Tables1 and 2 provide EDS analyses of solid phases precipitated in the pore spacesand capillary channels present in the lime mortar plaster sample taken from theSouthern Railway heritage structure.
An attempt was made to trace thecompositional variations of different calcium carbonate phases present asglobular vaterite, plates of aragonite and ikaite, radiating fibers and drusesof calcite. The EDS analysesshowed wide variations in their textures, crystal growth patterns,morphologies, habits and crystal structures. The carbonate minerals present inthe form of amorphous calcium carbonate, globular vaterite, plates ofaragonite, and fibers of calcite. Moreover, globular vaterite growth pattern ofperipheral encrustations were also seen. Thin films and plates of aragonite andprisms of ikalite indicate their rapid stages of crystallization; relatively atelevated pore-pressure and low temperature. The co-existence of these mineralsdesignated the equilibrium state of their formation.
Theradiating fibers and druses of calcium carbonate indicated that theyincorporate significant amount of water in their lattices; similar to theformation of zeolites in their cavities. Though these minerals do not show anydistinct chemical variations among the paragenesis of carbonate minerals, theyexhibited continuous compositional chemical variations; implied that they werederived from the same source of groundwater seepage through capillary pores.Figure 1 shows such a positive linear variation between aluminum carbonate andsilicon carbonate. Both Al and Si dissolve and precipitate respectively highlyacidic and alkaline conditions.
This revealed that the original groundwatersource might be initially acidic with dissolution of enough CO2.They might have precipitated by the liberation of excessive CO2 fromseepage alkaline solution. The pH is the monitoring factor controlling theprecipitation of these components.
The carbonated seepage of water through thecapillary pores contain very low and limited concentration of dolomite (Figure2), Mg, Fe, Na and K (Figures 3 and 4). The carbonates are precipitated fromthe carbonated seepage water showed two distinct trends of precipitation; thenormal trend of precipitation took place with progressive precipitation atsaturated CO2 seepage water and the other linear trend movesnegatively during depletion of excessive CO2 with increasingprecipitation of carbonates (Figure 5). The progressive normative carbonateprecipitation causes depletion of gypsum components (Figure 6). The sodium ionsremain constant while enrichment of Ca ions. A negative correlation ofenrichment of Na ions against Ca ions was due to enrichment of salinity level(Figure 7). A Similar trend was observed for the distribution ofnormative alkali carbonates and normative calcite distribution (8).
Thedistribution of Ca/CO2 against Si/Al and Na+K against Ca/CO2 exhibitednegative correlations (Figures 9 and 10). All these diagrams in the Figures 1-10revealed that the dissolved CO2 in the seepage water played acritical role in dissolution of ionic materials. The depletion of CO2by escape of dissolved CO2 from the alkaline seepage water inducedprecipitation of carbonate minerals. The escaped CO2 from alkalineseepage water fills the empty spaces of partially filled pore spaces; alongwith air components induce and increase pore pressure.
The influx of incomingpore-fluids additionally imparts more pressure on the pore fluids. Therefore,relatively high-pressure minerals like aragonite and ikaite concentrate in thepore fluid. During the course of evaporation, the volume of pore fluid shrinkswith free growth of hydrated carbonates in voids and capillary channels. Atthat time, sudden increasing of volume of free spaces in voids and capillarychannels drastically reduces temperature of pore fluids that further promotescrystallization of ikaite like hydrated minerals. The pore-fluids withenrichment of Ca, CO2 and H2O also favor crystallizationof ikaite