineral, i.e. some secondary gypsums from Wapno Salt Dome consist only 159 ppm Sr (Jaworska Ratajczak, 2008), primary anhydrite from which it has been created consist 1700 p>pm Sr. 

Farnsworth M. 1925 - The hydration of Anhydrite. Industrial and Eneineering Chemistry, 17,9 %7-970. 

Sievert T., Wolter A. Singh N.B. 2005 - Hydratation of anhydrite of gypsum (CaSO4.II) in ball mill. Cement and Concerete Research, 35 623-630. 

Previous studies on hydration of anhydrite were summarized and discrepancy in the mechanism of anhydrite hydration between different researchers was analyzed. In our present study, the double salt mechanism by Budnikov and the z/r ratio law by Murat et al were examined based on investigation of the formation and hydration of sulfate calcium double salt. Our results showed that most of the soluble sulfate compounds can excite the hydration of anhydrite. However, only part of the sulfate reacted with anhydrite and formed double salts at ambient temperature, and the rate of reaction between sulfate and anhydrite was less than that between the sulfate and gypsum, this argues against the double salt mechanism by Budnikov. During the hydration process, crystallization of hydrate sulfate exerts influence on hydration of sulfate calcium instead of the z/r ratio of excitation agent. Hydration of anhydrite in fact follows the path of dissolntion-nucleation-ciystallization. Soluble sulfate compounds decrease the surface potential barrier of nucleation and act as the catalytic agent of nucleation. 

Meehanisms of hydration of anhydrite, especially the role of excitation agents in the hydration process, have been extensively investigated. It was generally accepted that excitation agents could exert great effect on hydration of anhydrite, however, the hydration process is still in large debate. Mainly two models of hydration of anhydrite were proposed to explain the hydration process, namely dissolution-crystallization and double salt models. 

According to the nucleation-growth theory (Murat et al.), the salt with a z/r value larger than that of Ca should restrain the hydration of anhydrite. However, Yang et al l found that the Cd, Cu, Mn, and Fe sulphates were able to excite the hydration of anhydrite, while those with a z/r value less than that of Ca such as LF, Na, and K showed different excitation properties when combining different anion, suggesting that during the hydration of anhydrite the influence of anion should be taken into consideration. 

The double salt theory of hydration of anhydrite was proposed by Budnikov in 1930s. It was suggested that in water and salt eonditions the surfaces of anhydrite particles formed imstable hydrates (salts mCaS04 nH20), and the hydrates then decomposed into hydrous salt and gypsum, the crystallization of the resultant gypsum resulted in the solidifying of the slurry. The reaction equations can be expressed as follows ... 

At room temperature, those soluble sulphates which could react with calcium sulphate to produce double salt were only Na2S04, K2SO4, Rb2S04, CS2SO4, and (NH4)2S04. The hydration excitation for anhydrous calcium sulphate of these sulphates depended on their concentration and the rate of double salt formation. Table 2 showed the effect of K-sulphate solution with certain concentrations on hydration of anhydrite, and Figure 4 showed the XRD patterns of their hydration products. 

As shown in the Figure 6, the excitation effect of the sulphates was poor at low concentration condition, and the hydration rate of anhydrite increased as the concentration of sulphate raised. All the sulphates showed the similar excitation effect on hydration of anhydrite for 3 days except for that of CdS04. The hydration rate of anhydrite was closely related to the concentration of sulphate crystallization, the lower the concentration of sulphate crystallization, the higher the hydration rate of anhydrite. Obviously, the hydration excitation of anhydrite was related to crystallization of hydrated sulphate. Figure 8 showed that the Ca concentration decreased as the concentration of sulphate increased after hydration of anhydrite in the sulphate solution for 3 days, and in solution of the same concentration value of sulphate, the Ca concentration was basically similar and was less than the equilibrium concentration value of gypsum, suggesting that common-ion effect restrained the dissolution of calcium sulphate. 

The results of XPS analysis suggested that the concentration of the salt when metal cations could be detected by XPS analysis in the product of anhydrite hydration equaled to the one when it speeded up the hydration of anhydrite, suggesting that formation of double salt or hydrated sulphate acted as the heterogeneous nucleus in the hydration process, and nucleation played an important role in hydration of anhydrite. 

All kinds of sulphates could excite the hydration of anhydrite in a certain extent. Table 12 showed that the excitation effect on hydration of anhydrite of different cations was obviously different. However, for the same cation the excitation effect on hydration of anhydrite varied as presence of different anion or multi-cation. It can be seen from Figure 12 that salts with the same cation but different anion had different effect in excitation of hydration of anhydrite. Except for sulphates, hydration rate of anhydrite in other kinds of K-salts showed no obvious change within 60 hours. But after 3 days, the hydration rate showed distinct difference, in a... 

The excitation effect on anhydrite hydration of double cation sulphates was evidently better than that of K2SO4, for this kind of sulphates can release the second cation into the solution and, thus, enhance the excitation for hydration of anhydrite. In the experiments, all the three kinds of double cation sulphates produced hydrated proton when they hydrated in the solution, which increased the acidity of the solution and improved the dissolution of anhydrite so that enhanced the hydration rate of anhydrite. The reaction equations were written as follows ... 

Observation of the hydration process indicated the following. In the first few minutes of the dissolution of free lime, the hydration of anhydrite and hemihydrate and the formation of calcium aluminate hydrate and monosulfate hydrate occur. Ettringite is formed within one hour, monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours, with the maximum heat ofhydration of C3S at about 10 hours. Measurements of the non-evaporable water indicate that the amount of combined water is 60-80% of the theoretically determined total amount of combined water at complete hydration. (The total amount of combined water was estimated to be 36%.) The amount of ettringite in the paste is estimated to be about 18-25% for the period one hourto seven days. The monosulfate content increases from about 10% at six hours to about 15-25%inone day. The alite in Reg Set cement paste is approximately 65-70% hydrated in one day and 8 0-95% hydrated in seven days. It is suggested that fluoride is tied up as A1(0H)2F. The possibility of fluoride-substituted ettringite and the formation of halo-aluminate hydrates of the form ( A CaXj wHjO is conceivable. 

Calorimetry eurves forthe two Jet Seteements deseribed above are shown in Fig. 21. There are four main peaks in the heat evolution curves. The first peak appears irmnediately and is due to the following dissolution of free lime, hydration of anhydrite and hemihydrate, and the formation of C-A-H and monosulfate hydrate. The seeond peak is attributed to the formation of ettringite, the third to the formation of monosulfate hydrate, and the fourth peak to the formation of C-S-H. The overlap of the second and third peaks (cement B) and the larger third peak are attributed to active eonversion of ettringite to monosulfate hydrate. The broader fourth peak (eement B) occurred later indicating a less active formation of C-S-H gel than for cement A. 

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