Hydratesare composed with water of crystallization in their structures.
When a hydrateis heated, it absorbs enormous quantity of heat (endothermic) and formsanhydrous mineral. When an anhydrate is immersed into water; it absorbs waterand releases enormous quantity of heat transforming into a hydrate mineral 6.In other way, it can be expressed that a hydrate is formed by releasingenormous quantity of heat from its anhydrous product. For an example; theformation snow from freezing water releases heat and snowfalls warms up theatmosphere may be cited. The heat released into the pore space might promotefurther evaporation of pore fluids. Most of the hydrates are stable and solublein water at room temperature.
Some hydrates spontaneously loss water ofcrystallization by efflorescence. Others absorb water into their structureforming hydroscopic hydrates. Some deliquescent mineral like sodium hydroxideabsorb huge quantities of water and form as liquid. The decomposition of carbohydratesgenerally releases water. Thus water of crystallization in a hydrateminerals play critical role on their changes in their specific gravities and inturning their volumes 7.
Hydration is not a reversible reaction;however, the environment crystallization of hydrates plays critical role forthe formation of hydrates and anhydrates 8. The repeated hydration and dehydration changes the volume of salineminerals which in turn affects the volume of pore spaces and hairline cracksare induced. Most pores are partially or completely filled with saline porefluids and repeated influxes of saline fluids and evaporations play criticalrole on the evolution of saline precipitates 9. The evaporation of saline fluids precipitatessaline minerals initially at peripheral portions of saline droplets inside thepores.
The ionic components of chloride, carbonate and sulphate, hydroxide andwater play critical role in the formation of mixed crystals of mineralcomponents rather than individual minerals. The scope of the investigationmainly lies to trace the trend of changes of chemical composition duringsuccessive crystallization of chloride components from the primary carbonatesource. Generally, the composition of groundwater is enriched with bicarbonatesand depleted in carbonates. Similar condition is true in the case of salinewaters interlocked in the pore spaces of lime-mortars.
Ikaite(CaCO3.6H2O) has recently been reported from the polarice sediments both in Antarctic and Artic Oceans 10-12. The calcium carbonate has three polymorphs (aragonite, calcite andvaterite) and three other hydrous phases (mono hydrocalcite, ikaite andamorphous calcium). The solution super saturation is a key factorcontributing to the stabilization of the polymorphic phases of calciumcarbonate 13. The Super saturation ratio of carbonate polymorphs increaseswith pH. The rapidly changing supersaturation during the course of calcium carbonate precipitation, play criticalrole on the growth and nucleation of ikaite formation under favourableenvironment. The study of precipitation and crystallization of calciumcarbonate polymorphic phases is rather complex and not much research is doneregarding their stability condition nucleation and crystal growth 14 and 15.The water enriched with Ca2+ and HCO3- maycontinue to flow through the void and capillary pore spaces.
When the waterenters these voids; pore spaces partial pressure of CO2 decreases and CO2 isreleased. Thedegassing of CO2 drives the precipitation of carbonate minerals. Inpore spaces resembling a closed system, CO2 and air cannot migratein and out of the system. Therefore, the total carbonate concentration will beconstant (the concentration of HCO3 as a function of pH). Thebreathing effect of pore spaces will be stopped over time. The influx ofbicarbonate water produces carbonate ions. HCO3- + H+ <--------> CO32-+ H+ ……………………………………………..
(Equation 1) Ca(HCO3)2(aq) ——–> CO2 (g) + H2O(l) +CaCO3 (s)……………………………(Equation 2)The Hydrogen ions are molecularions with formula H3O+(H2O). A concentrationof hydrogen ions lower than 10-7 is alkaline comprises more of OHions. The reactionbetween the hydroxide ion and the hydrogen ion removes the hydrogen ion fromthe solution, making the solution less acidic and more basic. At an atmospheric of pressure of one, pure CO2gas over distilled water may yield asolution with a pH near 3.
6 16. With the increasing pressure pH valuedecreases further and the solubility of solution increases considerably. Withlarge amount of dissolved Ca, pH value moves to the side of alkalinity and morequantity of Ca will be precipitated at the prevalence of optimum environment.
According to Elfiland Roques 14 two hydrated forms of amorphous calcium carbonate andmonohydrate calcium carbonate are precursors of calcite precipitation. Under water rich environment at lowtemperature <20oC and pressure around <4 kbar, ikaiteprecipitates from CaCO3.6H2O 10 concentrated seepagewaters. A reversible reaction would form at favourable thermodynamic condition:CaCO3. 6H2O <--------> CaCO3+6H2O…………………………………..…(Equation 3)ikaite aragoniteThe presence significant amount of normativesilicon and aluminum carbonates indicate that the pore fluids are subjected tohigh pressure environment 17. Similar to ikaite, kieserite (MgSO4.
H2O),epsomite (MgSO4.7H2O) and mirabiite (MgSO4. 10H2O) form at decreasing concentration of Mg and increasingconcentration of SO4 ions in the saline solution 18. The evaporation of saline fluids precipitatessaline minerals initially at peripheral portions of saline droplets inside thepores. Identification ofsaline hydrates in the pore spaces is rather difficult. They usually occur inthe form of mixtures of one or two forms minerals.
If the mineral phases areidentified more quantitative information may be obtained about the sequences ofcrystallization of evaporates and the minerals filled or partially filled inthe pore spaces. The cavities and porespaces >10?m fluid flows by gravitational forces and the pores less than 1?mgenerally adsorption process takes place 19. Though all these pores occur inthe lime plaster, the pores of sizes between 1 and 10?m play vital role in theprecipitation /deposition of saline minerals in pore spaces.
Though pore spacesin the heritage structure improve advantageous thermal and acoustic barrier,they also have some disadvantages by their groundwater and repeatedprecipitation and evaporation of fossil fluids forming hydrates and anhydratesinducing cracks.