Gypsum-anhydrite reaction

Figure 20.5. Effect of NaCl concentration on the equilibrium temperature of the anhydrite-gypsum reaction at 1 bar. Data from Ref. 16.

Calcium Sulfate. [CAS 10101-41-4], Gypsum. CaS04 2H 0. plaster of Paris. CaSOj HiO anhydrite CaSOj. white solid, slightly soluble (about 0.2 g per 100 ml of H 0), formed by reaction of soluble calcium salL solution with a sulfate solution. pK,p of CaS04 = 4.62s- See also Anhydrite Gypsum and Calcium Sulfate. 

When the activity of each species in a reaction is known, we can determine the temperature (or temperatures) at which the reaction is in equilibrium. As an example, we calculate the temperature at which gypsum (CaS04 2 H2O) dehydrates to form anhydrite (CaS04). The RXN commands... 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO3 and the... 

Sulfate is principally derived from the weathering of CaS04 minerals (gypsum and anhydrite) in sedimentary rocks. Some sulfate in rivers, however, comes from the weathering of magnesium sulfate salts in sedimentary rocks and from oxidation of sulfides (primarily FeS2. pyrite) in sedimentary and crystalline rocks. The latter process also liberates small amounts of the cations Ca2+, Mg2+, Na+, and K+ by reactions like ... 

Effect of Climatic Conditions on Plaster Composition. The temperature and humidity of a tomb determine whether the gypsum form or the orthorhombic anhydrite form is most thermodynamically stable. At 28 °C and 25% RH, orthorhombic anhydrite is the stable form in the tomb of Nefertari (38), Early measurements of the temperature and humidity in six royal tombs in the Valley of the Kings indicate that the climatic conditions in all tombs inspected also favor the formation of orthorhombic anhydrite (45), Although Horemheb s tomb was not one of those studied, it was built at the same depth, in the same rock, and with a plan similar to that of the tomb of Seti I for which measurements of temperature and humidity exist (20 °C and 40% RH at the door and 25 °C and 34% RH at the far end) (45). Therefore, on the basis of this evidence alone, we expect that the climatic conditions within Horemheb s tomb also favor the formation of orthorhombic anhydrite from the original plaster. On the basis of our analyses, this reaction did not occur as completely as it did in the tomb of Nefertari (see previous section). 

Effect of Impurities on CaS04 Transformation. The transition from gypsum to orthorhombic anhydrite is slow but occurs even at ambient temperatures (44). The relatively large concentration of finely divided sodium chloride present in the plaster in the tomb of Nefertari may have facilitated the dehydration process. The presence of a hygroscopic material, such as sodium chloride, can help promote dehydration reactions. Also, impurities within the lattice of a crystalline structure can weaken the lattice (46, 47) and thereby accelerate thermodynamically favored reactions. These points suggest a strong correlation between the extent to which the dehydration reaction proceeds and sodium chloride concentration, but they do not exclude the possibility that dehydration can take place in the absence of salt. 

In addition to ion exchange with rock surfaces, alkali can react directly with specific rock minerals. When divalents, Ca and Mg ", exist, alkali will react with them and precipitation can occur. One example is the incongruent dissolution of anhydrite or gypsum in the rock to produce the less soluble calcium hydroxide (CaS04(s) -F NaOH Ca(OH)2(s) + Na2S04). Another simple example is Ca -F COs " CaC03(s). Alkali can also dissolve other minerals from a rock, for example, silica. These reactions could cause plugging. 

In the presence of alkali metals, thermal sulfate reduction will result in precipitation of carbonate cements (mainly calcite and dolomite) or carbonate replacement of dissolving sulfates (gypsum/anhydrite) (2. The reaction of polysulfides with bicarbonate has been suggested as a cause of calcite precipitation. Further, transition and base metals present in formation waters during thermal sulfate reduction could lead to the deposition of disseminated or stratiform base... 

When plaster of Paris or y-anhydrite is mixed with water, the reverse of the above reactions occurs water is reabsorbed with the formation of gypsum -... 

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

This reaction is linked with a decrease of volume however, as the crystalhzation of gypsum is taken into account, the overall volume is increasing and can be estimated as 18 cm /mole of AFm. But the crystallization of gypsum is less probable in the concentrated solution of chlorides and rather the formation of anhydrite will be more probable, with the decrease of total volume. These examples are showing that the effect of chloroaluminate hydrate on the formation of microcracks and deterioration of concrete is negligible. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

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