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Hydrates
are composed with water of crystallization in their structures. When a hydrate
is heated, it absorbs enormous quantity of heat (endothermic) and forms
anhydrous mineral. When an anhydrate is immersed into water; it absorbs water
and releases enormous quantity of heat transforming into a hydrate mineral [6].
In other way, it can be expressed that a hydrate is formed by releasing
enormous quantity of heat from its anhydrous product. For an example; the
formation snow from freezing water releases heat and snowfalls warms up the
atmosphere may be cited. The heat released into the pore space might promote
further evaporation of pore fluids. Most of the hydrates are stable and soluble
in water at room temperature. Some hydrates spontaneously loss water of
crystallization by efflorescence. Others absorb water into their structure
forming hydroscopic hydrates. Some deliquescent mineral like sodium hydroxide
absorb huge quantities of water and form as liquid. The decomposition of carbohydrates
generally releases water. Thus water of crystallization in a hydrate
minerals play critical role on their changes in their specific gravities and in
turning their volumes [7]. Hydration is not a reversible reaction;
however, the environment crystallization of hydrates plays critical role for
the formation of hydrates and anhydrates [8]. 
The repeated hydration and dehydration changes the volume of saline
minerals which in turn affects the volume of pore spaces and hairline cracks
are induced. Most pores are partially or completely filled with saline pore
fluids and repeated influxes of saline fluids and evaporations play critical
role on the evolution of saline precipitates [9].  The evaporation of saline fluids precipitates
saline minerals initially at peripheral portions of saline droplets inside the
pores. The ionic components of chloride, carbonate and sulphate, hydroxide and
water play critical role in the formation of mixed crystals of mineral
components rather than individual minerals. The scope of the investigation
mainly lies to trace the trend of changes of chemical composition during
successive crystallization of chloride components from the primary carbonate
source. Generally, the composition of groundwater is enriched with bicarbonates
and depleted in carbonates. Similar condition is true in the case of saline
waters interlocked in the pore spaces of lime-mortars.

Ikaite
(CaCO3.6H2O) has recently been reported from the polar
ice sediments both in Antarctic and Artic Oceans [10-12]. The calcium carbonate has three polymorphs (aragonite, calcite and
vaterite) and three other hydrous phases (mono hydrocalcite, ikaite and
amorphous calcium). The solution super saturation is a key factor
contributing to the stabilization of the polymorphic phases of calcium
carbonate [13]. The Super saturation ratio of carbonate polymorphs increases
with pH.  The rapidly changing super
saturation during the course of calcium carbonate precipitation, play critical
role on the growth and nucleation of ikaite formation under favourable
environment. The study of precipitation and crystallization of calcium
carbonate polymorphic phases is rather complex and not much research is done
regarding their stability condition nucleation and crystal growth [14 and 15].
The water enriched with Ca2+ and HCO3- may
continue to flow through the void and capillary pore spaces. When the water
enters these voids; pore spaces partial pressure of CO2 decreases and CO2 is
released. The
degassing of CO2 drives the precipitation of carbonate minerals. In
pore spaces resembling a closed system, CO2 and air cannot migrate
in and out of the system. Therefore, the total carbonate concentration will be
constant (the concentration of HCO3 as a function of pH). The
breathing effect of pore spaces will be stopped over time. The influx of
bicarbonate water produces carbonate ions.

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HCO3- + H+                                                         CO32-
+ H+ ……………………………………………..(Equation 1)

Ca(HCO3)2(aq)                 ——–>                           CO2 (g) + H2O
(l) +CaCO3 (s)……………………………(Equation 2)

The Hydrogen ions are molecular
ions with formula H3O+(H2O). A concentration
of hydrogen ions lower than 10-7 is alkaline comprises more of OH
ions. The reaction
between the hydroxide ion and the hydrogen ion removes the hydrogen ion from
the solution, making the solution less acidic and more basic.  At an atmospheric of pressure of one, pure CO2
gas over distilled water may yield  a
solution with a pH near 3.6 [16]. With the increasing pressure pH value
decreases further and the solubility of solution increases considerably. With
large amount of dissolved Ca, pH value moves to the side of alkalinity and more
quantity of Ca will be precipitated at the prevalence of optimum environment. According to Elfil
and Roques [14] two hydrated forms of amorphous calcium carbonate and
monohydrate calcium carbonate are precursors of calcite precipitation.  Under water rich environment at low
temperature                              CaCO3
+6H2O…………………………………..…(Equation 3)

ikaite                                                             aragonite

The presence significant amount of normative
silicon and aluminum carbonates indicate that the pore fluids are subjected to
high pressure environment [17]. Similar to ikaite, kieserite (MgSO4.H2O),
epsomite (MgSO4.7H2O) and mirabiite (MgSO4. 10
H2O) form at decreasing concentration of Mg and increasing
concentration of SO4 ions in the saline solution [18].  The evaporation of saline fluids precipitates
saline minerals initially at peripheral portions of saline droplets inside the
pores. Identification of
saline hydrates in the pore spaces is rather difficult. They usually occur in
the form of mixtures of one or two forms minerals. If the mineral phases are
identified more quantitative information may be obtained about the sequences of
crystallization of evaporates and the minerals filled or partially filled in
the pore spaces.  The cavities and pore
spaces >10?m fluid flows by gravitational forces and the pores less than 1?m
generally adsorption process takes place [19]. Though all these pores occur in
the lime plaster, the pores of sizes between 1 and 10?m play vital role in the
precipitation /deposition of saline minerals in pore spaces. Though pore spaces
in the heritage structure improve advantageous thermal and acoustic barrier,
they also have some disadvantages by their groundwater and repeated
precipitation and evaporation of fossil fluids forming hydrates and anhydrates
inducing cracks.

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