The The mode of occurrences of sulphate

The
chemical analyses carried out by energy dispersive X-ray analyses, structural
formula on the basis of 4(O) ions and Rittman’s normative calculations are
given in the Table 7. The mode of occurrences of sulphate bearing minerals are
discussed here. With increasing solubility of CaCO3 and NaCl;
crystallization of gypsum increases in pore-fluid 4.  It was sucked by capillary action and
adsorption and circulated through minute pores and voids present in the lime
mortar plaster. The height of suction capillary pores inversely proportional to
the dimension of capillary pore sizes. The wide compositional variations of sulphate
minerals may be due to crystal field stabilization energy possessed within the
crystals. Accordingly, solid angles and edges of crystals exposed to saturated
solution grow faster than the flat surfaces 27. Thus, orientations of solid
angles and edges exposed to super saturated pore-fluids during the course of
time of growing crystals play critical roles for compositional variations. As
water of crystallization increases differential expansions and shrinkages are
also increases in rainy and summer seasons. Gypsum dehydration reduces the
volume 39%. Similarly, the hydration of anhydrite increase volume 30 to 50%
28. Pore-fluids circulated through lime mortar is low viscous and can easily
penetrate and circulate through intermittent layers of lime mortar. Earlier
formed carbonates and sulphates can form as seeds to form gypsum.  During rainy season more, quantity fluids
circulated into the building structure. At the end of the season, precipitation
of lime materials took place by evaporations along inter-granular boundaries of
sands/mineral grains, pore-walls, voids, and cavities. Deposition of crust or
films of mineral materials generally takes place where the circulating fluid
passing through the grains of higher specific surface area. Crystal growths of
ettringite or thaumasite expand the volume of cavities creating hairline cracks
in the adjoining masonry structures. Scales of gypsum and other sulphate
minerals precipitated at super-saturation by mixing of groundwater and
estuarine water and also by changes in temperature, pressure, salinity levels,
pH and CO2 29. The compositional changes of pore-fluids were
restricted to seasonal rainfall, dryness, influx of pore-fluids and their
evaporation.  Most pores are partially or
completely filled with sulphate bearing pore fluids and repeated influxes of
saline fluids and evaporations play critical role on the evolution of sulphate
precipitates 9. The influx of pore-fluids in permeable open channels or into
isolated air voids under closed system cause changes of temperature and
pressure and dissolution and precipitation.