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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
Southern Railway heritage 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 minerals. 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

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