1 monsoon to rainy season. The groundwater is hard

1 Introduction

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.

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2 Methodologies Adopted

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.

 

3 Results and Discussions

3.1 Carbonate Phases

3.1.1
Compositional variations of carbonate minerals

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.