This is slightly lower in rainy seasons to the

This study presents
observation from a comprehensive Scanning Electron Microscopy (SEM) study of
lime mortar from a heritage structure. Microstructural and geo-chemical
characterizations of carbonate, chloride and sulphate phases were performed based
on energy dispersive X-ray analysis. The hydrated carbonates underwent
polymorphic changes from vaterite, aragonite and calcite. The habit of
crystallization calcite widely varies depending upon its environment of
crystallization amidst pore spaces and voids. The repeated influxes of and
evaporations of capillary seepage ground water and storm water into the lime
mortar; precipitated carbonate materials of wide compositional variations. The
study traced presence of halite mineral crystallized as interstitial skeletal
or hopper crystals.  A linear channel of
5×2 µm filled with these crystals indicated that the capillary forces
played a critical role in the influxes and evaporation of fossil pore fluids
with successive chemical variations from bicarbonate ground water source
contaminated with chloride ions. Presence of portlandite, anhydrite, and gypsum
was confirmed along with minor traces ettringite and thaumasite. Minerals of
higher water of crystallization trend towards from anhydrite, gypsum, thaumasite
to ettringite with increasing volumes during wet monsoon periods.  Dehydrated minerals form in reversible order
of decreasing volumes in dry seasons.  A
linear variation of chemical compositions of these minerals indicated that they
were crystallized from a common source of pore fluids of meteoric water
composed with carbonates, chlorides and sulphates. The repeated differential
order volume change by expansions and shrinkages has induced development of
hairline cracks and deformations in lime mortar. Use of lime and lime mortars
for construction of buildings was a very common practice in the past. For
renovation and reconstruction of heritage structures, it is necessary to
understand physicochemical characteristics of pore-fluids and growth of
hydrated and dehydrated minerals leading to volume changes which induce
structural deformation during the course of time. The present study reports
geo-chemical characterization of sulfate phases in lime mortar obtained from a
heritage structure. The present investigations are focused on crystal
morphology and geo-chemical characterization of carbonate, chloride and sulphate
bearing minerals present in the capillary channels and voids of lime mortars
used in a heritage structure. Along the coastal tract in which the structure is
located; the groundwater is contaminated with saline estuarine water. The pH
level varies between slightly alkaline (pH 7.04) to strongly alkaline (pH 8.36)
in pre-monsoon periods. It is slightly lower in rainy seasons to the extent of
pH 6.85. The slope gradient of the terrain is lower than 1:1000. The elevation
of the ground level is 3.5m above mean sea level (MSL). The groundwater table
lies 6 m below the ground level. It fluctuates ±5m from severe dry monsoon to
rainy season. The groundwater is hard saline water enriched with bicarbonates,
sulphates and chlorides 1-3.Representative mortar samples were collected from the heritage
structure and comprehensive geochemical analyses were carried out. The
scientific interpretations were made based on the energy dispersive X-ray
analysis (EDS) of Scanning Electron Microscope images. These images were
captured at various magnification levels by studying the morphology and crystal
habit of mineral grains deposited in the pore-spaces and cavities present in
the mortar sample. The carbonate, chloride and sulphate bearing minerals were
analyzed in first, second and third phases respectively. Among the 95 EDS
analyses, 70 analyses enriched with carbonate ions were selected for the first
phase study. The EDS analyses made in different parts
of the samples show genetic relationship with each other. The spot EDS analyses
were more specific with reference to bulk analyses in the same sample. The EDS
analyses made on lime mortar in different parts of the samples encompass the
compositional variations and genetic relationships with each other. The trend
of chemical variation between site and bulk analyses was also traced with help
of structural and textural variation of EDS images. Taking the oxygen (O)
content as standard estimation, the excess oxygen content was allotted to CO2
content for correction. Assuming that carbonate formation takes place under
oxygen enriched state; all ferrous iron was converted into ferric state. In
some cases, when the oxygen content is insufficient, the ferric iron is reduced
to ferrous state particularly when the saline material is enriched with
chloride components with depletion of carbonates. The elements determined were
reduced to 100% and redistributed to respective elements. These elements were
recalculated to their oxide form, cationic distribution (Rittman, 4) and for
determination of their structural formula on the basis of 6 (O) ions (Deer
et.al. 5). The presence of excessive CO2 content in many of the
samples indicate that; the water molecules were present in significant amount
in the composition of normative carbonate minerals. Perhaps, this may be due to
presence of hydrated carbonate, sulphate and chloride minerals.  The EDS analyses of both bulk and spot
positions are vary in their chemical compositions. Geochemical binary
distribution of chemical constituents was drawn to show the trend of evolution
of pore-fluids.  Among the 95 EDS analyses and images 24 analyses enriched with
chloride ions were selected for the second phase analysis. Among 95 analyzes were
made 18 analyzes enriched with S constituents were selected and studied. These
analyzes were calculated into oxides then recalculated on the basis of 4 (O)
atoms for gypsum molecules 5. Using excessive oxygen presented in the
analyzed sample, water contents were estimated, assuming that all excessive
oxygen presented in the EDS analyses for water content of HOH+ and
HOH-. The norms indicate a continuous compositional variation from
anhydrite, gypsum, thaumasite to ettringite. 
The composition of solid materials deposited in the pore spaces
determined by SEM-EDS observation represents for a fixed point or specific area
of nonmetric scale. However, the EDS analysis may not suitable for quantifying
hydrogen and other elements with atomic number less than 6 (including carbon).
Quantification of hydrogen and other elements with atomic number below 6 is difficult
with EDS (even carbon may be erroneous). So the detection of moisture content,
combined water and carbon may not be possible.  Due to the oxidation at the time of analytical
processes, the carbon may be oxidized to CO2 and eventually express more amount than the exact one. On
another similar note; the iron will be quantified as total content in ferrous
from irrespective of degree of oxidation.Tables 1 and 2 provide EDS analyses of
solid phases precipitated in the pore spaces and capillary channels present in
the lime mortar plaster sample taken from the structure.  An attempt was made to trace the compositional
variations of different calcium carbonate phases present as globular vaterite,
plates of aragonite and ikaite, radiating fibers and druses of calcite. The EDS
analyses showed wide variations in their textures, crystal growth patterns,
morphologies, habits and crystal structures. The carbonate minerals present in
the form of amorphous calcium carbonate, globular vaterite, plates of
aragonite, and fibers of calcite. Moreover, globular vaterite growth pattern of
peripheral encrustations were also seen. Thin films and plates of aragonite and
prisms of ikalite indicate their rapid stages of crystallization; relatively at
elevated pore-pressure and low temperature. The co-existence of these minerals
designated the equilibrium state of their formation. The radiating fibers and
druses of calcium carbonate indicated that they incorporate significant amount
of water in their lattices; similar to the formation of zeolites in their
cavities. Though these minerals do not show any distinct chemical variations
among the paragenesis of carbonate minerals, they exhibited continuous
compositional chemical variations; implied that they were derived from the same
source of groundwater seepage through capillary pores. Figure 1 shows such a
positive linear variation between aluminum carbonate and silicon carbonate. Both
Al and Si dissolve and precipitate respectively highly acidic and alkaline
conditions. This revealed that the original groundwater source might be
initially acidic with dissolution of enough CO2. They might have
precipitated by the liberation of excessive CO2 from seepage
alkaline solution. The pH is the monitoring factor controlling the
precipitation of these components. The carbonated seepage of water through the
capillary pores contain very low and limited concentration of dolomite (Figure
2), Mg, Fe, Na and K (Figures 3 and 4). The carbonates are precipitated from
the carbonated seepage water showed two distinct trends of precipitation; the
normal trend of precipitation took place with progressive precipitation at
saturated CO2 seepage water and the other linear trend moves
negatively during depletion of excessive CO2 with increasing
precipitation of carbonates (Figure 5). The progressive normative carbonate
precipitation causes depletion of gypsum components (Figure 6). The sodium ions
remain constant while enrichment of Ca ions. A negative correlation of
enrichment of Na ions against Ca ions was due to enrichment of salinity level
(Figure 7). A Similar trend was observed for the distribution of normative
alkali carbonates and normative calcite distribution (8). The distribution of
Ca/CO2 against Si/Al and Na+K against Ca/CO2 exhibited
negative correlations (Figures 9 and 10). All these diagrams in the Figures
1-10 revealed that the dissolved CO2 in the seepage water played a critical
role in dissolution of ionic materials. The depletion of CO2 by escape
of dissolved CO2 from the alkaline seepage water induced precipitation
of carbonate mineral phases. The escaped CO2 from alkaline seepage
water fills the empty spaces of partially filled pore spaces; along with air
components induce and increase pore pressure. The influx of incoming
pore-fluids additionally imparts more pressure on the pore fluids. Therefore,
relatively high-pressure minerals like aragonite and ikaite concentrate in the
pore fluid. During the course of evaporation, the volume of pore fluid shrinks
with free growth of hydrated carbonates in voids and capillary channels. At
that time, sudden increasing of volume of free spaces in voids and capillary
channels drastically reduces temperature of pore fluids that further promotes
crystallization of ikaite like hydrated minerals. The pore-fluids with
enrichment of Ca, CO2 and H2O also favor crystallization
of ikaite. Figure 11 represents
SEM secondary electrons images of lime mortars studied along with sites for
analysis considered. The images describe the habits of different carbonate
phases which were present in the system.  
The salient features are; images 11a to 11f – kidney shaped vaterite
encrustations; figures 11g to 11n -globular grains of vaterites; figures 11o to
11s – amorphous calcium carbonates; figures 11t to 11w – prism like carbonates
along the margin of cavity channels; figures 11x to 11ab globular carbonate
mineral; Figures 11 ac and 11ad – saccharoid of calcium carbonates. Figure 12 presents continuation of figure 11 depicting crystal
habits and textures of calcite mineral.  
The images 12a to 12c represents saccharoids of calcium carbonates; 12d
to 12g elucidate druses of calcium carbonate; 12h to 12i give druses under
higher magnifications; 12j to 12k illustrate plate like crystals at the edges
of cavities; 12l to 12ab prism like carbonates along the margin of cavity
channels and amorphous phases; 12ac to 12ai provides massive amorphous calcium
carbonate precipitate.Hydrates
are composed with water of crystallization in their structures. When a hydrate
is thermally exposed, it absorbs enormous quantity of heat (endothermic) and
forms anhydrous mineral. When an anhydrate is immersed into water; it absorbs
water and releases huge 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.  Efflorescence
may cause spontaneous loss of water of crystallization in some hydrates.   Others
absorb water into their structure forming hydroscopic hydrates. 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
specific gravities and 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. Aragonite, calcite and vaterite are three
polymorphs of calcium carbonate. Similarly, other hydrous phases of calcium
carbonates are mono hydrocalcite, ikaite and amorphous calcium. The super
saturation of the solution is a key factor interposing to the stabilization of
the polymorphic forms of calcium carbonate 13. Generally, the super
saturation ratio of carbonate polymorphs increases with pH.  The rapid changes during the course of super
saturation 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.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 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.Formation of hydrated minerals from
calcium carbonate saturated solutions is rather complex, but they form at
natural condition at low temperature condition and at relatively high-pressure
conditions. Such an environment is possible to occur in pore spaces occurring
in building structures in tropical and temperate regions. The volume changes
during hydration and dehydration of these hydrates induce hairline cracks
during the course of time by repeated influxes and evaporation of capillary
seepage pore fluids. 

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