HCO3- OH ions. The reaction between the hydroxide ion

HCO3- + H+                              <-------->                           CO32-+ H+ ……………………………………………..(Equation 1) Ca(HCO3)2(aq)                 ——–>                           CO2 (g) +H2O (l) +CaCO3 (s)……………………………(Equation2)The Hydrogen ions are molecularions with formula H3O+(H2O).

A concentrationof hydrogen ions lower than 10-7 is alkaline comprises more of OHions. The reaction betweenthe hydroxide ion and the hydrogen ion removes the hydrogen ion from thesolution, 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.

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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.